AD AD7705BR 3 v/5 v, 1 mw 2-/3-channel 16-bit, sigma-delta adc Datasheet

a
3 V/5 V, 1 mW
2-/3-Channel 16-Bit, Sigma-Delta ADCs
AD7705/AD7706*
FEATURES
AD7705: Two Fully Differential Input Channel ADCs
AD7706: Three Pseudo Differential Input Channel ADCs
16 Bits No Missing Codes
0.003% Nonlinearity
Programmable Gain Front End
Gains from 1 to 128
Three-Wire Serial Interface
SPI™, QSPI™, MICROWIRE™ and DSP Compatible
Schmitt Trigger Input on SCLK
Ability to Buffer the Analog Input
2.7 V to 3.3 V or 4.75 V to 5.25 V Operation
Power Dissipation 1 mW max @ 3␣ V
Standby Current 8 ␮A max
16-Lead DIP, 16-Lead SOIC and TSSOP Packages
FUNCTIONAL BLOCK DIAGRAM
VDD
REF IN(–)
AD7705/AD7706
CHARGE
BALANCING
A/D CONVERTER
ANALOG
INPUT
CHANNELS
MAX
BUFFER
The AD7705/AD7706 operate from a single 2.7 V to 3.3 V or
4.75 V to 5.25 V supply. The AD7705 features two fully differential analog input channels while the AD7706 features three
pseudo differential input channels. Both devices feature a differential reference input. Input signal ranges of 0 mV to +20␣ mV
through 0 V to +2.5␣ V can be incorporated on both devices when
operating with a VDD of 5 V and a reference of 2.5 V. They can
also handle bipolar input signal ranges of ±20␣ mV through ±2.5␣ V,
which are referenced to the AIN(–) inputs on the AD7705 and to
the COMMON input on the AD7706. The AD7705/AD7706,
with 3 V supply and a 1.225 V reference, can handle unipolar
input signal ranges of 0 mV to +10␣ mV through 0 V to +1.225␣ V.
Its bipolar input signal ranges are ± 10␣ mV through ± 1.225␣ V.
The AD7705/AD7706 thus perform all signal conditioning and
conversion for a two- or three-channel system.
The AD7705/AD7706 are ideal for use in smart, microcontroller
or DSP-based systems. They feature a serial interface that can
be configured for three-wire operation. Gain settings, signal
polarity and update rate selection can be configured in software
*Protected by U.S. Patent Number 5,134,401.
SPI and QSPI are trademarks of Motorola, Inc.
MICROWIRE is a trademark of National Semiconductor.
REV. A
Information furnished by Analog Devices is believed to be accurate and
reliable. However, no responsibility is assumed by Analog Devices for its
use, nor for any infringements of patents or other rights of third parties
which may result from its use. No license is granted by implication or
otherwise under any patent or patent rights of Analog Devices.
S-D
MODULATOR
PGA
A = 1<128
DIGITAL FILTER
SERIAL INTERFACE
REGISTER BANK
GENERAL DESCRIPTION
The AD7705/AD7706 are complete analog front ends for low
frequency measurement applications. These two-/three-channel
devices can accept low level input signals directly from a transducer and produce a serial digital output. They employ a sigmadelta conversion technique to realize up to 16 bits of no missing
codes performance. The selected input signal is applied to a
proprietary programmable gain front end based around an analog modulator. The modulator output is processed by an onchip digital filter. The first notch of this digital filter can be
programmed via an on-chip control register allowing adjustment
of the filter cutoff and output update rate.
REF IN(+)
MCLK IN
MCLK OUT
SCLK
CLOCK
GENERATION
CS
DIN
DOUT
GND
DRDY
RESET
using the input serial port. The part contains self-calibration and
system calibration options to eliminate gain and offset errors on
the part itself or in the system.
CMOS construction ensures very low power dissipation, and the
power-down mode reduces the standby power consumption to
20␣ µW typ. These parts are available in a 16-lead, 0.3 inch-wide,
plastic dual-in-line package (DIP), a 16-lead wide body (0.3
inch) small outline (SOIC) package and also a low profile 16lead TSSOP.
PRODUCT HIGHLIGHTS
1. The AD7705/AD7706 consumes less than 1 mW at 3 V
supplies and 1␣ MHz master clock, making it ideal for use in
low power systems. Standby current is less than 8␣ µA.
2. The programmable gain input allows the AD7705/AD7706
to accept input signals directly from a strain gage or transducer, removing a considerable amount of signal conditioning.
3. The AD7705/AD7706 is ideal for microcontroller or DSP
processor applications with a three-wire serial interface reducing the number of interconnect lines and reducing the
number of opto-couplers required in isolated systems.
4. The part features excellent static performance specifications
with 16 bits, no missing codes, ± 0.003% accuracy and low
rms noise (<600␣ nV). Endpoint errors and the effects of
temperature drift are eliminated by on-chip calibration options, which remove zero-scale and full-scale errors.
One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A.
Tel: 781/329-4700
World Wide Web Site: http://www.analog.com
Fax: 781/326-8703
© Analog Devices, Inc., 1998
AD7705/AD7706–SPECIFICATIONS
(VDD = +3 V or 5 V, REF IN(+) = +1.225␣ V with VDD = 3 V and +2.5 V
with VDD = 5 V; REF␣ IN(–) = GND; MCLK IN = 2.4576␣ MHz unless otherwise noted. All specifications TMIN to TMAX unless otherwise noted.)
Parameter
STATIC PERFORMANCE
No Missing Codes
Output Noise
Integral Nonlinearity2
Unipolar Offset Error
Unipolar Offset Drift4
Bipolar Zero Error
Bipolar Zero Drift4
5
Positive Full-Scale Error
Full-Scale Drift4, 6
Gain Error7
Gain Drift4, 8
Bipolar Negative Full-Scale Error2
Bipolar Negative Full-Scale Drift4
ANALOG INPUTS/REFERENCE INPUTS
Input Common-Mode Rejection (CMR)2
VDD = 5 V
Gain = 1
Gain = 2
Gain = 4
Gain = 8v128
VDD = 3 V
Gain = 1
Gain = 2
Gain = 4
Gain = 8v128
Normal-Mode 50 Hz Rejection2
Normal-Mode 60 Hz Rejection2
Common-Mode 50 Hz Rejection2
Common-Mode 60 Hz Rejection2
Absolute/Common-Mode REF IN Voltage2
Absolute/Common-Mode AIN Voltage2, 9
B Version1
Units
Conditions/Comments
16
See Tables I and III
± 0.003
See Note 3
0.5
See Note 3
0.5
0.1
See Note 3
0.5
See Note 3
0.5
± 0.003
1
0.6
Bits min
Guaranteed by Design. Filter Notch < 60␣ Hz
Depends on Filter Cutoffs and Selected Gain
Filter Notch < 60␣ Hz. Typically ± 0.0003%
% of FSR max
µV/°C typ
µV/°C typ
µV/°C typ
µV/°C typ
ppm of FSR/°C typ
% of FSR typ
µV/°C typ
µV/°C typ
dB typ
dB typ
dB typ
dB typ
105
110
120
130
98
98
150
150
GND to VDD
GND – 30 mV
VDD + 30␣ mV
GND + 50␣ mV
VDD – 1.5␣ V
1
10
0 to +VREF/GAIN11
± VREF/GAIN
GAIN × fCLKIN /64
fCLKIN /8
dB typ
dB typ
dB typ
dB typ
dB typ
dB typ
dB typ
dB typ
V min to V max
V min
V max
V min
V max
nA max
pF max
nom
nom
1/1.75
V min/max
REF IN(+) – REF IN(–) Voltage
1/3.5
V min/max
REF IN Input Sampling Rate, fS
fCLKIN /64
AIN DC Input Current2
AIN Sampling Capacitance2
AIN Differential Voltage Range10
AIN Input Sampling Rate, fS
Reference Input Range
REF IN(+) – REF IN(–) Voltage
LOGIC INPUTS
Input Current
All Inputs Except MCLK IN
MCLK
All Inputs Except SCLK and MCLK IN
VINL, Input Low Voltage
VINH, Input High Voltage
SCLK Only (Schmitt Triggered Input)
VT+
VT–
VT+ – VT–
SCLK Only (Schmitt Triggered Input)
VT+
VT–
VT+ – VT–
MCLK IN Only
VINL, Input Low Voltage
VINH, Input High Voltage
MCLK IN Only
VINL, Input Low Voltage
VINH, Input High Voltage
Typically ± 0.001%
For Gains of 1 to 4
For Gains of 8 to 128
Specifications for AIN and REF IN Unless Noted
96
105
110
130
Absolute/Common-Mode AIN Voltage2, 9
For Gains 1, 2 and 4
For Gains 8, 16, 32, 64 and 128
For Filter Notches of 25 Hz, 50 Hz, ± 0.02 × fNOTCH
For Filter Notches of 20 Hz, 60 Hz, ± 0.02 × fNOTCH
For Filter Notches of 25 Hz, 50 Hz, ± 0.02 × fNOTCH
For Filter Notches of 20 Hz, 60 Hz, ± 0.02 × fNOTCH
BUF Bit of Setup Register = 0
BUF Bit of Setup Register = 1
Unipolar Input Range (B/U Bit of Setup Register = 1)
Bipolar Input Range (B/U Bit of Setup Register = 0)
For Gains of 1 to 4
For Gains of 8 to 128
VDD = 2.7 V to 3.3 V. VREF = 1.225 ± 1% for Specified
Performance
VDD = 4.75 V to 5.25 V. VREF = 2.5 ± 1% for Specified
Performance
±1
± 10
µA max
µA max
Typically ± 20 nA
Typically ± 2 µA
0.8
0.4
2.0
V max
V max
V min
VDD = 5 V
VDD = 3 V
VDD = 3 V and 5 V
VDD = 5 V NOMINAL
1.4/3
0.8/1.4
0.4/0.8
V min/V max
V min/V max
V min/V max
1/2.5
0.4/1.1
0.375/0.8
V min/V max
V min/V max
V min/V max
0.8
3.5
V max
V min
0.4
2.5
V max
V min
VDD = 3 V NOMINAL
VDD = 5 V NOMINAL
VDD = 3 V NOMINAL
–2–
REV. A
AD7705/AD7706
Parameter
LOGIC OUTPUTS (Including MCLK OUT)
VOL, Output Low Voltage
VOL, Output Low Voltage
VOH, Output High Voltage
VOH, Output High Voltage
Floating State Leakage Current
Floating State Output Capacitance13
Data Output Coding
SYSTEM CALIBRATION
Positive Full-Scale Calibration Limit14
Negative Full-Scale Calibration Limit14
Offset Calibration Limit14
Input Span15
POWER REQUIREMENTS
VDD Voltage
Power Supply Currents16
VDD Voltage
Power Supply Currents16
Standby (Power-Down) Current17
Power Supply Rejection18
B Version1
Units
Conditions/Comments
0.4
0.4
4
VDD–0.6
± 10
9
Binary
Offset Binary
V max
V max
V min
V min
µA max
pF typ
ISINK = 800␣ µA Except for MCLK OUT.12 VDD = 5 V.
ISINK = 100␣ µA Except for MCLK OUT.12 VDD = 3 V.
ISOURCE = 200 µA Except for MCLK OUT.12 VDD = 5 V.
ISOURCE = 100␣ µA Except for MCLK OUT.12 VDD = 3 V.
(1.05 × VREF)/GAIN
–(1.05 × VREF)/GAIN
–(1.05 × VREF)/GAIN
(0.8 × VREF)/GAIN
(2.1 × VREF)/GAIN
V max
V max
V max
V min
V max
GAIN Is the Selected PGA Gain (1 to 128)
GAIN Is the Selected PGA Gain (1 to 128)
GAIN Is the Selected PGA Gain (1 to 128)
GAIN Is the Selected PGA Gain (1 to 128)
GAIN Is the Selected PGA Gain (1 to 128)
+2.7 to +3.3
V min to V max
0.32
0.6
0.4
0.6
0.7
1.1
+4.75 to +5.25
mA max
mA max
mA max
mA max
mA max
mA max
V min to V max
0.45
0.7
0.6
0.85
0.9
1.3
16
8
See Note 19
mA max
mA max
mA max
mA max
mA max
mA max
µA max
µA max
dB typ
For Specified Performance
Digital I/Ps = 0␣ V or VDD. External MCLK IN and
CLK DIS = 1
BUF Bit = 0. fCLKIN = 1␣ MHz. Gains of 1 to 128
BUF Bit = 1. fCLKIN = 1␣ MHz. Gains of 1 to 128
BUF Bit = 0. fCLKIN = 2.4576␣ MHz. Gains of 1 to 4
BUF Bit = 0. fCLKIN = 2.4576␣ MHz. Gains of 8 to 128
BUF Bit = 1. fCLKIN = 2.4576␣ MHz. Gains of 1 to 4
BUF Bit = 1. fCLKIN = 2.4576␣ MHz. Gains of 8 to 128
For Specified Performance
Digital I/Ps = 0␣ V or VDD. External MCLK IN and
CLK DIS = 1.
BUF Bit = 0. fCLKIN = 1␣ MHz. Gains of 1 to 128
BUF Bit = 1. fCLKIN = 1␣ MHz. Gains of 1 to 128
BUF Bit = 0. fCLKIN = 2.4576␣ MHz. Gains of 1 to 4
BUF Bit = 0. fCLKIN = 2.4576␣ MHz. Gains of 8 to 128
BUF Bit = 1. fCLKIN = 2.4576␣ MHz. Gains of 1 to 4
BUF Bit = 1. fCLKIN = 2.4576␣ MHz. Gains of 8 to 128
External MCLK IN = 0 V or VDD. VDD = 5 V. See Figure 9
External MCLK IN = 0 V or VDD. VDD = 3 V
Unipolar Mode
Bipolar Mode
NOTES
1
Temperature range as follows: B Version, –40°C to +85°C.
2
These numbers are established from characterization or design at initial product release.
3
A calibration is effectively a conversion so these errors will be of the order of the conversion noise shown in Tables I and III. This applies after calibration at the
temperature of interest.
4
Recalibration at any temperature will remove these drift errors.
5
Positive Full-Scale Error includes Zero-Scale Errors (Unipolar Offset Error or Bipolar Zero Error) and applies to both unipolar and bipolar input ranges.
6
Full-Scale Drift includes Zero-Scale Drift (Unipolar Offset Drift or Bipolar Zero Drift) and applies to both unipolar and bipolar input ranges.
7
Gain Error does not include Zero-Scale Errors. It is calculated as Full-Scale Error–Unipolar Offset Error for unipolar ranges and Full-Scale Error–Bipolar Zero Error for
bipolar ranges.
8
Gain Error Drift does not include Unipolar Offset Drift/Bipolar Zero Drift. It is effectively the drift of the part if zero scale calibrations only were performed.
9
This common-mode voltage range is allowed provided that the input voltage on analog inputs does not go more positive than V DD + 30 mV or go more negative than
GND – 30␣ mV. Parts are functional with voltages down to GND – 200 mV, but with increased leakage at high temperature.
10
The analog input voltage range on AIN(+) is given here with respect to the voltage on AIN(–) on the AD7705 and is given with respect to the COMMON input on the
AD7706. The absolute voltage on the analog inputs should not go more positive than V DD + 30␣ mV, or go more negative than GND␣ – 30␣ mV for specified performance, input
voltages of GND – 200 mV can be accommodated, but with increased leakage at high temperature.
11
VREF = REF IN(+) – REF IN(–).
12
These logic output levels apply to the MCLK OUT only when it is loaded with one CMOS load.
13
Sample tested at +25°C to ensure compliance.
14
After calibration, if the analog input exceeds positive full scale, the converter will output all 1s. If the analog input is less than negative full scale, the device will output all 0s.
15
These calibration and span limits apply provided the absolute voltage on the analog inputs does not exceed V DD + 30␣ mV or go more negative than GND – 30␣ mV. The offset
calibration limit applies to both the unipolar zero point and the bipolar zero point.
16
When using a crystal or ceramic resonator across the MCLK pins as the clock source for the device, the V DD current and power dissipation will vary depending on the crystal or
resonator type (see Clocking and Oscillator Circuit section).
17
If the external master clock continues to run in standby mode, the standby current increases to 150␣ µA typical at 5 V and 75 µA at 3 V. When using a crystal or ceramic
resonator across the MCLK pins as the clock source for the device, the internal oscillator continues to run in standby mode and the power dissipation depends on the crystal
or resonator type (see Standby Mode section).
18
Measured at dc and applies in the selected passband. PSRR at 50␣ Hz will exceed 120␣ dB with filter notches of 25 Hz or 50␣ Hz. PSRR at 60␣ Hz will exceed 120␣ dB with filter
notches of 20 Hz or 60␣ Hz.
19
PSRR depends on both gain and V DD.
Gain
VDD = 3 V
VDD = 5 V
1
86
90
2
78
78
4
85
84
8–128
93
91
Specifications subject to change without notice.
REV. A
–3–
AD7705/AD7706
TIMING CHARACTERISTICS1, 2
Parameter
fCLKIN
3, 4
tCLKIN LO
tCLKIN HI
t1
t2
Read Operation
t3
t4
t5 5
t6
t7
t8
t9 6
t10
Write Operation
t11
t12
t13
t14
t15
t16
(VDD = +2.7␣ V to +5.25␣ V; GND = 0 V; fCLKIN = 2.4576␣ MHz; Input Logic 0 = 0 V, Logic 1 = VDD
unless otherwise noted.)
Limit at TMIN, TMAX
(B Version)
Units
Conditions/Comments
400
2.5
0.4 × tCLKIN
0.4 × tCLKIN
500 × tCLKIN
100
kHz min
MHz max
ns min
ns min
ns nom
ns min
Master Clock Frequency: Crystal Oscillator or Externally Supplied
for Specified Performance
Master Clock Input Low Time. tCLKIN = 1/fCLKIN
Master Clock Input High Time
DRDY High Time
RESET Pulsewidth
0
120
0
80
100
100
100
0
10
60
100
100
ns min
ns min
ns min
ns max
ns max
ns min
ns min
ns min
ns min
ns max
ns max
ns max
DRDY to CS Setup Time
CS Falling Edge to SCLK Rising Edge Setup Time
SCLK Falling Edge to Data Valid Delay
VDD = +5␣ V
VDD = +3.0␣ V
SCLK High Pulsewidth
SCLK Low Pulsewidth
CS Rising Edge to SCLK Rising Edge Hold Time
Bus Relinquish Time after SCLK Rising Edge
VDD = +5␣ V
VDD = +3.0␣ V
SCLK Falling Edge to DRDY High7
120
30
20
100
100
0
ns min
ns min
ns min
ns min
ns min
ns min
CS Falling Edge to SCLK Rising Edge Setup Time
Data Valid to SCLK Rising Edge Setup Time
Data Valid to SCLK Rising Edge Hold Time
SCLK High Pulsewidth
SCLK Low Pulsewidth
CS Rising Edge to SCLK Rising Edge Hold Time
NOTES
1
Sample tested at +25°C to ensure compliance. All input signals are specified with tr = tf = 5 ns (10% to 90% of VDD) and timed from a voltage level of 1.6 V.
2
See Figures 16 and 17.
3
fCLKIN Duty Cycle range is 45% to 55%. f CLKIN must be supplied whenever the AD7705/AD7706 is not in Standby mode. If no clock is present in this case, the device
can draw higher current than specified and possibly become uncalibrated.
4
The AD7705/AD7706 is production tested with f CLKIN at 2.4576␣ MHz (1␣ MHz for some I DD tests). It is guaranteed by characterization to operate at 400␣ kHz.
5
These numbers are measured with the load circuit of Figure 1 and defined as the time required for the output to cross the VOL or VOH limits.
6
These numbers are derived from the measured time taken by the data output to change 0.5␣ V when loaded with the circuit of Figure 1. The measured number is
then extrapolated back to remove effects of charging or discharging the 50 pF capacitor. This means that the times quoted in the timing characteristics are the
true bus relinquish times of the part and as such are independent of external bus loading capacitances.
7
DRDY returns high after the first read from the device after an output update. The same data can be read again, if required, while DRDY is high, although care
should be taken that subsequent reads do not occur close to the next output update.
ISINK (800mA AT VDD = +5V
100mA AT VDD = +3V)
TO OUTPUT
PIN
+1.6V
50pF
ISOURCE (200mA AT VDD = +5V
100mA AT VDD = +3V)
Figure 1. Load Circuit for Access Time and Bus Relinquish Time
–4–
REV. A
AD7705/AD7706
SOIC Package, Power Dissipation . . . . . . . . . . . . . . . 450 mW
θJA Thermal Impedance . . . . . . . . . . . . . . . . . . . . . 75°C/W
Lead Temperature, Soldering
Vapor Phase (60 sec) . . . . . . . . . . . . . . . . . . . . . . +215°C
Infrared (15 sec) . . . . . . . . . . . . . . . . . . . . . . . . . .+220°C
SSOP Package, Power Dissipation . . . . . . . . . . . . . . . 450 mW
θJA Thermal Impedance . . . . . . . . . . . . . . . . . . . . 139°C/W
Lead Temperature, Soldering
Vapor Phase (60 sec) . . . . . . . . . . . . . . . . . . . . . . +215°C
␣ ␣ ␣ ␣ Infrared (15 sec) . . . . . . . . . . . . . . . . . . . . . . . . . .+220°C
ESD Rating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . >4000␣ V
ABSOLUTE MAXIMUM RATINGS*
(TA = +25°C unless otherwise noted)
VDD to GND . . . . . . . . . . . . . . . . . . . . . . . . . . –0.3␣ V to +7␣ V
Analog Input Voltage to GND . . . . . . . . –0.3 V to VDD + 0.3␣ V
Reference Input Voltage to GND . . . . . –0.3 V to VDD + 0.3␣ V
Digital Input Voltage to GND . . . . . . . . –0.3 V to VDD + 0.3 V
Digital Output Voltage to GND . . . . . . –0.3 V to VDD + 0.3 V
Operating Temperature Range
Commercial (B Version) . . . . . . . . . . . . . . –40°C to +85°C
Storage Temperature Range . . . . . . . . . . . . –65°C to +150°C
Junction Temperature . . . . . . . . . . . . . . . . . . . . . . . . . +150°C
Plastic DIP Package, Power Dissipation . . . . . . . . . . 450 mW
θJA Thermal Impedance . . . . . . . . . . . . . . . . . . . . 105°C/W
Lead Temperature, (Soldering, 10 sec) . . . . . . . . . . +260°C
*Stresses above those listed under Absolute Maximum Ratings may cause permanent damage to the device. This is a stress rating only; functional operation of the
device at these or any other conditions above those indicated in the operational
section of this specification is not implied. Exposure to absolute maximum rating
conditions for extended periods may affect device reliability.
ORDERING GUIDE
VDD
Supply
Temperature
Range
Package
Description
Package
Options
AD7705BN
AD7705BR
AD7705BRU
EVAL-AD7705EB
2.7 V to 5.25 V
2.7 V to 5.25 V
2.7 V to 5.25 V
–40°C to +85°C
–40°C to +85°C
–40°C to +85°C
Evaluation Board
Plastic DIP
SOIC
TSSOP
N-16
R-16
RU-16
AD7706BN
AD7706BR
AD7706BRU
EVAL-AD7706EB
2.7 V to 5.25 V
2.7 V to 5.25 V
2.7 V to 5.25 V
–40°C to +85°C
–40°C to +85°C
–40°C to +85°C
Evaluation Board
Plastic DIP
SOIC
TSSOP
N-16
R-16
RU-16
Model
REV. A
–5–
AD7705/AD7706
PIN CONFIGURATIONS
SCLK 1
16
GND
SCLK 1
16
GND
MCLK IN 2
15
VDD
MCLK IN 2
15
VDD
MCLK OUT 3
14
DIN
MCLK OUT 3
14
DIN
13
DOUT
CS 4
AD7705
TOP VIEW
RESET 5 (Not to Scale) 12 DRDY
CS
4
RESET
5
AD7706
DOUT
TOP VIEW
12 DRDY
(Not to Scale)
13
AIN2(+) 6
11
AIN2(–)
AIN1 6
11
AIN3
AIN1(+) 7
10
REF IN(–)
AIN2 7
10
REF IN(–)
AIN1(–) 8
9
REF IN(+)
COMMON 8
9
REF IN(+)
PIN FUNCTION DESCRIPTIONS
Pin No.
Mnemonic
Function
1
SCLK
2
MCLK IN
3
MCLK OUT
4
CS
5
RESET
6
AIN2(+)[AIN1]
7
AIN1(+)[AIN2]
8
AIN1(–)[COMMON]
9
REF IN(+)
Serial Clock. Schmitt-Triggered Logic Input. An external serial clock is applied to this input
to access serial data from the AD7705/AD7706. This serial clock can be a continuous clock
with all data transmitted in a continuous train of pulses. Alternatively, it can be a noncontinuous clock with the information being transmitted to the AD7705/AD7706 in smaller
batches of data.
Master Clock signal for the device. This can be provided in the form of a crystal/resonator or
external clock. A crystal/resonator can be tied across the MCLK IN and MCLK OUT pins.
Alternatively, the MCLK IN pin can be driven with a CMOS-compatible clock and MCLK
OUT left unconnected. The part can be operated with clock frequencies in the range
500 kHz to 5 MHz.
When the master clock for the device is a crystal/resonator, the crystal/resonator is connected
between MCLK IN and MCLK␣ OUT. If an external clock is applied to MCLK IN, MCLK
OUT provides an inverted clock signal. This clock can be used to provide a clock source for
external circuitry and is capable of driving one CMOS load. If the user does not require it,
this MCLK OUT can be turned off via the CLK DIS bit of the Clock Register. This ensures
that the part is not burning unnecessary power driving capacitive loads on MCLK OUT.
Chip Select. Active low Logic Input used to select the AD7705/AD7706. With this input
hard-wired low, the AD7705/AD7706 can operate in its three-wire interface mode with
SCLK, DIN and DOUT used to interface to the device. CS can be used to select the device
in systems with more than one device on the serial bus or as a frame synchronization signal in
communicating with the AD7705/AD7706.
Logic Input. Active low input that resets the control logic, interface logic, calibration
coefficients, digital filter and analog modulator of the part to power-on status.
AD7705: Positive input of the differential Analog Input Channel 2. AD7706: Analog Input
Channel 1.
AD7705: Positive input of the differential Analog Input Channel 1. AD7706: Analog Input
Channel 2.
AD7705: Negative input of the differential Analog Input Channel 1. AD7706: COMMON
Input. Analog inputs for Channels 1, 2 and 3 are referenced to this input.
Reference Input. Positive input of the differential Reference Input to the AD7705/AD7706.
The reference input is differential with the provision that REF IN(+) must be greater than
REF IN(–). REF␣ IN(+) can lie anywhere between VDD and GND.
–6–
REV. A
AD7705/AD7706
Pin No.
Mnemonic
Function
10
REF IN(–)
11
AIN2(–)[AIN3]
12
DRDY
13
DOUT
14
DIN
15
16
VDD
GND
Reference Input. Negative input of the differential reference input to the AD7705/AD7706.
The REF␣ IN(–) can lie anywhere between VDD and GND provided REF␣ IN(+) is greater
than REF␣ IN(–).
AD7705: Negative input of the differential analog Input Channel 2. AD7706: Analog Input
Channel 3.
Logic Output. A logic low on this output indicates that a new output word is available from
the AD7705/AD7706 data register. The DRDY pin will return high upon completion of a
read operation of a full output word. If no data read has taken place between output updates,
the DRDY line will return high for 500 × tCLK␣ IN cycles prior to the next output update.
While DRDY is high, a read operation should neither be attempted nor in progress to avoid
reading from the data register as it is being updated. The DRDY line will return low again
when the update has taken place. DRDY is also used to indicate when the AD7705/AD7706
has completed its on-chip calibration sequence.
Serial Data Output with serial data being read from the output shift register on the part. This
output shift register can contain information from the setup register, communications register, clock register or data register, depending on the register selection bits of the Communications Register.
Serial Data Input with serial data being written to the input shift register on the part. Data
from this input shift register is transferred to the setup register, clock register or communications register, depending, on the register selection bits of the Communications Register.
Supply Voltage, +2.7 V to +5.25 V operation.
Ground reference point for the AD7705/AD7706’s internal circuitry.
OUTPUT NOISE (5 V OPERATION)
Table I shows the AD7705/AD7706 output rms noise for the selectable notch and –3␣ dB frequencies for the part, as selected by FS0
and FS1 of the Clock Register. The numbers given are for the bipolar input ranges with a VREF of +2.5␣ V and VDD = 5 V. These
numbers are typical and are generated at an analog input voltage of 0␣ V with the part used in either buffered or unbuffered mode. Table II
meanwhile shows the output peak-to-peak noise for the selectable notch and –3 dB frequencies for the part. It is important to note that
these numbers represent the resolution for which there will be no code flicker. They are not calculated based on rms noise but on peak-to-peak
noise. The numbers given are for bipolar input ranges with a VREF of +2.5 V and for either buffered or unbuffered mode. These numbers are typical and are rounded to the nearest LSB. The numbers apply for the CLK DIV bit of the Clock Register set to 0.
Table I. Output RMS Noise vs. Gain and Output Update Rate @ 5 V
Filter First
Notch and O/P –3␣ dB
Data Rate
Frequency
MCLK IN = 2.4576 MHz
50␣ Hz
13.1␣ Hz
60␣ Hz
15.72␣ Hz
250␣ Hz
65.5␣ Hz
500␣ Hz
131␣ Hz
MCLK IN = 1 MHz
20␣ Hz
5.24␣ Hz
25␣ Hz
6.55␣ Hz
100␣ Hz
26.2␣ Hz
200␣ Hz
52.4␣ Hz
REV. A
Gain of
1
Gain of
2
Typical Output RMS Noise in ␮V
Gain of
Gain of
Gain of
4
8
16
Gain of
32
Gain of
64
Gain of
128
4.1
5.1
110
550
2.1
2.5
49
285
1.2
1.4
31
145
0.75
0.8
17
70
0.7
0.75
8
41
0.66
0.7
3.6
22
0.63
0.67
2.3
9.1
0.6
0.62
1.7
4.7
4.1
5.1
110
550
2.1
2.5
49
285
1.2
1.4
31
145
0.75
0.8
17
70
0.7
0.75
8
41
0.66
0.7
3.6
22
0.63
0.67
2.3
9.1
0.6
0.62
1.7
4.7
–7–
AD7705/AD7706
Table II. Peak-to-Peak Resolution vs. Gain and Output Update Rate @ 5 V
Filter First
Notch and O/P –3␣ dB
Data Rate
Frequency
MCLK IN = 2.4576 MHz
50␣ Hz
13.1␣ Hz
60␣ Hz
15.72␣ Hz
250␣ Hz
65.5␣ Hz
500␣ Hz
131␣ Hz
MCLK IN = 1 MHz
20␣ Hz
5.24␣ Hz
25␣ Hz
6.55␣ Hz
100␣ Hz
26.2␣ Hz
200␣ Hz
52.4␣ Hz
Gain of
1
Gain of
2
Typical Peak-to-Peak Resolution Bits
Gain of
Gain of
Gain of
Gain of
4
8
16
32
Gain of
64
Gain of
128
16
16
13
10
16
16
13
10
16
16
13
10
16
16
13
10
16
15
13
10
16
14
13
10
15
14
12
10
14
13
12
10
16
16
13
10
16
16
13
10
16
16
13
10
16
16
13
10
16
15
13
10
16
14
13
10
15
14
12
10
14
13
12
10
OUTPUT NOISE (3 V OPERATION)
Table III shows the AD7705/AD7706 output rms noise for the selectable notch and –3␣ dB frequencies for the part, as selected by
FS0 and FS1 of the Clock Register. The numbers given are for the bipolar input ranges with a VREF of +1.225␣ V and a VDD = 3 V.
These numbers are typical and are generated at an analog input voltage of 0␣ V with the part used in either buffered or unbuffered
mode. Table II meanwhile shows the output peak-to-peak noise for the selectable notch and –3 dB frequencies for the part. It is important to note that these numbers represent the resolution for which there will be no code flicker. They are not calculated based on rms noise but
on peak-to-peak noise. The numbers given are for bipolar input ranges with a VREF of +1.225 V and for either buffered or unbuffered
mode. These numbers are typical and are rounded to the nearest LSB. The numbers apply for the CLK DIV bit of the Clock Register set to 0.
Table III. Output RMS Noise vs. Gain and Output Update Rate @ 3 V
Filter First
Notch and O/P –3␣ dB
Data Rate
Frequency
MCLK IN = 2.4576 MHz
50␣ Hz
13.1␣ Hz
60␣ Hz
15.72␣ Hz
250␣ Hz
65.5␣ Hz
500␣ Hz
131␣ Hz
MCLK IN = 1 MHz
20␣ Hz
5.24␣ Hz
25␣ Hz
6.55␣ Hz
100␣ Hz
26.2␣ Hz
200␣ Hz
52.4␣ Hz
Gain of
1
Gain of
2
Typical Output RMS Noise in ␮V
Gain of
Gain of
Gain of
4
8
16
Gain of
32
Gain of
64
Gain of
128
3.8
5.1
50
270
2.4
2.9
25
135
1.5
1.7
14
65
1.3
1.5
9.9
41
1.1
1.2
5.1
22
1.0
1.0
2.6
9.7
0.9
0.9
2.3
5.1
0.9
0.9
2.0
3.3
3.8
5.1
50
270
2.4
2.9
25
135
1.5
1.7
14
65
1.3
1.5
9.9
41
1.1
1.2
5.1
22
1.0
1.0
2.6
9.7
0.9
0.9
2.3
5.1
0.9
0.9
2.0
3.3
Table IV.␣ Peak-to-Peak Resolution vs. Gain and Output Update Rate @ 3 V
Filter First
Notch and O/P –3␣ dB
Data Rate
Frequency
Gain of
1
␣ Gain of
␣␣2
Typical Peak-to-Peak Resolution in Bits
Gain of
Gain of
Gain of
Gain of
4
8
16
32
Gain of
64␣ ␣ ␣ ␣
Gain of
128
16
16
13
10
16
16
13
10
15
15
13
10
15
14
13
10
14
14
12
10
13
13
12
10
13
13
11
10
12
12
11
10
16
16
13
10
16
16
13
10
15
15
13
10
15
14
13
10
14
14
12
10
13
13
12
10
13
13
11
10
12
12
11
10
MCLK IN = 2.4576 MHz
50␣ Hz
13.1␣ Hz
60␣ Hz
15.72␣ Hz
250␣ Hz
65.5␣ Hz
500␣ Hz
131␣ Hz
MCLK IN = 1 MHz
20␣ Hz
5.24␣ Hz
25␣ Hz
6.55␣ Hz
100␣ Hz
26.2␣ Hz
200␣ Hz
52.4␣ Hz
–8–
REV. A
Typical Performance Characteristics–AD7705/AD7706
400
32771
VDD = 5V
VREF = 2.5V
GAIN = 128
50Hz UPDATE RATE
32770
TA = +258C
RMS NOISE = 600nV
300
OCCURRENCE
CODE READ
32769
32768
32767
32766
32765
200
100
32764
32763
0
200
100
300
400 500 600
READING NO.
700
800
0
900 1000
32764
Figure 2. Typical Noise Plot @ Gain = 128 with 50 Hz
Update Rate
32765
32766
32767
CODE
32768
32769
32770
Figure 5. Histogram of Data in Figure 2
1.2
1.2
VDD = 3V
TA = +258C
VDD = 5V
TA = +258C
BUFFERED MODE, GAIN = 128
1.0
1.0
0.8
0.8
BUFFERED MODE, GAIN = 128
IDD – mA
IDD – mA
BUFFERED MODE, GAIN = 1
BUFFERED MODE, GAIN = 1
0.6
0.6
0.4
0.4
UNBUFFERED MODE, GAIN = 128
UNBUFFERED MODE, GAIN = 128
0.2
0.2
UNBUFFERED MODE, GAIN = 1
UNBUFFERED MODE, GAIN = 1
0
0.4
0.6
0.8
1.0
1.2 1.4 1.6 1.8
FREQUENCY – MHz
2.0
2.2
2.4
0
0.4
2.6
Figure 3. Typical IDD vs. MCLKIN Frequency @ 3 V
1.0
1.2
BUFFERED MODE
fCLK = 2.4576MHz, CLKDIV = 0
UNBUFFERED MODE
fCLK = 5MHz, CLKDIV = 1
0.6
0.5
0.4
UNBUFFERED MODE
fCLK = 2.84MHz, CLKDIV = 0
0.3
VDD = 3V
EXTERNAL MCLK
CLKDIS = 1
TA = +258C
0.2
0.1
2.0
2.2
2.4
2.6
BUFFERED MODE
fCLK = 2.4576MHz, CLKDIV = 0
UNBUFFERED MODE
fCLK = 5MHz, CLKDIV = 1
0.6
UNBUFFERED MODE
fCLK = 2.4576MHz, CLKDIV = 0
0.4
VDD = 5V
EXTERNAL MCLK
CLKDIS = 1
TA = +258C
0.2
BUFFERED MODE
fCLK = 1MHz, CLKDIV = 0
BUFFERED MODE
fCLK = 1MHz, CLKDIV = 0
0
0
1
2
4
8
16
32
64
1
128
2
4
8
16
32
64
128
GAIN
GAIN
Figure 4. Typical IDD vs. Gain and Clock Frequency @ 3 V
REV. A
1.2 1.4 1.6 1.8
FREQUENCY – MHz
UNBUFFERED MODE
fCLK = 1MHz,
0.8 CLKDIV = 0
IDD – mA
IDD – mA
UNBUFFERED MODE
fCLK = 1MHz,
0.7 CLKDIV = 0
1.0
BUFFERED MODE
fCLK = 5MHz,
CLKDIV = 1
1.0
0.8
0.8
Figure 6. Typical IDD vs. MCLKIN Frequency @ 5 V
BUFFERED MODE
fCLK = 5MHz,
CLKDIV = 1
0.9
0.6
Figure 7. Typical IDD vs. Gain and Clock Frequency @ 5 V
–9–
AD7705/AD7706
TEK STOP: SINGLE SEQ 50.0kS/s
20
VDD
STANDBY CURRENT – mA
16
1
2
OSCILLATOR = 4.9152 MHz
MCLK IN = 0V OR VDD
12
VDD = 5V
8
VDD = 3V
4
2
OSCILLATOR = 2.4576 MHz
CH1 5.00V
CH2 2.00V
0
–40 –30 –20 –10
5ms/DIV
Figure 8. Typical Crystal Oscillator Power-Up Time
0
10 20 30 40
TEMPERATURE – 8C
50
60
70
80
Figure 9. Standby Current vs. Temperature
ON-CHIP REGISTERS
The AD7705/AD7706 contains eight on-chip registers which can be accessed via the serial port of the part. The first of these is a
Communications Register that controls the channel selection, decides whether the next operation is a read or write operation and
also decides which register the next read or write operation accesses. All communications to the part must start with a write operation to the Communications Register. After power-on or RESET, the device expects a write to its Communications Register. The
data written to this register determines whether the next operation to the part is a read or a write operation and also determines to
which register this read or write operation occurs. Therefore, write access to any of the other registers on the part starts with a write
operation to the Communications Register followed by a write to the selected register. A read operation from any other register on
the part (including the Communications Register itself and the output data register) starts with a write operation to the Communications Register followed by a read operation from the selected register. The Communications Register also controls the standby mode
and channel selection and the DRDY status is also available by reading from the Communications Register. The second register is a
Setup Register that determines calibration mode, gain setting, bipolar/unipolar operation and buffered mode. The third register is
labelled the Clock Register and contains the filter selection bits and clock control bits. The fourth register is the Data Register from
which the output data from the part is accessed. The final registers are the calibration registers which store channel calibration data.
The registers are discussed in more detail in the following sections.
Communications Register (RS2, RS1, RS0 = 0, 0, 0)
The Communications Register is an 8-bit register from which data can either be read or to which data can be written. All communications to the part must start with a write operation to the Communications Register. The data written to the Communications Register determines whether the next operation is a read or write operation and to which register this operation takes place. Once the
subsequent read or write operation to the selected register is complete, the interface returns to where it expects a write operation to
the Communications Register. This is the default state of the interface, and on power-up or after a RESET, the AD7705/AD7706 is
in this default state waiting for a write operation to the Communications Register. In situations where the interface sequence is lost,
if a write operation of sufficient duration (containing at least 32 serial clock cycles) takes place with DIN high, the AD7705 returns
to this default state. Table V outlines the bit designations for the Communications Register.
Table V.␣ Communications Register
0/DRDY (0)
RS2 (0)
RS1 (0)
RS0 (0)
R/W (0)
STBY (0)
CH1 (0)
CH0 (0)
0/DRDY
For a write operation, a “0” must be written to this bit so that the write operation to the Communications Register
actually takes place. If a “1” is written to this bit, the part will not clock on to subsequent bits in the register. It
will stay at this bit location until a “0” is written to this bit. Once a “0” is written to this bit, the next seven bits
will be loaded to the Communications Register. For a read operation, this bit provides the status of the DRDY flag
from the part. The status of this bit is the same as the DRDY output pin.
RS2–RS0
Register Selection Bits. These three bits select to which one of eight on-chip registers the next read or write operation takes place, as shown in Table VI, along with the register size. When the read or write operation to the selected register is complete, the part returns to where it is waiting for a write operation to the Communications
Register. It does not remain in a state where it will continue to access the register.
–10–
REV. A
AD7705/AD7706
Table VI. Register Selection
RS2
RS1
RS0
Register
Register Size
0
0
0
0
1
1
1
1
0
0
1
1
0
0
1
1
0
1
0
1
0
1
0
1
Communications Register
Setup Register
Clock Register
Data Register
Test Register
No Operation
Offset Register
Gain Register
8 Bits
8 Bits
8 Bits
16 Bits
8 Bits
24 Bits
24 Bits
R/W
Read/Write Select. This bit selects whether the next operation is a read or write operation to the selected register.
A “0” indicates a write cycle for the next operation to the appropriate register, while a “1” indicates a read operation from the appropriate register.
STBY
Standby. Writing a “1” to this bit puts the part into its standby or power-down mode. In this mode, the part consumes only 10 µA of power supply current. The part retains its calibration coefficients and control word information when in STANDBY. Writing a “0” to this bit places the part in its normal operating mode.
CH1–CH0
Channel Select. These two bits select a channel for conversion or for access to the calibration coefficients as outlined in Table VII. Three pairs of calibration registers on the part are used to store the calibration coefficients
following a calibration on a channel. They are shown in Tables VII for the AD7705 and Table VIII for the AD7706
to indicate which channel combinations have independent calibration coefficients. With CH1 at Logic 1 and CH0
at a Logic 0, the part looks at the AIN1(–) input internally shorted to itself on the AD7705 or at COMMON
internally shorted to itself on the AD7706. This can be used as a test method to evaluate the noise performance of
the part with no external noise sources. In this mode, the AIN1(–)/COMMON input should be connected to
an external voltage within the allowable common-mode range for the part.
Table VII. Channel Selection for AD7705
CH1
CH0
AIN(+)
AIN(–)
Calibration Register Pair
0
0
1
1
0
1
0
1
AIN1(+)
AIN2(+)
AIN1(–)
AIN1(–)
AIN1(–)
AIN2(–)
AIN1(–)
AIN2(–)
Register Pair 0
Register Pair 1
Register Pair 0
Register Pair 2
Table VIII. Channel Selection for AD7706
REV. A
CH1
CH0
AIN
Reference
Calibration Register Pair
0
0
1
1
0
1
0
1
AIN1
AIN2
COMMON
AIN3
COMMON
COMMON
COMMON
COMMON
Register Pair 0
Register Pair 1
Register Pair 0
Register Pair 2
–11–
AD7705/AD7706
Setup Register (RS2, RS1, RS0 = 0, 0, 1); Power-On/Reset Status: 01␣ Hex
The Setup Register is an eight bit register from which data can either be read or to which data can be written. Table IX outlines the
bit designations for the Setup Register.
Table IX. Setup Register
MD1 (0)
MD0 (0)
G2 (0)
G1 (0)
G0 (0)
B/U (0)
BUF (0)
FSYNC (1)
MD1
MD0
Operating Mode
0
0
Normal Mode: this is the normal mode of operation of the device whereby the device is performing normal
conversions.
0
1
Self-Calibration: this activates self-calibration on the channel selected by CH1 and CH0 of the Communications Register. This is a one-step calibration sequence and when complete the part returns to Normal Mode
with MD1 and MD0 returning to 0, 0. The DRDY output or bit goes high when calibration is initiated and
returns low when this self-calibration is complete and a new valid word is available in the data register. The
zero-scale calibration is performed at the selected gain on internally shorted (zeroed) inputs and the fullscale calibration is performed at the selected gain on an internally-generated VREF/Selected Gain.
1
0
Zero-Scale System Calibration: this activates zero scale system calibration on the channel selected by CH1
and CH0 of the Communications Register. Calibration is performed at the selected gain on the input voltage provided at the analog input during this calibration sequence. This input voltage should remain stable
for the duration of the calibration. The DRDY output or bit goes high when calibration is initiated and
returns low when this zero-scale calibration is complete and a new valid word is available in the data register.
At the end of the calibration, the part returns to Normal Mode with MD1 and MD0 returning to 0, 0.
1
1
Full-Scale System Calibration: this activates full-scale system calibration on the selected input channel.
Calibration is performed at the selected gain on the input voltage provided at the analog input during this
calibration sequence. This input voltage should remain stable for the duration of the calibration. Once
again, the DRDY output or bit goes high when calibration is initiated and returns low when this full-scale
calibration is complete and a new valid word is available in the data register. At the end of the calibration,
the part returns to Normal Mode with MD1 and MD0 returning to 0, 0.
G2–G0
Gain Selection Bits. These bits select the gain setting for the on-chip PGA as outlined in Table X.
Table X. Gain Selection
G2
G1
G0
Gain Setting
0
0
0
0
1
1
1
1
0
0
1
1
0
0
1
1
0
1
0
1
0
1
0
1
1
2
4
8
16
32
64
128
B/U
Bipolar/Unipolar Operation. A “0” in this bit selects Bipolar Operation. A “1” in this bit selects Unipolar Operation.
BUF
Buffer Control. With this bit at “0,” the on-chip buffer on the analog input is shorted out. With the buffer shorted
out, the current flowing in the VDD line is reduced. When this bit is high, the on-chip buffer is in series with the
analog input allowing the input to handle higher source impedances.
FSYNC
Filter Synchronization. When this bit is high, the nodes of the digital filter, the filter control logic and the calibration control logic are held in a reset state and the analog modulator is also held in its reset state. When this bit
goes low, the modulator and filter start to process data and a valid word is available in 3 × 1/(output update rate),
i.e., the settling time of the filter. This FSYNC bit does not affect the digital interface and does not reset the
DRDY output if it is low.
–12–
REV. A
AD7705/AD7706
Clock Register (RS2, RS1, RS0 = 0, 1, 0); Power-On/Reset Status: 05␣ Hex
The Clock Register is an 8-bit register from which data can either be read or to which data can be written. Table XI outlines the bit
designations for the Clock Register.
Table XI. Clock Register
ZERO (0)
ZERO (0)
ZERO (0)
CLKDIS (0)
CLKDIV (0)
CLK (1)
FS1 (0)
FS0 (1)
ZERO
Zero. A zero MUST be written to these bits to ensure correct operation of the AD7705/AD7706. Failure to do so
may result in unspecified operation of the device.
CLKDIS
Master Clock Disable Bit. A Logic 1 in this bit disables the master clock from appearing at the MCLK OUT pin.
When disabled, the MCLK OUT pin is forced low. This feature allows the user the flexibility of using the MCLK
OUT as a clock source for other devices in the system or of turning off the MCLK OUT as a power saving feature.
When using an external master clock on the MCLK IN pin, the AD7705/AD7706 continues to have internal
clocks and will convert normally with the CLKDIS bit active. When using a crystal oscillator or ceramic resonator
across the MCLK IN and MCLK OUT pins, the AD7705/AD7706 clock is stopped and no conversions take place
when the CLKDIS bit is active.
CLKDIV
Clock Divider Bit. With this bit at a Logic 1, the clock frequency appearing at the MCLK IN pin is divided by two
before being used internally by the AD7705/AD7706. For example, when this bit is set to 1, the user can operate
with a 4.9152 MHz crystal between MCLK IN and MCLK OUT and internally the part will operate with the
specified 2.4576 MHz. With this bit at a Logic 0, the clock frequency appearing at the MCLK IN pin is the frequency used internally by the part.
CLK
Clock Bit. This bit should be set in accordance with the operating frequency of the AD7705/AD7706. If the device
has a master clock frequency of 2.4576 MHz (CLKDIV = 0) or 4.9152 MHz (CLKDIV = 1), then this bit should
be set to a “1.” If the device has a master clock frequency of 1 MHz (CLKDIV = 0) or 2 MHz (CLKDIV = 1),
this bit should be set to a “0.” This bit sets up the appropriate scaling currents for a given operating frequency and
also chooses (along with FS1 and FS0) the output update rate for the device. If this bit is not set correctly for the
master clock frequency of the device, then the AD7705/AD7706 may not operate to specification.
FS1, FS0
Filter Selection Bits. Along with the CLK bit, FS1 and FS0 determine the output update rate, filter first notch and
–3 dB frequency as outlined in Table XII. The on-chip digital filter provides a sinc3 (or Sinx/x3) filter response. In
association with the gain selection, it also determines the output noise of the device. Changing the filter notch
frequency, as well as the selected gain, impacts resolution. Tables I to IV show the effect of filter notch frequency
and gain on the output noise and effective resolution of the part. The output data rate (or effective conversion
time) for the device is equal to the frequency selected for the first notch of the filter. For example, if the first notch
of the filter is selected at 50 Hz, a new word is available at a 50 Hz output rate or every 20 ms. If the first notch is
at 500 Hz, a new word is available every 2 ms. A calibration should be initiated when any of these bits are changed.
The settling time of the filter to a full-scale step input is worst case 4 × 1/(output data rate). For example, with the
filter first notch at 50 Hz, the settling time of the filter to a full-scale step input is 80 ms max. If the first notch is at
500 Hz, the settling time is 8 ms max. This settling time can be reduced to 3 × 1/(output data rate) by synchronizing the step input change to a reset of the digital filter. In other words, if the step input takes place with the FSYNC bit
high, the settling-time will be 3 × 1/(output data rate) from when the FSYNC bit returns low.
The –3 dB frequency is determined by the programmed first notch frequency according to the relationship:
filter –3 dB frequency = 0.262 × filter first notch frequency
Table XII. Output Update Rates
CLK*
0
0
0
0
1
1
1
1
FS1
FS0
Output Update Rate
–3 dB Filter Cutoff
0
0
1
1
0
0
1
1
0
1
0
1
0
1
0
1
20 Hz
25 Hz
100 Hz
200 Hz
50 Hz
60 Hz
250 Hz
500 Hz
5.24 Hz
6.55 Hz
26.2 Hz
52.4 Hz
13.1 Hz
15.7 Hz
65.5 Hz
131 Hz
*Assumes correct clock frequency on MCLK IN pin with CLKDIV bit set appropriately.
REV. A
–13–
AD7705/AD7706
Data Register (RS2, RS1, RS0 = 0, 1, 1)
The Data Register on the part is a 16-bit read-only register that contains the most up-to-date conversion result from the AD7705/
AD7706. If the Communications Register sets up the part for a write operation to this register, a write operation must actually take
place to return the part to where it is expecting a write operation to the Communications Register. However, the 16 bits of data
written to the part will be ignored by the AD7705/AD7706.
Test Register (RS2, RS1, RS0 = 1, 0, 0); Power-On/Reset Status: 00␣ Hex
The part contains a Test Register that is used when testing the device. The user is advised not to change the status of any of the bits
in this register from the default (Power-on or RESET) status of all 0s as the part will be placed in one of its test modes and will not
operate correctly.
Zero-Scale Calibration Register (RS2, RS1, RS0 = 1, 1, 0); Power-On/Reset Status: 1F4000␣ Hex
The AD7705/AD7706 contains independent sets of zero-scale registers, one for each of the input channels. Each of these registers is
a 24-bit read/write register; 24 bits of data must be written otherwise no data will be transferred to the register. This register is used
in conjunction with its associated full-scale register to form a register pair. These register pairs are associated with input channel
pairs as outlined in Table VII. While the part is set up to allow access to these registers over the digital interface, the part itself no
longer has access to the register coefficients to correctly scale the output data. As a result, there is a possibility that after accessing the
calibration registers (either read or write operation) the first output data read from the part may contain incorrect data. In addition, a
write to the calibration register should not be attempted while a calibration is in progress. These eventualities can be avoided by
taking the FSYNC bit in the mode register high before the calibration register operation and taking it low after the operation is
complete.
Full-Scale Calibration Register (RS2, RS1, RS0 = 1, 1, 1); Power-On/Reset Status: 5761AB␣ Hex
The AD7705/AD7706 contains independent sets of full-scale registers, one for each of the input channels. Each of these registers is a
24-bit read/write register; 24 bits of data must be written otherwise no data will be transferred to the register. This register is used in
conjunction with its associated zero-scale register to form a register pair. These register pairs are associated with input channel pairs
as outlined in Table VII. While the part is set up to allow access to these registers over the digital interface, the part itself no longer
has access to the register coefficients to correctly scale the output data. As a result, there is a possibility that after accessing the calibration registers (either read or write operation) the first output data read from the part may contain incorrect data. In addition, a
write to the calibration register should not be attempted while a calibration is in progress. These eventualities can be avoided by
taking FSYNC bit in the mode register high before the calibration register operation and taking it low after the operation is complete.
CALIBRATION SEQUENCES
The AD7705/AD7706 contains a number of calibration options as previously outlined. Table XIII summarizes the calibration types,
the operations involved and the duration of the operations. There are two methods of determining the end of calibration. The first is
to monitor when DRDY returns low at the end of the sequence. DRDY not only indicates when the sequence is complete, but also
that the part has a valid new sample in its data register. This valid new sample is the result of a normal conversion which follows the
calibration sequence. The second method of determining when calibration is complete is to monitor the MD1 and MD0 bits of the
Setup Register. When these bits return to 0 (0 following a calibration command), it indicates that the calibration sequence is complete. This method does not give any indication of there being a valid new result in the data register. However, it gives an earlier
indication than DRDY that calibration is complete. The duration to when the Mode Bits (MD1 and MD0) return to 0 (0 represents
the duration of the calibration carried out). The sequence to when DRDY goes low also includes a normal conversion and a pipeline
delay, tP, to correctly scale the results of this first conversion. tP will never exceed 2000 × tCLKIN. The time for both methods is given
in the table.
Table XIII. Calibration Sequences
Calibration Type
MD1, MD0
Calibration Sequence
Duration to Mode Bits
Duration to DRDY
Self-Calibration
0, 1
6 × 1/Output Rate
9 × 1/Output Rate + tP
ZS System Calibration
FS System Calibration
1, 0
1, 1
Internal ZS Cal @ Selected Gain +
Internal FS Cal @ Selected Gain
ZS Cal on AIN @ Selected Gain
FS Cal on AIN @ Selected Gain
3 × 1/Output Rate
3 × 1/Output Rate
4 × 1/Output Rate + tP
4 × 1/Output Rate + tP
–14–
REV. A
AD7705/AD7706
CIRCUIT DESCRIPTION
input is also incorporated in this sigma-delta modulator with the
input sampling frequency being modified to give the higher
gains. A sinc3 digital low-pass filter processes the output of the
sigma-delta modulator and updates the output register at a rate
determined by the first notch frequency of this filter. The output data can be read from the serial port randomly or periodically at any rate up to the output register update rate. The first
notch of this digital filter (and hence its –3␣ dB frequency) can
be programmed via the Setup Register bits FS0 and FS1. With
a master clock frequency of 2.4576␣ MHz, the programmable
range for this first notch frequency is from 50␣ Hz to 500␣ Hz,
giving a programmable range for the –3␣ dB frequency of
13.1␣ Hz to 131␣ Hz. With a master clock frequency of 1␣ MHz,
the programmable range for this first notch frequency is from
20␣ Hz to 200␣ Hz, giving a programmable range for the –3␣ dB
frequency of 5.24␣ Hz to 52.4␣ Hz.
The AD7705/AD7706 is a sigma-delta A/D converter with onchip digital filtering, intended for the measurement of wide
dynamic range, low frequency signals such as those in industrial
control or process control applications. It contains a sigma-delta
(or charge-balancing) ADC, a calibration microcontroller with
on-chip static RAM, a clock oscillator, a digital filter and a bidirectional serial communications port. The part consumes only
320␣ µA of power supply current, making it ideal for batterypowered or loop-powered instruments. These parts operate with
a supply voltage of 2.7 V to 3.3 V or 4.75 V to 5.25 V.
The AD7705 contains two programmable-gain fully differential
analog input channels, while the AD7706 contains three pseudo
differential analog input channels. The selectable gains on these
inputs are 1, 2, 4, 8, 16, 32, 64 and 128 allowing the part to
accept unipolar signals of between 0 mV to +20␣ mV and 0 V to
+2.5␣ V, or bipolar signals in the range from ± 20␣ mV to ± 2.5␣ V
when the reference input voltage equals +2.5␣ V. With a reference voltage of +1.225␣ V, the input ranges are from 0 mV to
+10␣ mV to 0 V to +1.225␣ V in unipolar mode, and from ± 10␣ mV
to ± 1.225 V in bipolar mode. Note that the bipolar ranges are
with respect to AIN(–) on the AD7705, and with respect to
COMMON on the AD7706, and not with respect to GND.
The basic connection diagram for the AD7705 is shown in
Figure 10. This shows the AD7705 being driven from the analog +5␣ V supply. An AD780, precision +2.5 V reference, provides the reference source for the part. On the digital side, the
part is configured for three-wire operation with CS tied to
GND. A quartz crystal or ceramic resonator provide the master
clock source for the part. In most cases, it will be necessary to
connect capacitors on the crystal or resonator to ensure that it
does not oscillate at overtones of its fundamental operating frequency. The values of capacitors will vary, depending on the
manufacturer’s specifications. The same setup applies to the
AD7706.
The input signal to the analog input is continuously sampled
at a rate determined by the frequency of the master clock,
MCLK␣ IN, and the selected gain. A charge-balancing A/D
converter (Sigma-Delta Modulator) converts the sampled signal
into a digital pulse train whose duty cycle contains the digital
information. The programmable gain function on the analog
ANALOG
+5V SUPPLY
10mF
0.1mF
VDD
DIFFERENTIAL
ANALOG
INPUT
AIN1(+)
DIFFERENTIAL
ANALOG
INPUT
ANALOG +5V
SUPPLY
AIN2(+)
AD7705
DRDY
DATA READY
DOUT
RECEIVE (READ)
AIN1(–)
DIN
SCLK
GND
SERIAL CLOCK
+5V
RESET
VIN
VOUT
AD780/
REF192
SERIAL DATA
AIN2(–)
REF IN(+)
10mF
0.1mF
REF IN(–)
CS
MCLK IN
GND
MCLK OUT
CRYSTAL OR
CERAMIC
RESONATOR
Figure 10. AD7705 Basic Connection Diagram
REV. A
–15–
AD7705/AD7706
Table XIV. External R, C Combination for No 16-Bit Gain
Error (Unbuffered Mode Only)
ANALOG INPUT
Analog Input Ranges
The AD7705 contains two differential analog input pairs
AIN1(+), AIN1(–) and AIN2(+), AIN2(–). These input pairs
provide programmable-gain, differential input channels that
can handle either unipolar or bipolar input signals. It should be
noted that the bipolar input signals are referenced to the respective AIN(–) input of each input pair. The AD7706 contains
three pseudo differential analog input pairs AIN1, AIN2 and
AIN3, which are referenced to the COMMON input on the part.
In unbuffered mode, the common-mode range of the input is
from GND to VDD, provided that the absolute value of the
analog input voltage lies between GND␣ –␣ 30␣ mV and VDD
+␣ 30␣ mV. This means that in unbuffered mode the part can
handle both unipolar and bipolar input ranges for all gains.
Absolute voltages of GND – 200 mV can be accommodated on
the analog inputs at 25°C without degradation in performance,
but leakage current increases appreciably with increasing temperature. In buffered mode, the analog inputs can handle
much larger source impedances, but the absolute input voltage
range is restricted to between GND␣ + 50␣ mV to VDD – 1.5 V
which also places restrictions on the common-mode range. This
means that in buffered mode there are some restrictions on the
allowable gains for bipolar input ranges. Care must be taken in
setting up the common-mode voltage and input voltage range
so that the above limits are not exceeded, otherwise there will
be a degradation in linearity performance.
In unbuffered mode, the analog inputs look directly into the
7␣ pF input sampling capacitor, CSAMP. The dc input leakage
current in this unbuffered mode is 1␣ nA maximum. As a result,
the analog inputs see a dynamic load that is switched at the
input sample rate (see Figure 11). This sample rate depends on
master clock frequency and selected gain. CSAMP is charged to
AIN(+) and discharged to AIN(–) every input sample cycle.
The effective on-resistance of the switch, RSW, is typically 7␣ kΩ.
CSAMP must be charged through RSW and through any external
source impedances every input sample cycle. Therefore, in
unbuffered mode, source impedances mean a longer charge time
for CSAMP and this may result in gain errors on the part. Table
XIV shows the allowable external resistance/capacitance values,
for unbuffered mode, such that no gain error to the 16-bit level
is introduced on the part. Note that these capacitances are
total capacitances on the analog input, external capacitance
plus 10 pF capacitance from the pins and lead frame of the device.
AIN(+)
AIN(–)
RSW (7kV TYP)
CSAMP
(7pF)
HIGH
IMPEDANCE
1G
VBIAS
SWITCHING FREQUENCY DEPENDS ON
fCLKIN AND SELECTED GAIN
Figure 11. Unbuffered Analog Input Structure
External Capacitance (pF)
100
500
1000
Gain
0
50
1
2
4
8–128
368 kΩ
177.2 kΩ
82.8 kΩ
35.2 kΩ
90.6 kΩ
44.2 kΩ
21.2 kΩ
9.6 kΩ
54.2 kΩ
26.4 kΩ
12.6 kΩ
5.8 kΩ
14.6 kΩ
7.2 kΩ
3.4 kΩ
1.58 Ω
5000
8.2 kΩ
4 kΩ
1.94 kΩ
880 Ω
2.2 kΩ
1.12 kΩ
540 Ω
240 Ω
In buffered mode, the analog inputs look into the high-impedance
inputs stage of the on-chip buffer amplifier. CSAMP is charged
via this buffer amplifier such that source impedances do not
affect the charging of CSAMP. This buffer amplifier has an offset
leakage current of 1 nA. In this buffered mode, large source
impedances result in a small dc offset voltage developed across
the source impedance, but not in a gain error.
Input Sample Rate
The modulator sample frequency for the AD7705/AD7706
remains at fCLKIN/128 (19.2␣ kHz @ fCLKIN = 2.4576␣ MHz) regardless of the selected gain. However, gains greater than 1 are
achieved by a combination of multiple input samples per modulator cycle and a scaling of the ratio of reference capacitor to
input capacitor. As a result of the multiple sampling, the input
sample rate of the device varies with the selected gain (see Table
XV). In buffered mode, the input is buffered before the input
sampling capacitor. In unbuffered mode, where the analog
input looks directly into the sampling capacitor, the effective
input impedance is 1/CSAMP × fS where CSAMP is the input sampling capacitance and fS is the input sample rate.
Table XV. Input Sampling Frequency vs. Gain
Gain
Input Sampling Frequency (fS)
1
2
4
8–128
fCLKIN/64 (38.4␣ kHz @ fCLKIN = 2.4576␣ MHz)
2 × fCLKIN/64 (76.8␣ kHz @ fCLKIN = 2.4576␣ MHz)
4 × fCLKIN/64 (76.8␣ kHz @ fCLKIN = 2.4576␣ MHz)
8 × fCLKIN/64 (307.2␣ kHz @ fCLKIN = 2.4576␣ MHz)
Bipolar/Unipolar Inputs
The analog inputs on the AD7705/AD7706 can accept either
unipolar or bipolar input voltage ranges. Bipolar input ranges do
not imply that the part can handle negative voltages on its analog
input, since the analog input cannot go more negative than
–30 mV to ensure correct operation of these parts. The input
channels are fully differential. As a result, on the AD7705, the
voltage to which the unipolar and bipolar signals on the AIN(+)
input are referenced is the voltage on the respective AIN(–)
input. On the AD7706, the voltages applied to the analog input
channels are referenced to the COMMON input. For example, if
AIN1(–) is +2.5␣ V and the AD7705 is configured for unipolar
operation with a gain of 2 and a VREF of +2.5␣ V, the input voltage
range on the AIN1(+) input is +2.5␣ V to +3.75␣ V. If AIN1(–) is
+2.5␣ V and the AD7705 is configured for bipolar mode with a
gain of 2 and a VREF of +2.5␣ V, the analog input range on the
AIN1(+) input is +1.25␣ V to +3.75 V (i.e., 2.5␣ V ± 1.25␣ V). If
AIN1(–) is at GND, the part cannot be configured for bipolar
ranges in excess of ± 30␣ mV.
–16–
REV. A
AD7705/AD7706
Bipolar or unipolar options are chosen by programming the B/U
bit of the Setup Register. This programs the channel for either
unipolar or bipolar operation. Programming the channel for
either unipolar or bipolar operation does not change any of the
input signal conditioning, it simply changes the data output
coding and the points on the transfer function where calibrations occur.
REFERENCE INPUT
The AD7705/AD7706’s reference inputs, REF␣ IN(+) and
REF␣ IN(–), provide a differential reference input capability.
The common-mode range for these differential inputs is from
GND to VDD. The nominal reference voltage, VREF (REF␣ IN(+)␣ –
REF␣ IN(–)), for specified operation, is +2.5␣ V for the AD7705/
AD7706 operated with a VDD of 5 V and +1.225␣ V for the
AD7705/AD7706 operated with a VDD of 3 V. The part is functional with VREF voltages down to 1 V, but with degraded performance as the output noise will, in terms of LSB size, be larger.
REF␣ IN(+) must always be greater than REF␣ IN(–) for correct
operation of the AD7705/AD7706.
Both reference inputs provide a high impedance, dynamic load
similar to the analog inputs in unbuffered mode. The maximum
dc input leakage current is ± 1 nA over temperature, and source
resistance may result in gain errors on the part. In this case, the
sampling switch resistance is 5␣ kΩ typ and the reference capacitor (CREF) varies with gain. The sample rate on the reference
inputs is fCLKIN/64 and does not vary with gain. For gains of 1
and 2, CREF is 8␣ pF; for a gain of 16, it is 5.5␣ pF, for a gain of
32, it is 4.25 pF, for a gain of 64, it is 3.625 pF and for a gain of
128, it is 3.3125␣ pF.
The output noise performance outlined in Tables I through IV
is for an analog input of 0␣ V, which effectively removes the
effect of noise on the reference. To obtain the same noise performance as shown in the noise tables over the full input range
requires a low noise reference source for the AD7705/AD7706.
If the reference noise in the bandwidth of interest is excessive, it
will degrade the performance of the AD7705/AD7706. In applications where the excitation voltage for the bridge transducer on
the analog input also derives the reference voltage for the part,
the effect of the noise in the excitation voltage will be removed
as the application is ratiometric. Recommended reference voltage sources for the AD7705 with a VDD of 5 V include the
AD780, REF43 and REF192, while the recommended reference
sources for the AD7705 operated with a VDD of 3 V include the
AD589 and AD1580. It is generally recommended to decouple
the output of these references in order to further reduce the
noise level.
DIGITAL FILTERING
The AD7705/AD7706 contains an on-chip low-pass digital filter
which processes the output of the part’s sigma-delta modulator.
Therefore, the part not only provides the analog-to-digital conversion function but also provides a level of filtering. There are a
number of system differences when the filtering function is
provided in the digital domain rather than the analog domain
and the user should be aware of these.
First, since digital filtering occurs after the A-to-D conversion
process, it can remove noise injected during the conversion
process. Analog filtering cannot do this. Also, the digital filter
can be made programmable far more readily than an analog
filter. Depending on the digital filter design, this gives the user
REV. A
the capability of programming cutoff frequency and output
update rate.
On the other hand, analog filtering can remove noise superimposed on the analog signal before it reaches the ADC. Digital
filtering cannot do this and noise peaks riding on signals near
full scale have the potential to saturate the analog modulator
and digital filter, even though the average value of the signal is
within limits. To alleviate this problem, the AD7705/AD7706
has overrange headroom built into the sigma-delta modulator
and digital filter, which allows overrange excursions of 5%
above the analog input range. If noise signals are larger than
this, consideration should be given to analog input filtering, or
to reducing the input channel voltage so that its full-scale is half
that of the analog input channel full-scale. This will provide an
overrange capability greater than 100% at the expense of reducing the dynamic range by 1 bit (50%).
In addition, the digital filter does not provide any rejection at
integer multiples of the digital filter’s sample frequency. However, the input sampling on the part provides attenuation at
multiples of the digital filter’s sampling frequency so that the
unattenuated bands actually occur around multiples of the
sampling frequency fS (as defined in Table XV). Thus the
unattenuated bands occur at n × fS (where n = 1, 2, 3 . . .). At
these frequencies, there are frequency bands, ± f3 dB wide f3 dB is
the cutoff frequency of the digital filter) at either side where
noise passes unattenuated to the output.
Filter Characteristics
The AD7705/AD7706’s digital filter is a low-pass filter with a
(sinx/x)3 response (also called sinc3). The transfer function for
this filter is described in the z-domain by:
H(z) =
1 1− Z –N
×
N 1− Z –1
3
and in the frequency domain by:
H( f ) =
1 SIN(N × π × f /f S )
×
SIN(π × f /f S )
N
3
where N is the ratio of the modulator rate to the output rate.
Phase Response:
∠H = –3 π (N – 2)× f /f S Rad
Figure 4 shows the filter frequency response for a cutoff frequency of 15.72␣ Hz, which corresponds to a first filter notch
frequency of 60␣ Hz. The plot is shown from dc to 390␣ Hz. This
response is repeated at either side of the digital filter’s sample
frequency and at either side of multiples of the filter’s sample
frequency.
The response of the filter is similar to that of an averaging filter,
but with a sharper roll-off. The output rate for the digital filter
corresponds with the positioning of the first notch of the filter’s
frequency response. Thus, for the plot of Figure 12 where the
output rate is 60␣ Hz, the first notch of the filter is at 60␣ Hz. The
notches of this (sinx/x)3 filter are repeated at multiples of the
first notch. The filter provides attenuation of better than 100␣ dB
at these notches.
–17–
AD7705/AD7706
The cutoff frequency of the digital filter is determined by the
value loaded to bits FS0 to FS1 in the CLOCK Register. Programming a different cutoff frequency via FS0 and FS1 does not
alter the profile of the filter response, it changes the frequency of
the notches. The output update of the part and the frequency of
the first notch correspond.
Since the AD7705/AD7706 contains this on-chip, low-pass
filtering, a settling time is associated with step function inputs
and data on the output will be invalid after a step change until
the settling time has elapsed. The settling time depends upon
the output rate chosen for the filter. The settling time of the
filter to a full-scale step input can be up to four times the output
data period. For a synchronized step input (using the FSYNC
function), the settling time is three times the output data period.
0
–20
–40
–60
GAIN – dB
–120
–140
–160
–180
–200
–220
0
60
120
180
240
FREQUENCY – Hz
300
The digital filter does not provide any rejection at integer multiples of the modulator sample frequency, as outlined earlier.
However, due to the AD7705/AD7706’s high oversampling
ratio, these bands occupy only a small fraction of the spectrum
and most broadband noise is filtered. This means that the analog filtering requirements in front of the AD7705/AD7706 are
considerably reduced versus a conventional converter with no
on-chip filtering. In addition, because the part’s common-mode
rejection performance of 100␣ dB extends out to several kHz,
common-mode noise in this frequency range will be substantially reduced.
Depending on the application, however, it may be necessary to
provide attenuation prior to the AD7705/AD7706 in order to
eliminate unwanted frequencies from these bands which the
digital filter will pass. It may also be necessary in some applications to provide analog filtering in front of the AD7705/AD7706
to ensure that differential noise signals outside the band of interest do not saturate the analog modulator.
If passive components are placed in front of the AD7705/AD7706
in unbuffered mode, care must be taken to ensure that the
source impedance is low enough not to introduce gain errors in
the system. This significantly limits the amount of passive antialiasing filtering which can be provided in front of the AD7705/
AD7706 when it is used in unbuffered mode. However, when
the part is used in buffered mode, large source impedances will
simply result in a small dc offset error (a 10␣ kΩ source resistance
will cause an offset error of less than 10␣ µV). Therefore, if the
system requires any significant source impedances to provide
passive analog filtering in front of the AD7705/AD7706, it is
recommended that the part be operated in buffered mode.
–80
–100
–240
ANALOG FILTERING
360
Figure 12. Frequency Response of AD7705 Filter
Post-Filtering
The on-chip modulator provides samples at a 19.2␣ kHz output
rate with fCLKIN at 2.4576␣ MHz. The on-chip digital filter decimates these samples to provide data at an output rate that corresponds to the programmed output rate of the filter. Since the
output data rate is higher than the Nyquist criterion, the output
rate for a given bandwidth will satisfy most application requirements. There may, however, be some applications which require
a higher data rate for a given bandwidth and noise performance.
Applications that need this higher data rate will require some
post-filtering following the digital filter of the AD7705/AD7706.
For example, if the required bandwidth is 7.86␣ Hz, but the
required update rate is 100␣ Hz, the data can be taken from the
AD7705/AD7706 at the 100␣ Hz rate giving a –3 dB bandwidth
of 26.2␣ Hz. Post-filtering can be applied to this to reduce the
bandwidth and output noise, to the 7.86␣ Hz bandwidth level,
while maintaining an output rate of 100␣ Hz.
Post-filtering can also be used to reduce the output noise from
the device for bandwidths below 13.1␣ Hz. At a gain of 128 and
a bandwidth of 13.1␣ Hz, the output rms noise is 450␣ nV. This
is essentially device noise or white noise and since the input is
chopped, the noise has a primarily flat frequency response. By
reducing the bandwidth below 13.1␣ Hz, the noise in the resultant passband can be reduced. A reduction in bandwidth by a
factor of 2 results in a reduction of approximately 1.25 in the
output rms noise. This additional filtering will result in a longer
settling-time.
CALIBRATION
The AD7705/AD7706 provides a number of calibration options
which can be programmed via the MD1 and MD0 bits of the
Setup Register. The different calibration options are outlined in
the Setup Register and Calibration Sequences sections. A calibration cycle may be initiated at any time by writing to these
bits of the Setup Register. Calibration on the AD7705/AD7706
removes offset and gain errors from the device. A calibration
routine should be initiated on the device whenever there is a
change in the ambient operating temperature or supply voltage.
It should also be initiated if there is a change in the selected
gain, filter notch or bipolar/unipolar input range.
The AD7705/AD7706 offers self-calibration and system calibration facilities. For full calibration to occur on the selected channel, the on-chip microcontroller must record the modulator
output for two different input conditions. These are “zeroscale” and “full-scale” points. These points are derived by
performing a conversion on the different input voltages provided
to the input of the modulator during calibration. As a result, the
accuracy of the calibration can only be as good as the noise level
that it provides in normal mode. The result of the “zero-scale”
calibration conversion is stored in the Zero-Scale Calibration
Register while the result of the “full-scale” calibration conversion is stored in the Full-Scale Calibration Register. With these
readings, the microcontroller can calculate the offset and the
gain slope for the input-to-output transfer function of the converter. Internally, the part works with a resolution of 33 bits to
determine its conversion result of 16 bits.
–18–
REV. A
AD7705/AD7706
Self-Calibration
A self-calibration is initiated on the AD7705/AD7706 by writing
the appropriate values (0, 1) to the MD1 and MD0 bits of
the Setup Register. In the self-calibration mode with a unipolar input range, the zero scale point used in determining the
calibration coefficients is with the inputs of the differential
pair internally shorted on the part (i.e., AIN(+) = AIN(–) =
Internal Bias Voltage in the case of the AD7705 and AIN =
COMMON = Internal Bias voltage on the AD7706). The PGA
is set for the selected gain (as per G1 and G0 bits in the Communications Register) for this zero-scale calibration conversion.
The full-scale calibration conversion is performed at the selected
gain on an internally-generated voltage of VREF/Selected Gain.
The duration time for the calibration is 6 × 1/Output Rate. This
is made up of 3 × 1/Output Rate for the zero-scale calibration
and 3 × 1/Output Rate for the full-scale calibration. At this time
the MD1 and MD0 bits in the Setup Register return to 0, 0.
This gives the earliest indication that the calibration sequence is
complete. The DRDY line goes high when calibration is initiated and does not return low until there is a valid new word in
the data register. The duration time from the calibration command being issued to DRDY going low is 9 × 1/Output Rate.
This is made up of 3 × 1/Output Rate for the zero-scale calibration, 3 × 1/Output Rate for the full-scale calibration, 3 × 1/Output
Rate for a conversion on the analog input and some overhead to
correctly set up the coefficients. If DRDY is low before (or goes
low during) the calibration command write to the Setup Register, it may take up to one modulator cycle (MCLK␣ IN/128)
before DRDY goes high to indicate that calibration is in progress.
Therefore, DRDY should be ignored for up to one modulator
cycle after the last bit is written to the Setup Register in the
calibration command.
For bipolar input ranges in the self-calibrating mode, the sequence is very similar to that just outlined. In this case, the two
points are exactly the same as above but, since the part is configured for bipolar operation, the shorted inputs point is actually
midscale of the transfer function.
System Calibration
System calibration allows the AD7705/AD7706 to compensate
for system gain and offset errors as well as its own internal errors. System calibration performs the same slope factor calculations as self-calibration, but uses voltage values presented by
the system to the AIN inputs for the zero- and full-scale points.
Full system calibration requires a two-step process, a ZS System
Calibration followed by an FS System Calibration.
For a full system calibration, the zero-scale point must be presented to the converter first. It must be applied to the converter
before the calibration step is initiated and remain stable until the
step is complete. Once the system zero-scale voltage has been
set up, a ZS System Calibration is then initiated by writing the
appropriate values (1, 0) to the MD1 and MD0 bits of the
Setup Register. The zero-scale system calibration is performed
at the selected gain. The duration of the calibration is 3 × 1/Output
Rate. At this time, the MD1 and MD0 bits in the Setup Register
return to 0, 0. This gives the earliest indication that the calibration sequence is complete. The DRDY line goes high when
calibration is initiated and does not return low until there is a
valid new word in the data register. The duration time from
the calibration command being issued to DRDY going low is
4 × 1/Output Rate as the part performs a normal conversion on
REV. A
the AIN voltage before DRDY goes low. If DRDY is low before
(or goes low during) the calibration command write to the Setup
Register, it may take up to one modulator cycle (MCLK␣ IN/128)
before DRDY goes high to indicate that calibration is in progress.
Therefore, DRDY should be ignored for up to one modulator
cycle after the last bit is written to the Setup Register in the
calibration command.
After the zero-scale point is calibrated, the full-scale point is
applied to AIN and the second step of the calibration process is
initiated by again writing the appropriate values (1, 1) to MD1
and MD0. Again, the full-scale voltage must be set up before
the calibration is initiated and it must remain stable throughout
the calibration step. The full-scale system calibration is performed at the selected gain. The duration of the calibration is
3 × 1/Output Rate. At this time, the MD1 and MD0 bits in the
Setup Register return to 0, 0. This gives the earliest indication
that the calibration sequence is complete. The DRDY line goes
high when calibration is initiated and does not return low until
there is a valid new word in the data register. The duration time
from the calibration command being issued to DRDY going low
is 4 × 1/Output Rate as the part performs a normal conversion
on the AIN voltage before DRDY goes low. If DRDY is low
before (or goes low during) the calibration command write to
the Setup Register, it may take up to one modulator cycle
(MCLK␣ IN/128) before DRDY goes high to indicate that calibration is in progress. Therefore, DRDY should be ignored for
up to one modulator cycle after the last bit is written to the
Setup Register in the calibration command.
In the unipolar mode, the system calibration is performed between the two endpoints of the transfer function; in the bipolar
mode, it is performed between midscale (zero differential voltage) and positive full-scale.
The fact that the system calibration is a two-step calibration
offers another feature. After the sequence of a full system calibration has been completed, additional offset or gain calibrations can be performed by themselves to adjust the system zero
reference point or the system gain. Calibrating one of the parameters, either system offset or system gain, will not affect the
other parameter.
System calibration can also be used to remove any errors from
source impedances on the analog input when the part is used in
unbuffered mode. A simple R, C antialiasing filter on the front
end may introduce a gain error on the analog input voltage, but
the system calibration can be used to remove this error.
Span and Offset Limits
Whenever a system calibration mode is used, there are limits on
the amount of offset and span which can be accommodated.
The overriding requirement in determining the amount of offset
and gain that can be accommodated by the part is the requirement that the positive full-scale calibration limit is < 1.05␣ ×
VREF/GAIN. This allows the input range to go 5% above the
nominal range. The built-in headroom in the AD7705/AD7706’s
analog modulator ensures that the part will still operate correctly with a positive full-scale voltage that is 5% beyond the
nominal.
The range of input span in both the unipolar and bipolar modes
has a minimum value of 0.8 ×␣ VREF/GAIN and a maximum
value of 2.1 ×␣ VREF/GAIN. However, the span (which is the
difference between the bottom of the AD7705/AD7706’s input
–19–
AD7705/AD7706
range and the top of its input range) has to take into account the
limitation on the positive full-scale voltage. The amount of
offset which can be accommodated depends on whether the
unipolar or bipolar mode is being used. Once again, the offset
has to take into account the limitation on the positive full-scale
voltage. In unipolar mode, there is considerable flexibility in
handling negative (with respect to AIN(–) on the AD7705 and
with respect to COMMON on the AD7706) offsets. In both
unipolar and bipolar modes, the range of positive offsets that
can be handled by the part depends on the selected span. Therefore, in determining the limits for system zero-scale and fullscale calibrations, the user has to ensure that the offset range
plus the span range does exceed 1.05 ×␣ VREF/GAIN. This is best
illustrated by looking at a few examples.
AD7705/AD7706
MCLK OUT
C2
The AD7705/AD7706 requires a master clock input, which
may be an external CMOS compatible clock signal applied to
the MCLK␣ IN pin with the MCLK␣ OUT pin left unconnected.
Alternatively, a crystal or ceramic resonator of the correct frequency can be connected between MCLK␣ IN and MCLK␣ OUT
as shown in figure 6, in which case the clock circuit will function
as an oscillator, providing the clock source for the part. The
input sampling frequency, the modulator sampling frequency,
the –3␣ dB frequency, output update rate and calibration time
are all directly related to the master clock frequency, fCLKIN.
Reducing the master clock frequency by a factor of 2 will halve
the above frequencies and update rate and double the calibration time. The current drawn from the VDD power supply is also
related to fCLKIN. Reducing fCLKIN by a factor of 2 will halve the
digital part of the total VDD current but will not affect the current drawn by the analog circuitry.
UPPER LIMIT ON
AD7705 INPUT VOLTAGE
GAIN CALIBRATIONS EXPAND
OR CONTRACT THE
AD7705/AD7706 INPUT RANGE
NOMINAL ZERO
SCALE POINT
OFFSET CALIBRATIONS MOVE
INPUT RANGE UP OR DOWN
LOWER LIMIT ON
AD7705/AD7706 INPUT VOLTAGE
–1.05 3 VREF/GAIN
Figure 13. Span and Offset Limits
If the part is used in bipolar mode with a required span of
± 0.4 ×␣ VREF/GAIN, the offset range the system calibration can
handle is from –0.65 ×␣ VREF/GAIN to +0.65 ×␣ VREF/GAIN.
If the part is used in bipolar mode with a required span of
± VREF/GAIN, then the offset range which the system calibration
can handle is from –0.05 ×␣ VREF/GAIN to +0.05 ×␣ VREF/GAIN.
Similarly, if the part is used in bipolar mode and required to
remove an offset of ± 0.2 ×␣ VREF/GAIN, the span range the system calibration can handle is ± 0.85 ×␣ VREF/GAIN.
Power-Up and Calibration
On power-up, the AD7705/AD7706 performs an internal reset
that sets the contents of the internal registers to a known state.
There are default values loaded to all registers after power-on or
reset. The default values contain nominal calibration coefficients
for the calibration registers. However, to ensure correct calibration for the device, a calibration routine should be performed
after power-up.
The power dissipation and temperature drift of the AD7705/
AD7706 are low and no warm-up time is required before the
initial calibration is performed. However, if an external reference is being used, this reference must have stabilized before
MCLK IN
C1
USING THE AD7705/AD7706
Clocking and Oscillator Circuit
1.05 3 VREF/GAIN
–0V DIFFERENTIAL
CRYSTAL OR
CERAMIC
RESONATOR
Figure 14. Crystal/Resonator Connection for the
AD7705/AD7706
If the part is used in unipolar mode with a required span of
0.8 ×␣ VREF/GAIN, the offset range the system calibration can
handle is from –1.05␣ ×␣ VREF/GAIN to +0.25 ×␣ VREF/GAIN. If
the part is used in unipolar mode with a required span of VREF/
GAIN, the offset range the system calibration can handle is
from –1.05 ×␣ VREF/GAIN to +0.05 ×␣ VREF/GAIN. Similarly, if
the part is used in unipolar mode and required to remove an
offset of 0.2 ×␣ VREF/GAIN, the span range the system calibration
can handle is 0.85 ×␣ VREF/GAIN.
AD7705/AD7706
INPUT RANGE
(0.8 3 VREF/GAIN TO
2.1 3 VREF/GAIN)
calibration is initiated. Similarly, if the clock source for the part
is generated from a crystal or resonator across the MCLK pins,
the start-up time for the oscillator circuit should elapse before a
calibration is initiated on the part (see below).
Using the part with a crystal or ceramic resonator between the
MCLK IN and MCLK OUT pins generally causes more current to be drawn from VDD than when the part is clocked from
a driven clock signal at the MCLK IN pin. This is because the
on-chip oscillator circuit is active in the case of the crystal or
ceramic resonator. Therefore, the lowest possible current on
the AD7705/AD7706 is achieved with an externally applied
clock at the MCLK IN pin with MCLK OUT unconnected,
unloaded and disabled.
The amount of additional current taken by the oscillator depends on a number of factors—first, the larger the value of
capacitor (C1 and C2) placed on the MCLK␣ IN and MCLK␣ OUT
pins, the larger the current consumption on the AD7705/
AD7706. Care should be taken not to exceed the capacitor
values recommended by the crystal and ceramic resonator
manufacturers to avoid consuming unnecessary current. Typical
values for C1 and C2 are recommended by crystal or ceramic
resonator manufacturers, these are in the range of 30␣ pF to
50␣ pF and if the capacitor values on MCLK IN and MCLK
OUT are kept in this range they will not result in any excessive
current. Another factor that influences the current is the effective series resistance (ESR) of the crystal that appears between
the MCLK IN and MCLK OUT pins of the AD7705/AD7706.
As a general rule, the lower the ESR value the lower the current
taken by the oscillator circuit.
–20–
REV. A
AD7705/AD7706
When operating with a clock frequency of 2.4576␣ MHz, there is
50␣ µA difference in the current between an externally applied
clock and a crystal resonator when operating with a VDD of
+3␣ V. With VDD = +5␣ V and fCLKIN = 2.4576␣ MHz, the typical
current increases by 250␣ µA for a crystal/resonator supplied
clock versus an externally applied clock. The ESR values for
crystals and resonators at this frequency tend to be low and as a
result there tends to be little difference between different crystal
and resonator types.
When operating with a clock frequency of 1␣ MHz, the ESR
value for different crystal types varies significantly. As a result,
the current drain varies across crystal types. When using a crystal with an ESR of 700␣ Ω or when using a ceramic resonator, the
increase in the typical current over an externally-applied clock is
20␣ µA with VDD = +3␣ V and 200␣ µA with VDD = +5␣ V. When
using a crystal with an ESR of 3␣ kΩ, the increase in the typical
current over an externally applied clock is again 100␣ µA with
VDD = +3␣ V but 400␣ µA with VDD = +5␣ V.
The on-chip oscillator circuit also has a start-up time associated
with it before it is oscillating at its correct frequency and correct
voltage levels. Typical start-up times with VDD = 5 V are 6 ms
using a 4.9512 MHz crystal, 16 ms with a 2.4576 MHz crystal
and 20 ms with a 1 MHz crystal oscillator. Start-up times are
typically 20% slower when the power supply voltage is reduced
to 3 V. At 3␣ V supplies, depending on the loading capacitances
on the MCLK pins, a 1␣ MΩ feedback resistor may be required
across the crystal or resonator in order to keep the start up times
around the 20␣ ms duration.
The AD7705/AD7706’s master clock appears on the MCLK
OUT pin of the device. The maximum recommended load on
this pin is one CMOS load. When using a crystal or ceramic
resonator to generate the AD7705/AD7706’s clock, it may be
desirable to use this clock as the clock source for the system.
In this case, it is recommended that the MCLK OUT signal is
buffered with a CMOS buffer before being applied to the rest of
the circuit.
System Synchronization
The FSYNC bit of the Setup Register allows the user to reset
the modulator and digital filter without affecting any of the
setup conditions on the part. This allows the user to start gathering samples of the analog input from a known point in time,
i.e., when the FSYNC is changed from 1 to 0.
With a 1 in the FSYNC bit of the Setup Register, the digital
filter and analog modulator are held in a known reset state and
the part is not processing any input samples. When a 0 is then
written to the FSYNC bit, the modulator and filter are taken
out of this reset state and the part starts to gather samples again
on the next master clock edge.
The FSYNC input can also be used as a software start convert
command allowing the AD7705/AD7706 to be operated in a
conventional converter fashion. In this mode, writing to the
FSYNC bit starts conversion and the falling edge of DRDY
indicates when conversion is complete. The disadvantage of this
scheme is that the settling time of the filter has to be taken into
account for every data register update. This means that the rate
at which the data register is updated is three times slower in this
mode.
REV. A
Since the FSYNC bit resets the digital filter, the full settling
time of 3 × 1/Output Rate has to elapse before there is a new
word loaded to the output register on the part. If the DRDY
signal is low when FSYNC goes to a 0, the DRDY signal will
not be reset high by the FSYNC command. This is because the
AD7705/AD7706 recognizes that there is a word in the data
register which has not been read. The DRDY line will stay low
until an update of the data register takes place, at which time it
will go high for 500 × tCLKIN before returning low again. A read
from the data register resets the DRDY signal high and it will
not return low until the settling time of the filter has elapsed
(from the FSYNC command) and there is a valid new word in
the data register. If the DRDY line is high when the FSYNC
command is issued, the DRDY line will not return low until the
settling time of the filter has elapsed.
Reset Input
The RESET input on the AD7705/AD7706 resets all the logic,
the digital filter and the analog modulator, while all on-chip
registers are reset to their default state. DRDY is driven high
and the AD7705/AD7706 ignores all communications to any of
its registers while the RESET input is low. When the RESET
input returns high, the AD7705/AD7706 starts to process data
and DRDY will return low in 3 × 1/Output Rate indicating a
valid new word in the data register. However, the AD7705/
AD7706 operates with its default setup conditions after a
RESET and it is generally necessary to set up all registers and
carry out a calibration after a RESET command.
The AD7705/AD7706’s on-chip oscillator circuit continues to
function even when the RESET input is low. The master clock
signal continues to be available on the MCLK OUT pin. Therefore, in applications where the system clock is provided by the
AD7705/AD7706’s clock, the AD7705/AD7706 produces an
uninterrupted master clock during RESET commands.
Standby Mode
The STBY bit in the Communications Register of the AD7705/
AD7706 allows the user to place the part in a power-down
mode when it is not required to provide conversion results. The
AD7705/AD7706 retains the contents of all its on-chip registers
(including the data register) while in standby mode. When released from standby mode, the part starts to process data and a
new word is available in the data register in 3 × 1/Output rate
from when a 0 is written to the STBY bit.
The STBY bit does not affect the digital interface, nor does it
affect the status of the DRDY line. If DRDY is high when the
STBY bit is brought low, it will remain high until there is a valid
new word in the data register. If DRDY is low when the STBY
bit is brought low, it will remain low until the data register is
updated, at which time the DRDY line will return high for
500␣ ×␣ tCLKIN before returning low again. If DRDY is low when
the part enters its standby mode (indicating a valid unread word
in the data register), the data register can be read while the part
is in standby. At the end of this read operation, the DRDY will
be reset high as normal.
–21–
AD7705/AD7706
Placing the part in standby mode reduces the total current to
9␣ µA typical with VDD = 5 V and 4 µA with VDD = 3 V when the
part is operated from an external master clock provided this
master clock is stopped. If the external clock continues to run in
standby mode, the standby current increases to 150␣ µA typical
with 5 V supplies and 75 µA typical with 3.3 V supplies. If a
crystal or ceramic resonator is used as the clock source, the total
current in standby mode is 400␣ µA typical with 5 V supplies and
90 µA with 3.3 V supplies. This is because the on-chip oscillator
circuit continues to run when the part is in its standby mode.
This is important in applications where the system clock is provided by the AD7705/AD7706’s clock, so that the AD7705/
AD7706 produces an uninterrupted master clock even when it is
in its standby mode.
Accuracy
Sigma-Delta ADCs, like VFCs and other integrating ADCs, do
not contain any source of nonmonotonicity and inherently offer
no missing codes performance. The AD7705/AD7706 achieves
excellent linearity by the use of high quality, on-chip capacitors,
which have a very low capacitance/voltage coefficient. The device also achieves low input drift through the use of chopperstabilized techniques in its input stage. To ensure excellent
performance over time and temperature, the AD7705/AD7706
uses digital calibration techniques that minimize offset and gain
error.
Supply Current
The current consumption on the AD7705/AD7706 is specified
for supplies in the range +2.7␣ V to +3.3␣ V and in the range +4.75␣ V
to +5.25␣ V. The part operates over a +2.7 V to +5.25␣ V supply
range and the IDD for the part varies as the supply voltage varies
over this range. There is an internal current boost bit on the
AD7705/AD7706 that is set internally in accordance with the
operating conditions. This affects the current drawn by the
analog circuitry within these devices. Minimum power consumption is achieved when the AD7705/AD7706 is operated with an
fCLKIN of 1 MHz or at gains of 1 to 4 with fCLKIN = 2.4575 MHz
as the internal boost bit is off reducing the analog current consumption. Figure 15 shows the variation of the typical IDD with
VDD voltage for both a 1␣ MHz crystal oscillator and a 2.4576 MHz
crystal oscillator at +25°C. The AD7705/AD7706 is operated in
unbuffered mode. The relationship shows that the IDD is minimized by operating the part with lower VDD voltages. IDD on the
AD7705/AD7706 is also minimized by using an external master
clock or by optimizing external components when using the
on-chip oscillator circuit. Figures 3, 4, 6 and 7 show variations
in IDD with gain, VDD and clock frequency using an external
clock.
1600
1400
Drift Considerations
1200
The AD7705/AD7706 uses chopper stabilization techniques to
minimize input offset drift. Charge injection in the analog
switches and dc leakage currents at the sampling node are the
primary sources of offset voltage drift in the converter. The dc
input leakage current is essentially independent of the selected
gain. Gain drift within the converter depends primarily upon
the temperature tracking of the internal capacitors. It is not
affected by leakage currents.
MCLK IN = CRYSTAL OSCILLATOR
TA = +258C
UNBUFFERED MODE
GAIN = 128
IDD – mA
1000
fCLK = 2.4576MHz
800
600
fCLK = 1MHz
400
200
Measurement errors due to offset drift or gain drift can be
eliminated at any time by recalibrating the converter. Using
the system calibration mode can also minimize offset and gain
errors in the signal conditioning circuitry. Integral and differential linearity errors are not significantly affected by temperature
changes.
POWER SUPPLIES
The AD7705/AD7706 operates with a VDD power supply between 2.7 V and 5.25 V. While the latch-up performance of the
AD7705/AD7706 is good, it is important that power is applied
to the AD7705/AD7706 before signals at REF␣ IN, AIN or the
logic input pins in order to avoid excessive currents. If this is not
possible, the current that flows in any of these pins should be
limited. If separate supplies are used for the AD7705/AD7706
and the system digital circuitry, the AD7705/AD7706 should be
powered up first. If it is not possible to guarantee this, current
limiting resistors should be placed in series with the logic
inputs to again limit the current. Latch-up current is greater
than 100 mA.
0
2.5
3.0
3.5
4.0
VDD
4.5
5.0
5.5
Figure 15. IDD vs. Supply Voltage
Grounding and Layout
Since the analog inputs and reference input are differential, most
of the voltages in the analog modulator are common-mode voltages. The excellent common-mode rejection of the part will
remove common-mode noise on these inputs. The digital filter
will provide rejection of broadband noise on the power supplies,
except at integer multiples of the modulator sampling frequency.
The digital filter also removes noise from the analog and reference inputs provided those noise sources do not saturate the
analog modulator. As a result, the AD7705/AD7706 is more
immune to noise interference than a conventional high resolution converter. However, because the resolution of the AD7705/
AD7706 is so high, and the noise levels from the AD7705/
AD7706 so low, care must be taken with regard to grounding
and layout.
–22–
REV. A
AD7705/AD7706
The printed circuit board that houses the AD7705 should be
designed so that the analog and digital sections are separated
and confined to certain areas of the board. This facilitates the
use of ground planes which can be separated easily. A minimum
etch technique is generally best for ground planes as it gives the
best shielding. Digital and analog ground planes should only be
joined in one place to avoid ground loops. If the AD7705/AD7706
is in a system where multiple devices require AGND-to-DGND
connections, the connection should be made at one point only,
a star ground point which should be established as close as
possible to the AD7705 GND.
performance of the part, independent of the analog input signal.
The scheme involves using a test mode on the part where the
inputs to the AD7705 are internally shorted together to provide
a zero differential voltage for the analog modulator. External to
the device, the AIN1(–) input on the AD7705 should be connected to a voltage that is within the allowable common-mode
range of the part. Similarly, on the AD7706 the COMMON
input should be connected to a voltage within its allowable
common-mode range for evaluation purposes. This scheme should
be used after a calibration has been performed on the part.
Avoid running digital lines under the device as these will couple
noise onto the die. The analog ground plane should be allowed
to run under the AD7705/AD7706 to avoid noise coupling. The
power supply lines to the AD7705/AD7706 should use as large
a trace as possible to provide low impedance paths and reduce
the effects of glitches on the power supply line. Fast switching
signals like clocks should be shielded with digital ground to
avoid radiating noise to other sections of the board and clock
signals should never be run near the analog inputs. Avoid crossover of digital and analog signals. Traces on opposite sides of
the board should run at right angles to each other. This will
reduce the effects of feedthrough through the board. A microstrip technique is by far the best, but is not always possible with
a double-sided board. In this technique, the component side of
the board is dedicated to ground planes while signals are placed
on the solder side.
DIGITAL INTERFACE
Good decoupling is important when using high resolution
ADCs. All analog supplies should be decoupled with 10␣ µF
tantalum in parallel with 0.1␣ µF ceramic capacitors to GND. To
achieve the best from these decoupling components, they have
to be placed as close as possible to the device, ideally right up
against the device. All logic chips should be decoupled with
0.1␣ µF disc ceramic capacitors to DGND.
Evaluating the AD7705/AD7706 Performance
The recommended layout for the AD7705 and AD7706 is outlined in their associated evaluation. These evaluation board
packages include a fully assembled and tested evaluation board,
documentation, software for controlling the board over the
printer port of a PC and software for analyzing their performance on the PC.
Noise levels in the signals applied to the AD7705/AD7706 may
also affect performance of the part. The AD7705/AD7706 software evaluation package allows the user to evaluate the true
REV. A
As previously outlined, the AD7705/AD7706’s programmable
functions are controlled using a set of on-chip registers. Data is
written to these registers via the part’s serial interface and read
access to the on-chip registers is also provided by this interface.
All communications to the part must start with a write operation
to the Communications Register. After power-on or RESET,
the device expects a write to its Communications Register. The
data written to this register determines whether the next operation to the part is a read or a write operation and also determines to which register this read or write operation occurs.
Therefore, write access to any of the other registers on the part
starts with a write operation to the Communications Register
followed by a write to the selected register. A read operation
from any other register on the part (including the output data
register) starts with a write operation to the Communications
Register followed by a read operation from the selected register.
The AD7705/AD7706’s serial interface consists of five signals,
CS, SCLK, DIN, DOUT and DRDY. The DIN line is used for
transferring data into the on-chip registers while the DOUT line
is used for accessing data from the on-chip registers. SCLK is
the serial clock input for the device and all data transfers (either
on DIN or DOUT) take place with respect to this SCLK signal.
The DRDY line is used as a status signal to indicate when data
is ready to be read from the AD7705/AD7706’s data register.
DRDY goes low when a new data word is available in the output register. It is reset high when a read operation from the data
register is complete. It also goes high prior to the updating of
the output register to indicate when not to read from the device
to ensure that a data read is not attempted while the register is
being updated. CS is used to select the device. It can be used to
decode the AD7705/AD7706 in systems where a number of
parts are connected to the serial bus.
–23–
AD7705/AD7706
Figures 16 and 17 show timing diagrams for interfacing to the
AD7705/AD7706 with CS used to decode the part. Figure 16 is
for a read operation from the AD7705/AD7706’s output shift
register while Figure 17 shows a write operation to the input
shift register. It is possible to read the same data twice from the
output register even though the DRDY line returns high after
the first read operation. Care must be taken, however, to ensure
that the read operations have been completed before the next
output update is about to take place.
The AD7705/AD7706 serial interface can operate in three-wire
mode by tying the CS input low. In this case, the SCLK, DIN
and DOUT lines are used to communicate with the AD7705/
AD7706 and the status of DRDY can be obtained by interrogating the MSB of the Communications Register. This scheme is
suitable for interfacing to microcontrollers. If CS is required as a
decoding signal, it can be generated from a port bit. For
microcontroller interfaces, it is recommended that the SCLK
idles high between data transfers.
The AD7705/AD7706 can also be operated with CS used as a
frame synchronization signal. This scheme is suitable for DSP
interfaces. In this case, the first bit (MSB) is effectively clocked
out by CS since CS would normally occur after the falling edge
of SCLK in DSPs. The SCLK can continue to run between
data transfers provided the timing numbers are obeyed.
The serial interface can be reset by exercising the RESET input
on the part. It can also be reset by writing a series of 1s on the
DIN input. If a Logic 1 is written to the AD7705/AD7706 DIN
line for at least 32 serial clock cycles the serial interface is reset.
This ensures that in three-wire systems, if the interface gets lost
either via a software error or by some glitch in the system, it can
be reset back to a known state. This state returns the interface
to where the AD7705/AD7706 is expecting a write operation to
its Communications Register. This operation in itself does not
reset the contents of any registers but since the interface was
lost, the information written to any of the registers is unknown
and it is advisable to set up all registers again.
Some microprocessor or microcontroller serial interfaces have a
single serial data line. In this case, it is possible to connect the
AD7705/AD7706’s DATA OUT and DATA IN lines together
and connect them to the single data line of the processor. A
10 kΩ pull-up resistor should be used on this single data line. In
this case, if the interface gets lost, because the read and write
operations share the same line the procedure to reset it back to a
known state is somewhat different than previously described. It
requires a read operation of 24 serial clocks followed by a write
operation where a Logic 1 is written for at least 32 serial clock
cycles to ensure that the serial interface is back into a known
state.
DRDY
t10
t3
CS
t4
t8
t6
SCLK
t9
t7
t5
DOUT
LSB
MSB
Figure 16. Read Cycle Timing Diagram
CS
t11
t16
t14
SCLK
t12
DIN
t13
t15
LSB
MSB
Figure 17. Write Cycle Timing Diagram
–24–
REV. A
AD7705/AD7706
CONFIGURING THE AD7705/AD7706
The AD7705/AD7706 contains six on-chip registers that the
user can accesses via the serial interface. Communication with
any of these registers is initiated by writing to the Communications Register first. Figure 18 outlines a flow diagram of the
sequence used to configure all registers after a power-up or reset
on the AD7705, similar procedures apply to the AD7706. The
flowchart also shows two different read options—the first where
the DRDY pin is polled to determine when an update of the
data register has taken place, the second where the DRDY bit of
the Communications Register is interrogated to see if a data
register update has taken place. Also included in the flowing diagram is a series of words that should be written to the registers
for a particular set of operating conditions. These conditions
are gain of one, no filter sync, bipolar mode, buffer off, clock of
4.9512␣ MHz and an output rate of 50 Hz.
START
POWER-ON/RESET FOR AD7705
CONFIGURE & INITIALIZE mC/mP SERIAL PORT
WRITE TO COMMUNICATIONS REGISTER SELECTING
CHANNEL & SETTING UP NEXT OPERATION TO BE A
WRITE TO THE CLOCK REGISTER (20 HEX)
WRITE TO CLOCK REGISTER SETTING THE CLOCK
BITS IN ACCORDANCE WITH THE APPLIED MASTER
CLOCK SIGNAL AND SELECT UPDATE RATE FOR
SELECTED CHANNEL (0C HEX)
WRITE TO COMMUNICATIONS REGISTER SELECTING
CHANNEL & SETTING UP NEXT OPERATION TO BE A
WRITE TO THE SETUP REGISTER (10 HEX)
WRITE TO SETUP REGISTER CLEARING F SYNC,
SETTING UP GAIN, OPERATING CONDITIONS &
INITIATING A SELF-CALIBRATION ON SELECTED
CHANNEL (40 HEX)
POLL DRDY PIN
NO
WRITE TO COMMUNICATIONS REGISTER SETTING UP NEXT
OPERATION TO BE A READ FROM THE COMMUNICATIONS
REGISTER (08 HEX)
DRDY
LOW?
YES
READ FROM COMMUNICATIONS REGISTER
WRITE TO COMMUNICATIONS REGISTER SETTING UP
NEXT OPERATION TO BE A READ FROM THE DATA
REGISTER (38 HEX)
POLL DRDY BIT OF COMMUNICATIONS REGISTER
READ FROM DATA REGISTER
NO
DRDY
LOW?
YES
WRITE TO COMMUNICATIONS REGISTER SETTING UP
NEXT OPERATION TO BE A READ FROM THE DATA
REGISTER (38 HEX)
READ FROM DATA REGISTER
Figure 18. Flowchart for Setting Up and Reading from the AD7705
REV. A
–25–
AD7705/AD7706
MICROCOMPUTER/MICROPROCESSOR INTERFACING
VDD
The AD7705/AD7706’s flexible serial interface allows for easy
interface to most microcomputers and microprocessors. The
flowchart of Figure 10 outlines the sequence that should be
followed when interfacing a microcontroller or microprocessor
to the AD7705/AD7706. Figures 19, 20 and 21 show some
typical interface circuits.
VDD
AD7705/AD7706
SS
RESET
68HC11
SCK
The serial interface on the AD7705/AD7706 is capable of operating from just three wires and is compatible with SPI interface
protocols. The three-wire operation makes the part ideal for
isolated systems where minimizing the number of interface lines
minimizes the number of opto-isolators required in the system.
The serial clock input is a Schmitt triggered input to accommodate slow edges from opto-couplers. The rise and fall times of
other digital inputs to the AD7705/AD7706 should be no longer
than 1␣ µs.
Most of the registers on the AD7705/AD7706 are 8-bit registers, which facilitates easy interfacing to the 8-bit serial ports of
microcontrollers. The Data Register on the AD7705/AD7706 is
16␣ bits, and the offset and gain registers are 24-bit registers but
data transfers to these registers can consist of multiple 8-bit
transfers to the serial port of the microcontroller. DSP processors and microprocessors generally transfer 16 bits of data in a
serial data operation. Some of these processors, such as the
ADSP-2105, have the facility to program the amount of cycles
in a serial transfer. This allows the user to tailor the number of
bits in any transfer to match the register length of the required
register in the AD7705/AD7706.
SCLK
MISO
DATA OUT
MOSI
DATA IN
CS
Figure 19. AD7705/AD7706 to 68HC11 Interface
The 68HC11 is configured in the master mode with its CPOL
bit set to a logic one and its CPHA bit set to a logic one. When
the 68HC11 is configured like this, its SCLK line idles high
between data transfers. The AD7705/AD7706 is not capable of
full duplex operation. If the AD7705/AD7706 is configured for
a write operation, no data appears on the DATA OUT lines
even when the SCLK input is active. Similarly, if the AD7705/
AD7706 is configured for a read operation, data presented to
the part on the DATA IN line is ignored even when SCLK is
active.
Even though some of the registers on the AD7705/AD7706 are
only eight bits in length, communicating with two of these registers in successive write operations can be handled as a single 16bit data transfer if required. For example, if the Setup Register
is to be updated, the processor must first write to the Communications Register (saying that the next operation is a write to the
Setup Register) and then write eight bits to the Setup Register.
If required, this can all be done in a single 16-bit transfer because once the eight serial clocks of the write operation to the
Communications Register have been completed, the part immediately sets itself up for a write operation to the Setup Register.
Coding for an interface between the 68HC11 and the AD7705/
AD7706 is given in Table XV. In this example, the DRDY
output line of the AD7705/AD7706 is connected to the PC0 port
bit of the 68HC11 and is polled to determine its status.
VDD
8XC51
VDD
P3.0
AD7705/AD7706 to 68HC11 Interface
Figure 19 shows an interface between the AD7705/AD7706 and
the 68HC11 microcontroller. The diagram shows the minimum
(three-wire) interface with CS on the AD7705/AD7706 hardwired low. In this scheme, the DRDY bit of the Communications Register is monitored to determine when the Data Register
is updated. An alternative scheme, which increases the number
of interface lines to four, is to monitor the DRDY output line
from the AD7705/AD7706. The monitoring of the DRDY line
can be done in two ways. First, DRDY can be connected to one
of the 68HC11’s port bits (such as PC0), which is configured as
an input. This port bit is then polled to determine the status of
DRDY. The second scheme is to use an interrupt driven system,
in which case the DRDY output is connected to the IRQ input
of the 68HC11. For interfaces that require control of the CS
input on the AD7705/AD7706, one of the port bits of the
68HC11 (such as PC1), which is configured as an output, can
be used to drive the CS input.
AD7705/AD7706
RESET
DATA OUT
DATA IN
P3.1
SCLK
CS
Figure 20. AD7705/AD7706 to 8XC51 Interface
AD7705/AD7706 to 8051 Interface
An interface circuit between the AD7705/AD7706 and the
8XC51 microcontroller is shown in Figure 20. The diagram
shows the minimum number of interface connections with CS
on the AD7705/AD7706 hard-wired low. In the case of the
8XC51 interface the minimum number of interconnects is just
two. In this scheme, the DRDY bit of the Communications
Register is monitored to determine when the Data Register is
updated. The alternative scheme, which increases the number of
–26–
REV. A
AD7705/AD7706
interface lines to three, is to monitor the DRDY output line
from the AD7705/AD7706. The monitoring of the DRDY line
can be done in two ways. First, DRDY can be connected to one
of the 8XC51’s port bits (such as P1.0) which is configured as
an input. This port bit is then polled to determine the status of
DRDY. The second scheme is to use an interrupt-driven system,
in which case the DRDY output is connected to the INT1 input
of the 8XC51. For interfaces that require control of the CS
input on the AD7705/AD7706, one of the port bits of the
8XC51 (such as P1.1), which is configured as an output, can be
used to drive the CS input. The 8XC51 is configured in its
Mode 0 serial interface mode. Its serial interface contains a
single data line. As a result, the DATA OUT and DATA IN
pins of the AD7705/AD7706 should be connected together with
a 10 kΩ pull-up resistor. The serial clock on the 8XC51 idles
high between data transfers. The 8XC51 outputs the LSB first
in a write operation, while the AD7705/AD7706 expects the
MSB first so the data to be transmitted has to be rearranged
before being written to the output serial register. Similarly,
the AD7705/AD7706 outputs the MSB first during a read operation while the 8XC51 expects the LSB first. Therefore, the
data read into the serial buffer needs to be rearranged before the
correct data word from the AD7705/AD7706 is available in the
accumulator.
VDD
ADSP-2103/
ADSP-2105
AD7705/AD7706
RESET
RFS
CS
TFS
DR
DATA OUT
DT
DATA IN
SCLK
SCLK
Figure 21. AD7705/AD7706 to ADSP-2103/ADSP-2105
Interface
AD7705/AD7706 to ADSP-2103/ADSP-2105 Interface
Figure 21 shows an interface between the AD7705/AD7706 and
the ADSP-2103/ADSP-2105 DSP processor. In the interface
shown, the DRDY bit of the Communications Register is again
monitored to determine when the Data Register is updated. The
alternative scheme is to use an interrupt-driven system, in which
case the DRDY output is connected to the IRQ2 input of the
ADSP-2103/ADSP-2105. The serial interface of the ADSP2103/ADSP-2105 is set up for alternate framing mode. The
RFS and TFS pins of the ADSP-2103/ADSP-2105 are configured as active low outputs and the ADSP-2103/ADSP-2105
serial clock line, SCLK, is also configured as an output. The CS
for the AD7705/AD7706 is active when either the RFS or TFS
outputs from the ADSP-2103/ADSP-2105 are active. The serial
clock rate on the ADSP-2103/ADSP-2105 should be limited to
3␣ MHz to ensure correct operation with the AD7705/AD7706.
CODE FOR SETTING UP THE AD7705/AD7706
Table XVII gives a set of read and write routines in C code for
interfacing the 68HC11 microcontroller to the AD7705. The
sample program sets up the various registers on the AD7705
and reads 1000 samples from the part into the 68HC11. The
setup conditions on the part are exactly the same as those outlined for the flowchart of Figure 18. In the example code given
here, the DRDY output is polled to determine if a new valid
word is available in the data register. The very same sequence is
applicable for the AD7706.
The sequence of the events in this program are as follows:
1. Write to the Communications Register, selecting channel one
as the active channel and setting the next operation to be a
write to the clock register.
2. Write to Clock Register setting the CLK DIV bit which
divides the external clock internally by two. This assumes
that the external crystal is 4.9512 MHz. The update rate is
selected to be 50 Hz.
3. Write to Communication Register selecting Channel 1 as the
active channel and setting the next operation to be a write to
the Setup Register.
4. Write to the Setup Register, setting the gain to 1, setting
bipolar mode, buffer off, clearing the filter synchronization
and initiating a self-calibration.
5. Poll the DRDY output.
6. Read the data from the Data Register.
7. Loop around doing Steps 5 and 6 until the specified number
of samples have been taken from the selected channel.
REV. A
–27–
AD7705/AD7706
Table XVII. C Code for Interfacing AD7705 to 68HC11
/* This program has read and write routines for the 68HC11 to interface to the AD7705 and the sample program sets the various
registers and then reads 1000 samples from one channel. */
#include <math.h>
#include <io6811.h>
#define NUM_SAMPLES 1000 /* change the number of data samples */
#define MAX_REG_LENGTH 2 /* this says that the max length of a register is 2 bytes */
Writetoreg (int);
Read (int,char);
char *datapointer = store;
char store[NUM_SAMPLES*MAX_REG_LENGTH + 30];
void main()
{
/* the only pin that is programmed here from the 68HC11 is the /CS and this is why the PC2 bit of PORTC is made as
an output */
char a;
DDRC = 0x04; /* PC2 is an output the rest of the port bits are inputs */
PORTC | = 0x04; /* make the /CS line high */
Writetoreg(0x20); /* Active Channel is Ain1(+)/Ain1(-), next operation as write to the clock register */
Writetoreg(0x0C); /* master clock enabled, 4.9512MHz Clock, set output rate to 50Hz*/
Writetoreg(0x10); /* Active Channel is Ain1(+)/Ain1(-), next operation as write to the setup register */
Writetoreg(0x40); /* gain = 1, bipolar mode, buffer off, clear FSYNC and perform a Self Calibration*/
while(PORTC & 0x10); /* wait for /DRDY to go low */
for(a=0;a<NUM_SAMPLES;a++);
{
Writetoreg(0x38); /*set the next operation for 16 bit read from the data register */
Read(NUM_SAMPES,2);
}
}
Writetoreg(int byteword);
{
int q;
SPCR = 0x3f;
SPCR = 0X7f; /* this sets the WiredOR mode(DWOM=1), Master mode(MSTR=1), SCK idles high(CPOL=1), /SS can be low
always (CPHA=1), lowest clock speed(slowest speed which is master clock /32 */
DDRD = 0x18; /* SCK, MOSI outputs */
q = SPSR;
q = SPDR; /* the read of the staus register and of the data register is needed to clear the interrupt which tells the user that the
data transfer is complete */
PORTC &= 0xfb; /* /CS is low */
SPDR = byteword; /* put the byte into data register */
while(!(SPSR & 0x80)); /* wait for /DRDY to go low */
PORTC |= 0x4; /* /CS high */
}
Read(int amount, int reglength)
{
int q;
SPCR = 0x3f;
SPCR = 0x7f; /* clear the interupt */
DDRD = 0x10; /* MOSI output, MISO input, SCK output */
while(PORTC & 0x10); /* wait for /DRDY to go low */
PORTC & 0xfb ; /* /CS is low */
for(b=0;b<reglength;b++)
{
SPDR = 0;
while(!(SPSR & 0x80)); /* wait until port ready before reading */
*datapointer++=SPDR; /* read SPDR into store array via datapointer */
}
PORTC|=4; /* /CS is high */
}
–28–
REV. A
AD7705/AD7706
Using the part with a programmed gain of 128 results in the
full-scale input span of the AD7705 being 15␣ mV, which corresponds with the output span from the transducer. The second
channel on the AD7705 can be used as an auxiliary channel to
measure a secondary variable such as temperature as shown in
Figure 22. This secondary channel can be used as a means of
adjusting the output of the primary channel, thus removing
temperature effects in the system.
APPLICATIONS
The AD7705 provides a dual channel, low cost, high resolution
analog-to-digital function. Because the analog-to-digital function is provided by a sigma-delta architecture, it makes the part
more immune to noisy environments thus making the part ideal
for use in industrial and process control applications. It also
provides a programmable gain amplifier, a digital filter and
calibration options. Thus, it provides far more system level
functionality than off-the-shelf integrating ADCs without the
disadvantage of having to supply a high quality integrating capacitor. In addition, using the AD7705 in a system allows the
system designer to achieve a much higher level of resolution
because noise performance of the AD7705 is better than that of
the integrating ADCs.
+5V
VDD
THERMOCOUPLE
JUNCTION
AIN1(+)
MCLK IN
AIN1(–)
The on-chip PGA allows the AD7705 to handle an analog input
voltage range as low as 10 mV full-scale with VREF = +1.25␣ V.
The differential inputs of the part allow this analog input range
to have an absolute value anywhere between GND and VDD
when the part is operated in unbuffered mode. It allows the user
to connect the transducer directly to the input of the AD7705.
The programmable gain front end on the AD7705 allows the
part to handle unipolar analog input ranges from 0␣ mV to
+20␣ mV to 0␣ V to +2.5␣ V and bipolar inputs of ± 20␣ mV to
± 2.5␣ V. Because the part operates from a single supply these
bipolar ranges are with respect to a biased-up differential input.
+5V
AD7705
REF IN(+)
REF192
MCLK OUT
OUTPUT
REF IN(–)
RESET
GND
GND
DRDY
DOUT DIN
CS
SCLK
Figure 23. Temperature Measurement Using the AD7705
EXCITATION VOLTAGE = +5V
+5V
VDD
IN+
OUT(+)
Temperature Measurement
AIN1(+)
OUT(–)
AIN1(–)
IN–
AD7705
24kV
MCLK IN
AIN2(+)
AIN2(–)
THERMOCOUPLE
JUNCTION
MCLK OUT
REF IN(+)
15kV
RESET
REF IN(–)
DRDY
GND
DOUT DIN
CS SCLK
Figure 22. Pressure Measurement Using the AD7705
Pressure Measurement
One typical application of the AD7705 is pressure measurement. Figure 22 shows the AD7705 used with a pressure
transducer, the BP01 from Sensym. The pressure transducer
is arranged in a bridge network and gives a differential output
voltage between its OUT(+) and OUT(–) terminals. With rated
full-scale pressure (in this case 300␣ mmHg) on the transducer,
the differential output voltage is 3␣ mV/V of the input voltage
(i.e., the voltage between its IN(+) and IN(–) terminals).
Assuming a 5␣ V excitation voltage, the full-scale output range
from the transducer is 15␣ mV. The excitation voltage for the
bridge is also used to generate the reference voltage for the
AD7705. Therefore, variations in the excitation voltage do
not introduce errors in the system. Choosing resistor values
of 24␣ kΩ and 15␣ kΩ, as per Figure 22, gives a 1.92␣ V reference
voltage for the AD7705 when the excitation voltage is 5␣ V.
REV. A
Another application area for the AD7705 is in temperature
measurement. Figure 23 outlines a connection from a thermocouple to the AD7705. In this application, the AD7705 is operated in its buffered mode to allow large decoupling capacitors
on the front end to eliminate any noise pickup that may have
been in the thermocouple leads. When the AD7705 is operated
in buffered mode, it has a reduced common-mode range. In
order to place the differential voltage from the thermocouple on
a suitable common-mode voltage, the AIN1(–) input of the
AD7705 is biased up at the reference voltage, +2.5␣ V.
Figure 23 shows another temperature measurement application
for the AD7705. In this case, the transducer is an RTD (Resistive Temperature Device), a PT100. The arrangement is a
4-lead RTD configuration. There are voltage drops across the
lead resistances RL1 and RL4 but these simply shift the commonmode voltage. There is no voltage drop across lead resistances
RL2 and RL3 as the input current to the AD7705 is very low.
The lead resistances present a small source impedance so it
would not generally be necessary to turn on the buffer on the
AD7705. If the buffer is required, the common-mode voltage
should be set accordingly by inserting a small resistance between the bottom end of the RTD and GND of the AD7705.
In the application shown, an external 400␣ µA current source
provides the excitation current for the PT100 and also generates
the reference voltage for the AD7705 via the 6.25␣ kΩ resistor.
Variations in the excitation current do not affect the circuit as
both the input voltage and the reference voltage vary radiometrically with the excitation current. However, the 6.25␣ kΩ resistor
must have a low temperature coefficient to avoid errors in the
reference voltage over temperature.
–29–
AD7705/AD7706
Smart Transmitters
+5V
VDD
Another area where the low power, single supply, three-wire
interface capabilities is of benefit is in smart transmitters. Here,
the entire smart transmitter must operate from the 4␣ mA to
20␣ mA loop. Tolerances in the loop mean that the amount of
current available to power the transmitter is as low as 3.5␣ mA.
The AD7705 consumes only 320␣ µA, leaving at least 3␣ mA
available for the rest of the transmitter. Figure 25 shows a block
diagram of a smart transmitter which includes the AD7705. The
AD7705 with its dual input channel is ideally suited to systems
requiring an auxiliary channel whose measured variable is used
to correct that of the primary channel.
400mA
REF IN(+)
6.25kV
REF IN(–)
MCLK IN
RL1
RL2
AD7705
AIN1(+)
RTD
MCLK OUT
RL3
AIN1(–)
RESET
RL4
DRDY
GND
DOUT DIN
CS
SCLK
Figure 24. RTD Measurement Using the AD7705
MAIN TRANSMITTER ASSEMBLY
ISOLATION
BARRIER
ISOLATED SUPPLY
VDD
REF IN
4.7mF
100kV
VCC
VCC
BOOST
REF OUT1
COMP
REF OUT2
DRIVE
MICROCONTROLLER UNIT
SENSORS
RTD
mV
V
TC
AD7705
MCLK IN
DN25D
2.2mF 0.1mF
•PID
•RANGE SETTING
•CALIBRATION
•LINEARIZATION
•OUTPUT CONTROL
•SERIAL COMMUNICATION
•HART PROTOCOL
0.01mF
4mA
TO
20mA
1kV
REF IN
1000pF
4.7mF
AD421
C1 C2 C3
LOOP
RTN
COM
COM
MCLK OUT
0.01mF
0.0033mF
GND
ISOLATED GROUND
0.01mF
Figure 25. Smart Transmitter Using the AD7705
–30–
REV. A
AD7705/AD7706
Battery Monitoring
Another area where the low power, single supply operation is a
requirement is battery monitoring in portable equipment applications. Figure 26 shows a block diagram of a battery monitor
that includes the AD7705 and an external multiplexer used to
differentially measure the voltage across a single cell. The second channel on the AD7705 is used to monitor current drain
from the battery. The AD7705 with its dual input channel is
ideally suited to measurement systems requiring two input channels, as in this case, to monitor voltage and current. Since the
AD7705 can accommodate very low input signals the RSENSE
can be kept low reducing undesired power dissipation. Operating with a gain of 128, a ± 9.57 mV full-scale signal can be
measured with a resolution of 2 µV, giving 13.5 bits of flickerfree performance in such a system. In order to obtain specified
performance in unbuffered mode, the common mode range of
the input is from GND to VDD provided that the absolute value
of the analog input voltage lies between GND – 30␣ mV and
VDD␣ +␣ 30␣ mV. Absolute voltages of GND – 200 mV can be
accommodated on the AD7705 at 25°C without any degradation in performance, but leakage current increases significantly
at elevated temperatures.
ON/OFF SWITCH
4-TO-1
DIFFERENTIAL
MULTIPLEXER
VCELL 1
+3V
VDIFF1
VCELL 2
DC BATTERY
CHARGING
SOURCE
VCELL 3
VDIFF2
VDD
AD7705
VDIFF3
AIN1(+)
AIN1(–) REF IN(+)
VCELL 4
VDIFF4
1.225V
REF
AIN2(+)
AIN2(–)
RSENSE
REF IN(–)
GND
Figure 26. Battery Monitoring Using the AD7705
REV. A
+5V
VOLTAGE
REGULATORS
–31–
LOAD
AD7705/AD7706
OUTLINE DIMENSIONS
Dimensions shown in inches and (mm).
16-Pin Plastic DIP
(N-16)
16
9
1
8
PIN 1
0.280 (7.11)
0.240 (6.10)
0.325 (8.26)
0.300 (7.62) 0.195 (4.95)
0.115 (2.93)
0.060 (1.52)
0.015 (0.38)
0.210 (5.33)
MAX
0.130
(3.30)
MIN
0.160 (4.06)
0.115 (2.93)
0.015 (0.381)
0.008 (0.204)
0.070 (1.77) SEATING
0.045 (1.15) PLANE
0.100
(2.54)
BSC
0.022 (0.558)
0.014 (0.356)
C3253a–0–11/98
0.840 (21.34)
0.745 (18.92)
16-Lead SOIC
(R-16)
9
1
8
0.4193 (10.65)
0.3937 (10.00)
16
0.2992 (7.60)
0.2914 (7.40)
0.4133 (10.50)
0.3977 (10.00)
0.1043 (2.65)
0.0926 (2.35)
PIN 1
0.0118 (0.30)
0.0040 (0.10)
0.0500
(1.27)
BSC
0.0192 (0.49)
SEATING
0.0138 (0.35) PLANE
0.0291 (0.74)
x 458
0.0098 (0.25)
88 0.0500 (1.27)
0.0125 (0.32) 08 0.0157 (0.40)
0.0091 (0.23)
16-Lead TSSOP
(RU-16)
0.201 (5.10)
0.193 (4.90)
9
1
PRINTED IN U.S.A.
0.256 (6.50)
0.246 (6.25)
0.177 (4.50)
0.169 (4.30)
16
8
PIN 1
0.006 (0.15)
0.002 (0.05)
0.0256
SEATING (0.65)
PLANE BSC
0.0433
(1.10)
MAX
0.0118 (0.30)
0.0075 (0.19)
8°
0°
0.0079 (0.20)
0.0035 (0.090)
–32–
0.028 (0.70)
0.020 (0.50)
REV. A