AD AD7711AR-REEL7 Lc2mos signal conditioning adc with rtd excitation current Datasheet

LC2MOS Signal Conditioning ADC
with RTD Excitation Currents
AD7711
FEATURES
Charge-Balancing ADC
24 Bits, No Missing Codes
ⴞ0.0015% Nonlinearity
2-Channel Programmable Gain Front End
Gains from 1 to 128
1 Differential Input
1 Single-Ended Input
Low-Pass Filter with Programmable Filter Cutoffs
Ability to Read/Write Calibration Coefficients
RTD Excitation Current Sources
Bidirectional Microcontroller Serial Interface
Internal/External Reference Option
Single- or Dual-Supply Operation
Low Power (25 mW typ) with Power-Down Mode
(7 mW typ)
APPLICATIONS
RTD Transducers
Process Control
Smart Transmitters
Portable Industrial Instruments
GENERAL DESCRIPTION
The AD7711 is a complete analog front end for low frequency
measurement applications. The device accepts low level signals
directly from a transducer and outputs a serial digital word. It
employs a ⌺-⌬ conversion technique to realize up to 24 bits of
no missing codes performance. The input signal is applied to a
proprietary programmable gain front end based around an analog modulator. The modulator output is processed by an on-chip
digital filter. The first notch of this digital filter can be programmed via the on-chip control register, allowing adjustment
of the filter cutoff and settling time.
The part features one differential analog input and one singleended analog input as well as a differential reference input.
Normally, one of the input channels will be used as the main
channel with the second channel used as an auxiliary input to
periodically measure a second voltage. It can be operated from a
single supply (by tying the VSS pin to AGND), provided that the
input signals on the analog inputs are more positive than –30 mV.
By taking the VSS pin negative, the part can convert signals
down to –VREF on its inputs. The part provides two current
sources that can be used to provide excitation in 3-wire and 4-wire
RTD configurations. The AD7711 thus performs all signal
conditioning and conversion for a single- or dual-channel system.
REV. G
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 that
may result from its use. No license is granted by implication or otherwise
under any patent or patent rights of Analog Devices. Trademarks and
registered trademarks are the property of their respective owners.
FUNCTIONAL BLOCK DIAGRAM
AVDD
REF
IN (–)
DVDD
REF
IN (+)
VBIAS
REF OUT
AVDD
2.5V REFERENCE
4.5␮A
CHARGE-BALANCING A/D
CONVERTER
AIN1(+)
M
U
X
AIN1(–)
AUTO-ZEROED
⌺-⌬
MODULATOR
PGA
DIGITAL
FILTER
SYNC
A = 1–128
AIN2
200␮A
CLOCK
GENERATION
AVDD
RTD1
MCLK
IN
MCLK
OUT
SERIAL INTERFACE
200␮A
CONTROL
REGISTER
RTD2
OUTPUT
REGISTER
AD7711
AGND DGND
VSS
RFS TFS MODE SDATA SCLK DRDY A0
The AD7711 is ideal for use in smart, microcontroller based
systems. Gain settings, signal polarity, input channel selection,
and RTD current control can be configured in software using
the bidirectional serial port. The AD7711 contains selfcalibration, system calibration, and background calibration
options, and also allows the user to read and write the on-chip
calibration registers.
CMOS construction ensures low power dissipation, and a software
programmable power-down mode reduces the standby power
consumption to only 7 mW typical. The part is available in a
24-lead, 0.3-inch-wide, plastic and hermetic dual-in-line package (DIP) as well as a 24-lead small outline (SOIC) package.
PRODUCT HIGHLIGHTS
1. The programmable gain front end allows the AD7711 to
accept input signals directly from an RTD transducer,
removing a considerable amount of signal conditioning.
On-chip current sources provide excitation for 3-wire and
4-wire RTD configurations.
2. No missing codes ensure true, usable, 23-bit dynamic range
coupled with excellent ± 0.0015% accuracy. The effects of
temperature drift are eliminated by on-chip self-calibration,
which removes zero-scale and full-scale errors.
3. The AD7711 is ideal for microcontroller or DSP processor
applications with an on-chip control register that allows
control over filter cutoff, input gain, channel selection, signal
polarity, RTD current control, and calibration modes.
4. The AD7711 allows the user to read and to write the on-chip
calibration registers. This means that the microcontroller has
much greater control over the calibration procedure.
One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A.
Tel: 781/329-4700
www.analog.com
Fax: 781/326-8703
© 2004 Analog Devices, Inc. All rights reserved.
AD7711–SPECIFICATIONS
(AVDD = +5 V ⴞ 5%; DVDD = +5 V ⴞ 5%; VSS = 0 V or –5 V ⴞ 5%; REF IN(+) =
+2.5 V; REF IN(–) = AGND; MCLK IN = 10 MHz unless otherwise stated. All specifications TMIN to TMAX, unless otherwise noted.)
Parameter
STATIC PERFORMANCE
No Missing Codes
Output Noise
Integral Nonlinearity @ 25∞C
TMIN to TMAX
Positive Full-Scale Error2, 3
Full-Scale Drift5
Unipolar Offset Error2
Unipolar Offset Drift5
Bipolar Zero Error2
Bipolar Zero Drift5
Gain Drift
Bipolar Negative Full-Scale Error2 @ 25∞C
TMIN to TMAX
Bipolar Negative Full-Scale Drift5
ANALOG INPUTS/REFERENCE INPUTS
Normal Mode 50 Hz Rejection6
Normal Mode 60 Hz Rejection6
DC Input Leakage Current @ 25∞C6
TMIN to TMAX
Sampling Capacitance6
AIN1/REF IN
Common-Mode Rejection (CMR)
Common-Mode Rejection (CMR)
Common-Mode 50 Hz Rejection6
Common-Mode 60 Hz Rejection6
Common-Mode Voltage Range7
Analog Inputs8
Input Voltage Range9
Input Sampling Rate, fS
AIN2 Offset Error
AIN2 Offset Drift
Reference Inputs
REF IN(+) – REF IN(–) Voltage11
Input Sampling Rate, fS
REFERENCE OUTPUT
Output Voltage
Initial Tolerance @ 25∞C
Drift
Output Noise
Line Regulation (AVDD)
Load Regulation
External Current
A, S Versions1
Unit
Conditions/Comments
24
22
18
15
12
See Tables I and II
± 0.0015
± 0.003
See Note 4
1
0.3
See Note 4
0.5
0.25
See Note 4
0.5
0.25
2
± 0.003
± 0.006
1
0.3
Bits min
Bits min
Bits min
Bits min
Bits min
mV/∞C typ
mV/∞C typ
Guaranteed by Design. For Filter Notches £ 60 Hz
For Filter Notch = 100 Hz
For Filter Notch = 250 Hz
For Filter Notch = 500 Hz
For Filter Notch = 1 kHz
Depends on Filter Cutoffs and Selected Gain
Filter Notches £ 60 Hz
Typically ± 0.0003%
Excluding Reference
Excluding Reference. For Gains of 1, 2
Excluding Reference. For Gains of 4, 8, 16, 32, 64, 128
mV/∞C typ
mV/∞C typ
For Gains of 1, 2
For Gains of 4, 8, 16, 32, 64, 128
mV/∞C typ
mV/∞C typ
ppm/∞C typ
% FSR max
% FSR max
mV/∞C typ
mV/∞C typ
For Gains of 1, 2
For Gains of 4, 8, 16, 32, 64, 128
% FSR max
% FSR max
Excluding Reference
Typically ± 0.0006%
Excluding Reference. For Gains of 1, 2
Excluding Reference. For Gains of 4, 8, 16, 32, 64, 128
100
100
10
1
20
dB min
dB min
pA max
nA max
pF max
For Filter Notches of 10 Hz, 25 Hz, 50 Hz, ± 0.02 ¥ fNOTCH
For Filter Notches of 10 Hz, 30 Hz, 60 Hz, ± 0.02 ¥ fNOTCH
100
90
150
150
VSS to AVDD
dB min
dB min
dB min
dB min
V min to V max
At DC and AVDD = 5 V
At DC and AVDD = 10 V
For Filter Notches of 10 Hz, 25 Hz, 50 Hz, ±0.02 ¥ fNOTCH
For Filter Notches of 10 Hz, 30 Hz, 60 Hz, ±0.02 ¥ fNOTCH
0 to +VREF10
± VREF
See Table III
2.5
1.5
max
max
mV max
mV/∞C typ
Removed by System Calibrations but not by Self-Calibration
+2.5 to +5
V min to V max
For Specified Performance. Part Is Functional with
Lower VREF Voltages
For Normal Operation. Depends on Gain Selected
Unipolar Input Range (B/U Bit of Control Register = 1)
Bipolar Input Range (B/U Bit of Control Register = 0)
fCLK IN/256
2.5
±1
20
30
1
1.5
1
V nom
% max
ppm/∞C typ
mV typ
mV/V max
mV/mA max
mA max
Peak-to-Peak Noise. 0.1 Hz to 10 Hz Bandwidth
Maximum Load Current 1 mA
NOTES
1
Temperature range is as follows: A Version = – 40∞C to +85∞C; S Version = –55∞C to +125∞C. See also Note 16.
2
Applies after calibration at the temperature of interest.
3
Positive full-scale error applies to both unipolar and bipolar input ranges.
4
These errors will be of the order of the output noise of the part, as shown in Table I, after system calibration. These errors will be 20 mV typical after self-calibration
or background calibration.
5
Recalibration at any temperature or use of the background calibration mode will remove these drift errors.
6
These numbers are guaranteed by design and/or characterization.
7
This common-mode voltage range is allowed, provided the input voltage on AIN(+) and AIN(–) does not exceed AVDD + 30 mV and V SS – 30 mV.
8
The analog inputs present a very high impedance dynamic load that varies with clock frequency and input sample rate. The maximum recommended source resistance depends on the selected gain (see Tables IV and V).
9
The analog input voltage range on the AIN1(+) input is given here with respect to the voltage on the AIN1(–) input. The input voltage range on the AIN2 input is
with respect to AGND. The absolute voltage on the analog inputs should not go more positive than AVDD + 30 mV, or more negative than VSS – 30 mV.
10
VREF = REF IN(+) – REF IN(–).
11
The reference input voltage range may be restricted by the input voltage range requirement on the VBIAS input.
–2–
REV. G
AD7711
Parameter
A, S Versions1
Unit
AVDD – 0.85 ¥ VREF
or AVDD – 3.5
V max
Conditions/Comments
12
VBIAS INPUT
Input Voltage Range
or AVDD – 2.1
VSS + 0.85 ¥ VREF
or VSS + 3
V max
or VSS + 2.1
65 to 85
V min
dB typ
± 10
mA max
0.8
2.0
V max
V min
0.8
3.5
V max
V min
LOGIC OUTPUTS
VOL, Output Low Voltage
VOH, Output High Voltage
Floating State Leakage Current
Floating State Output Capacitance13
0.4
4.0
± 10
9
V max
V min
mA max
pF typ
TRANSDUCER BURNOUT
Current
Initial Tolerance @ 25∞C
Drift
4.5
± 10
0.1
mA nom
% typ
%/∞C typ
RTD EXCITATION CURRENTS (RTD1, RTD2)
Output Current
Initial Tolerance @ 25∞C
Drift
Initial Matching @ 25∞C
Drift Matching
Line Regulation (AVDD)
Load Regulation
Output Compliance
200
± 20
20
±1
3
200
200
AVDD – 2
mA nom
% max
ppm/∞C typ
% max
ppm/∞C typ
nA/V max
nA/V max
V max
(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
VBIAS Rejection
LOGIC INPUTS
Input Current
All Inputs except MCLK IN
VINL, Input Low Voltage
VINH, Input High Voltage
MCLK IN Only
VINL, Input Low Voltage
VINH, Input High Voltage
SYSTEM CALIBRATION
Positive Full-Scale Calibration Limit14
Negative Full-Scale Calibration Limit14
Offset Calibration Limit15
Input Span15
V min
See VBIAS Input Section
Whichever Is Smaller; +5 V/–5 V or +10 V/0 V
Nominal AVDD/VSS
Whichever Is Smaller; +5 V/0 V Nominal AVDD/VSS
See VBIAS Input Section
Whichever Is Greater; +5 V/–5 V or +10 V/0 V
Nominal AVDD/VSS
Whichever Is Greater; +5 V/0 V Nominal AVDD/VSS
Increasing with Gain
ISINK = 1.6 mA
ISOURCE = 100 mA
Matching between RTD1 and RTD2 Currents
Matching between RTD1 and RTD2 Current Drift
AVDD = 5 V
GAIN Is the Selected PGA Gain (between 1 and 128)
GAIN Is the Selected PGA Gain (between 1 and 128)
GAIN Is the Selected PGA Gain (between 1 and 128)
GAIN Is the Selected PGA Gain (between 1 and 128)
GAIN Is the Selected PGA Gain (between 1 and 128)
NOTES
12
The AD7711 is tested with the following V BIAS voltages. With AVDD = 5 V and VSS = 0 V, VBIAS = 2.5 V; with AV DD = 10 V and VSS = 0 V, VBIAS = 5 V, and
with AVDD = 5 V and VSS = –5 V, VBIAS = 0 V.
13
Guaranteed by design, not production tested.
14
After calibration, if the analog input exceeds positive full scale, the converter outputs all 1s. If the analog input is less than negative full scale, the device outputs all 0s.
15
These calibration and span limits apply, provided the absolute voltage on the analog inputs does not exceed AVDD + 30 mV or go more negative than V SS – 30 mV.
The offset calibration limit applies to both the unipolar zero point and the bipolar zero point.
REV. G
–3–
AD7711–SPECIFICATIONS
Parameter
POWER REQUIREMENTS
Power Supply Voltages
AVDD Voltage16
DVDD Voltage17
AVDD – VSS Voltage
Power Supply Currents
AVDD Current
DVDD Current
VSS Current
Power Supply Rejection18
Positive Supply (AVDD and DVDD)
Negative Supply (VSS)
Power Dissipation
Normal Mode
Standby (Power-Down) Dissipation
A, S Versions1
Unit
Conditions/Comments
5 to 10
5
10.5
V nom
V nom
V max
± 5% for Specified Performance
± 5% for Specified Performance
For Specified Performance
4
4.5
1.5
mA max
mA max
mA max
See Note 19
90
dB typ
dB typ
45
52.5
15
mW max
mW max
mW max
VSS = –5 V
Rejection w.r.t. AGND; Assumes VBIAS Is Fixed
AVDD = DVDD = +5 V, VSS = 0 V; Typically 25 mW
AVDD = DVDD = +5 V, VSS = –5 V; Typically 30 mW
AVDD = DVDD = +5 V, VSS = 0 V or –5 V; Typically 7 mW
NOTES
16
The AD7711 is specified with a 10 MHz clock for AV DD voltages of 5 V ± 5%. It is specified with an 8 MHz clock for AV DD voltages greater than 5.25 V and less
than 10.5 V. Operating with AV DD voltages in the range 5.25 V to 10.5 V is only guaranteed over the 0∞C to 70∞C temperature range.
17
The ± 5% tolerance on the DV DD input is allowed provided DV DD does not exceed AVDD by more than 0.3 V.
18
Measured at dc and applies in the selected pass band. PSRR at 50 Hz will exceed 120 dB with filter notches of 10 Hz, 25 Hz, or 50 Hz. PSRR at 60 Hz will exceed
120 dB with filter notches of 10 Hz, 30 Hz, or 60 Hz.
19
PSRR depends on gain: Gain of 1 = 70 dB typ; Gain of 2: 75 dB typ; Gain of 4 = 80 dB typ; Gains of 8 to 128 = 85 dB typ. These numbers can be improved (to
95 dB typ) by deriving the V BIAS voltage (via Zener diode or reference) from the AV DD supply.
Specifications subject to change without notice.
ABSOLUTE MAXIMUM RATINGS*
(TA = 25∞C, unless otherwise noted.)
AVDD to DVDD . . . . . . . . . . . . . . . . . . . . . . . –0.3 V to +12 V
AVDD to VSS . . . . . . . . . . . . . . . . . . . . . . . . . . –0.3 V to +12 V
AVDD to AGND . . . . . . . . . . . . . . . . . . . . . . –0.3 V to +12 V
AVDD to DGND . . . . . . . . . . . . . . . . . . . . . . –0.3 V to +12 V
DVDD to AGND . . . . . . . . . . . . . . . . . . . . . . . –0.3 V to +6 V
DVDD to DGND . . . . . . . . . . . . . . . . . . . . . . . –0.3 V to +6 V
VSS to AGND . . . . . . . . . . . . . . . . . . . . . . . . . +0.3 V to –6 V
VSS to DGND . . . . . . . . . . . . . . . . . . . . . . . . . +0.3 V to –6 V
Analog Input Voltage to AGND
. . . . . . . . . . . . . . . . . . . . . . . . . VSS – 0.3 V to AVDD + 0.3 V
Reference Input Voltage to AGND
. . . . . . . . . . . . . . . . . . . . . . . . . VSS – 0.3 V to AVDD + 0.3 V
REF OUT to AGND . . . . . . . . . . . . . . . . . . . . –0.3 V to AVDD
Digital Input Voltage to DGND . . . . . –0.3 V to AVDD + 0.3 V
Digital Output Voltage to DGND . . . –0.3 V to DVDD + 0.3 V
Operating Temperature Range
Commercial (A Version) . . . . . . . . . . . . . . . . –40∞C to +85∞C
Extended (S Version) . . . . . . . . . . . . . . . . . –55∞C to +125∞C
Storage Temperature Range . . . . . . . . . . . . . –65∞C to +150∞C
Lead Temperature (Soldering, 10 secs) . . . . . . . . . . . . . 300∞C
Power Dissipation (Any Package) to 75∞C . . . . . . . . . . 450 mW
*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 listed in the operational
sections of the specification is not implied. Exposure to absolute maximum rating
conditions for extended periods may affect device reliability.
ORDERING GUIDE
Model
Temperature Range
Package Option*
AD7711AN
AD7711AR
AD7711AR-REEL
AD7711AR-REEL7
AD7711AQ
AD7711SQ
EVAL-AD7711EB
–40∞C to +85∞C
–40∞C to +85∞C
–40∞C to +85∞C
–40∞C to +85∞C
–40∞C to +85∞C
–55∞C to +125∞C
Evaluation Board
N-24
R-24
R-24
R-24
Q-24
Q-24
*N = Plastic DIP, Q = CERDIP, R = SOIC.
CAUTION
ESD (electrostatic discharge) sensitive device. Electrostatic charges as high as 4000 V readily
accumulate on the human body and test equipment and can discharge without detection. Although the
AD7711 features proprietary ESD protection circuitry, permanent damage may occur on devices
subjected to high energy electrostatic discharges. Therefore, proper ESD precautions are recommended
to avoid performance degradation or loss of functionality.
–4–
REV. G
AD7711
3
1, 2 (DVDD = +5 V ⴞ 5%; AVDD = +5 V or +10 V ⴞ 5%; VSS = 0 V or –5 V ⴞ 10%; AGND = DGND =
TIMING CHARACTERISTICS
0 V; fCLK IN = 10 MHz; Input Logic 0 = 0 V, Logic 1 = DVDD, unless otherwise noted.)
Parameter
Limit at TMIN, TMAX
(A, S Versions)
Unit
Conditions/Comments
fCLK IN4, 5
400
kHz min
Master Clock Frequency: Crystal Oscillator or Externally
Supplied for Specified Performance
10
0.4 × tCLK IN
0.4 × tCLK IN
50
50
1000
MHz max
ns min
ns min
ns max
ns max
ns min
0
0
2 × tCLK IN
0
4 × tCLK IN + 20
4 × tCLK IN + 20
tCLK IN/2
tCLK IN/2 + 30
tCLK IN/2
3 × tCLK IN/2
50
0
4 × tCLK IN + 20
4 × tCLK IN
0
10
ns min
ns min
ns min
ns min
ns max
ns max
ns min
ns max
ns nom
ns nom
ns min
ns min
ns max
ns min
ns min
ns min
tCLK IN LO
tCLK IN HI
tr6
tf 6
t1
Self-Clocking Mode
t2
t3
t4
t5
t6
t7 7
t8 7
t9
t10
t14
t15
t16
t17
t18
t19
Master Clock Input Low Time; tCLK IN = 1/fCLK IN
Master Clock Input High Time
Digital Output Rise Time. Typically 20 ns
Digital Output Fall Time. Typically 20 ns
SYNC Pulse Width
DRDY to RFS Setup Time
DRDY to RFS Hold Time
A0 to RFS Setup Time
A0 to RFS Hold Time
RFS Low to SCLK Falling Edge
Data Access Time (RFS Low to Data Valid)
SCLK Falling Edge to Data Valid Delay
SCLK High Pulse Width
SCLK Low Pulse Width
A0 to TFS Setup Time
A0 to TFS Hold Time
TFS to SCLK Falling Edge Delay Time
TFS to SCLK Falling Edge Hold Time
Data Valid to SCLK Setup Time
Data Valid to SCLK Hold Time
NOTES
1
Guaranteed by design, not production tested. All input signals are specified with tr = tf = 5 ns (10% to 90% of 5 V) and timed from a voltage level of 1.6 V.
2
See Figures 10 to 13.
3
The AD7711 is specified with a 10 MHz clock for AV DD voltages of 5 V ± 5%. It is specified with an 8 MHz clock for AV DD voltages greater than 5.25 V and less
than 10.5 V.
4
CLK IN duty cycle range is 45% to 55%. CLK IN must be supplied whenever the AD7711 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.
5
The AD7711 is production tested with f CLK IN at 10 MHz (8 MHz for AV DD > 5.25 V). It is guaranteed by characterization to operate at 400 kHz.
6
Specified using 10% and 90% points on waveform of interest.
7
These numbers are measured with the load circuit of Figure 1 and defined as the time required for the output to cross 0.8 V or 2.4 V.
REV.G
–5–
2
AD7711
TIMING CHARACTERISTICS
Parameter
External Clocking Mode
fSCLK
t20
t21
t22
t23
t247
t257
t26
t27
t28
t298
t30
t318
t32
t33
t34
t35
t36
Limit at TMIN, TMAX
(A, S Versions)
Unit
Conditions/Comments
fCLK IN/5
0
0
2 ¥ tCLK IN
0
4 ¥ tCLK IN
10
2 ¥ tCLK IN + 20
2 ¥ tCLK IN
2 ¥ tCLK IN
tCLK IN + 10
10
tCLK IN + 10
10
5 ¥ tCLK IN/2 + 50
0
0
4 ¥ tCLK IN
2 ¥ tCLK IN – SCLK High
30
MHz max
ns min
ns min
ns min
ns min
ns max
ns min
ns max
ns min
ns min
ns max
ns min
ns max
ns min
ns max
ns min
ns min
ns min
ns min
ns min
Serial Clock Input Frequency
DRDY to RFS Setup Time
DRDY to RFS Hold Time
A0 to RFS Setup Time
A0 to RFS Hold Time
Data Access Time (RFS Low to Data Valid)
SCLK Falling Edge to Data Valid Delay
SCLK High Pulse Width
SCLK Low Pulse Width
SCLK Falling Edge to DRDY High
SCLK to Data Valid Hold Time
RFS/TFS to SCLK Falling Edge Hold Time
RFS to Data Valid Hold Time
A0 to TFS Setup Time
A0 to TFS Hold Time
SCLK Falling Edge to TFS Hold Time
Data Valid to SCLK Setup Time
Data Valid to SCLK Hold Time
NOTES
8
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 100 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.
Specifications subject to change without notice.
PIN CONFIGURATION
DIP AND SOIC
1.6mA
TO OUTPUT
PIN
+2.1V
100pF
200␮A
Figure 1. Load Circuit for Access Time and Bus
Relinquish Time
–6–
SCLK
1
24
DGND
MCLK IN
2
23
DVDD
MCLK OUT
3
22
SDATA
A0
4
21
DRDY
20
RFS
SYNC
5
MODE
6
AIN1(+)
7
AIN1(–)
8
17
AIN2
RTD1
9
16
REF OUT
RTD2 10
15
REF IN(+)
VSS 11
14
REF IN(–)
AVDD 12
13
VBIAS
AD7711
TFS
TOP VIEW
(Not to Scale) 18 AGND
19
REV. G
AD7711
PIN FUNCTION DESCRIPTIONS
Pin Mnemonic
1
SCLK
Function
Serial Clock. Logic input/output, depending on the status of the MODE pin. When MODE is high, the
device is in its self-clocking mode, and the SCLK pin provides a serial clock output. This SCLK becomes
active when RFS or TFS goes low, and goes high impedance when either RFS or TFS returns high or when
the device has completed transmission of an output word. When MODE is low, the device is in its external
clocking mode and the SCLK pin acts as an input. This input 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 AD7711 in smaller batches of data.
2
MCLK IN
Master Clock Signal for the Device. This can be provided in the form of a crystal or external clock. A crystal 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 clock input frequency is nominally 10 MHz.
3
MCLK OUT
When the master clock for the device is a crystal, the crystal is connected between MCLK IN and MCLK OUT.
4
A0
Address Input. With this input low, reading and writing to the device is to the control register. With this input
high, access is to either the data register or the calibration registers.
5
SYNC
Logic Input. Allows for synchronization of the digital filters when using a number of AD7711s. It resets
the nodes of the digital filter.
6
MODE
Logic Input. When this pin is high, the device is in its self-clocking mode; with this pin low, the device is in its
external clocking mode.
7
AIN1(+)
Analog Input Channel 1. Positive input of the programmable gain differential analog input. The AIN1(+) input
is connected to an output current source that can be used to check that an external transducer has burned out
or gone open circuit. This output current source can be turned on/off via the control register.
8
AIN1(–)
Analog Input Channel 1. Negative input of the programmable gain differential analog input.
9
RTD1
Constant Current Output. A nominal 200 mA constant current is provided at this pin; this current can be
used as the excitation current for RTDs. This current can be turned on or off via the control register.
10
RTD2
Constant Current Output. A nominal 200 mA constant current is provided at this pin; this current can be
used as the excitation current for RTDs. This current can be turned on or off via the control register, and
can be used to eliminate lead resistance errors in 3-wire RTD configurations.
11
VSS
Analog Negative Supply, 0 V to –5 V. Tied to AGND for single-supply operation. The input voltage on AIN1
or AIN2 should not go > 30 mV negative w.r.t. VSS for correct operation of the device.
12
AVDD
Analog Positive Supply Voltage, 5 V to 10 V.
13
VBIAS
Input Bias Voltage. This input voltage should be set such that VBIAS + 0.85 ¥ VREF < AVDD and VBIAS – 0.85
¥ VREF > VSS where VREF is REF IN(+) – REF IN(–). Ideally, this should be tied halfway between AVDD
and VSS. Thus with AVDD = 5 V and VSS = 0 V, it can be tied to REF OUT; with AVDD = +5 V and
VSS = –5 V, it can be tied to AGND; with AVDD = 10 V, it can be tied to 5 V.
14
REF IN(–)
Reference Input. The REF IN(–) can lie anywhere between AVDD and VSS provided REF IN(+) is greater
than REF IN(–).
15
REF IN(+)
Reference Input. The reference input is differential provided REF IN(+) is greater than REF IN(–).
REF IN(+) can lie anywhere between AVDD and VSS.
16
REF OUT
Reference Output. The internal 2.5 V reference is provided at this pin. This is a single-ended output
that is referred to AGND. It is a buffered output capable of providing 1 mA to an external load.
17
AIN2
Analog Input Channel 2. Single-ended programmable gain analog input.
18
AGND
Ground Reference Point for Analog Circuitry.
19
TFS
Transmit Frame Synchronization. Active low logic input used to write serial data to the device with serial
data expected after the falling edge of this pulse. In the self-clocking mode, the serial clock becomes active
after TFS goes low. During a write operation to the AD7711, the SDATA line should not return to high
impedance until after TFS returns high.
REV.G
–7–
2
AD7711
Pin Mnemonic
Function
20
RFS
Receive Frame Synchronization. Active low logic input used to access serial data from the device. In the
self-clocking mode, the SCLK and SDATA lines both become active after RFS goes low. In the external
clocking mode, the SDATA line becomes active after RFS goes low.
21
DRDY
Logic Output. A falling edge indicates that a new output word is available for transmission. The DRDY pin
will return high upon completion of transmission of a full output word. DRDY is also used to indicate
when the AD7711 has completed its on-chip calibration sequence.
22
SDATA
Serial Data. Input/output with serial data being written to either the control register or the calibration
registers and serial data being accessed from the control register, calibration registers, or the data register.
During an output data read operation, serial data becomes active after RFS goes low (provided DRDY is
low). During a write operation, valid serial data is expected on the rising edges of SCLK when TFS is low.
The output data coding is natural binary for unipolar inputs and offset binary for bipolar inputs.
23
DVDD
Digital Supply Voltage, 5 V. DVDD should not exceed AVDD by more than 0.3 V in normal operation.
24
DGND
Ground Reference Point for Digital Circuitry.
TERMINOLOGY
Positive Full-Scale Overrange
Intergral Nonlinearity
Positive full-scale overrange is the amount of overhead available to handle input voltages on the AIN1(+) input greater
than AIN1(–) + V REF/GAIN or on the AIN2 input greater
than + V REF /GAIN (for example, noise peaks or excess
voltages due to system gain errors in system calibration routines) without introducing errors due to overloading the analog
modulator or to overflowing the digital filter.
This is the maximum deviation of any code from a straight line
passing through the endpoints of the transfer function. The endpoints of the transfer function are zero-scale (not to be confused
with bipolar zero), a point 0.5 LSB below the first code transition (000 . . . 000 to 000 . . . 001) and full scale, a point 0.5 LSB
above the last code transition (111 . . . 110 to 111 . . . 111). The
error is expressed as a percentage of full scale.
Negative Full-Scale Overrange
Positive Full-Scale Error
Positive full-scale error is the deviation of the last code transition (111 . . . 110 to 111 . . . 111) from the ideal input full-scale
voltage. For AIN1(+), the ideal full-scale input voltage is
(AIN1(–) + VREF/GAIN – 3/2 LSBs); for AIN2, the ideal fullscale input voltage is VREF/GAIN – 3/2 LSBs. It applies to both
unipolar and bipolar analog input ranges.
This is the amount of overhead available to handle voltages on
AIN1(+) below AIN1(–) – VREF/GAIN or on AIN2 below
–VREF/GAIN without overloading the analog modulator or overflowing the digital filter. Note that the analog input will accept
negative voltage peaks on AIN1(+) even in the unipolar mode
provided that AIN1(+) is greater than AIN1(–) and greater than
VSS – 30 mV.
Unipolar Offset Error
Offset Calibration Range
Unipolar offset error is the deviation of the first code transition
from the ideal voltage. For AIN1(+), the ideal input voltage is
(AIN1(–) + 0.5 LSB); for AIN2, the ideal input is 0.5 LSB
when operating in the unipolar mode.
In the system calibration modes, the AD7711 calibrates its
offset with respect to the analog input. The offset calibration
range specification defines the range of voltages that the AD7711
can accept and still calibrate offset accurately.
Bipolar Zero Error
Full-Scale Calibration Range
This is the deviation of the midscale transition (0111 . . . 111
to 1000 . . . 000) from the ideal input voltage. For AIN1(+), the
ideal input voltage is (AIN1(–) – 0.5 LSB); for AIN2, the ideal
input is – 0.5 LSB when operating in the bipolar mode.
This is the range of voltages that the AD7711 can accept in the
system calibration mode and still calibrate full-scale correctly.
Input Span
In system calibration schemes, two voltages applied in sequence
to the AD7711’s analog input define the analog input range.
The input span specification defines the minimum and maximum input voltages from zero- to full-scale that the AD7711
can accept and still calibrate gain accurately.
Bipolar Negative Full-Scale Error
This is the deviation of the first code transition from the ideal
input voltage. For (AIN1(+), the ideal input voltage is (AIN1(–)
– VREF/GAIN + 0.5 LSB); for AIN2 the ideal input is – VREF/GAIN
+ 0.5 LSB when operating in the bipolar mode.
–8–
REV. G
AD7711
CONTROL REGISTER (24 BITS)
A write to the device with the A0 input low writes data to the control register. A read to the device with the A0 input low accesses the
contents of the control register. The control register is 24 bits wide; 24 bits of data must be written to the registers or the data will not
be loaded. In other words, it is not possible to write just the first 12-bits of data into the control register. If more than 24 clock pulses
are provided before TFS returns high, then all clock pulses after the 24th clock pulse are ignored. Similarly, a read operation from the
control register should access 24 bits of data.
MSB
MD2
MD1
MD0
G2
G1
G0
CH
PD
WL
RO
BO
B/U
FS11
FS10
FS9
FS8
FS7
FS6
FS5
FS4
FS3
FS2
FS1
FS0
LSB
Operating Mode
MD2
MD1
MD0
Operating Mode
0
0
0
Normal Mode. This is the normal mode of operation where a read to the device with A0 high accesses
data from the data register. This is the default condition of these bits after the internal power-on reset.
0
0
1
Activate Self-Calibration. This activates self-calibration on the channel selected by CH. This is a one-step
calibration sequence, and when complete, the part returns to normal mode (with MD2, MD1, MD0 of
the control register returning to 0, 0, 0). The DRDY output indicates when this self-calibration is complete.
For this calibration type, the zero-scale calibration is done internally on shorted (zeroed) inputs, and the
full-scale calibration is done internally on VREF.
0
1
0
Activate System Calibration. This activates system calibration on the channel selected by CH. This is a
two-step calibration sequence, with the zero-scale calibration done first on the selected input channel and
DRDY indicating when this zero-scale calibration is complete. The part returns to normal mode at the
end of this first step in the two-step sequence.
0
1
1
Activate System Calibration. This is the second step of the system calibration sequence with full-scale
calibration being performed on the selected input channel. Once again, DRDY indicates when the fullscale calibration is complete. When this calibration is complete, the part returns to normal mode.
1
0
0
Activate System Offset Calibration. This activates system offset calibration on the channel selected by
CH. This is a one-step calibration sequence and, when complete, the part returns to normal mode with
DRDY indicating when this system offset calibration is complete. For this calibration type, the zero-scale
calibration is done on the selected input channel, and the full-scale calibration is done internally on VREF.
1
0
1
Activate Background Calibration. This activates background calibration on the channel selected by CH. If
the background calibration mode is on, the AD7711 provides continuous self-calibration of the
reference and shorted (zeroed) inputs. This calibration takes place as part of the conversion sequence,
extending the conversion time and reducing the word rate by a factor of 6. The major advantage is that
the user does not have to recalibrate the device when there is a change in the ambient temperature. In this
mode, the shorted (zeroed) inputs and VREF, as well as the analog input voltage, are continuously monitored and the calibration registers of the device are automatically updated.
1
1
0
Read/Write Zero-Scale Calibration Coefficients. A read to the device with A0 high accesses the contents
of the zero-scale calibration coefficients of the channel selected by CH. A write to the device with A0 high
writes data to the zero-scale calibration coefficients of the channel selected by CH. The word length for
reading and writing these coefficients is 24 bits, regardless of the status of the WL bit of the control
register. Therefore, 24 bits of data must be written to the calibration register or the new data will not be
transferred to the calibration register.
1
1
1
Read/Write Full-Scale Calibration Coefficients. A read to the device with A0 high accesses the contents of
the full-scale calibration coefficients of the channel selected by CH. A write to the device with A0 high
writes data to the full-scale calibration coefficients of the channel selected by CH. The word length for
reading and writing these coefficients is 24 bits, regardless of the status of the WL bit of the control
register. Therefore, 24 bits of data must be written to the calibration register, or the new data will not be
transferred to the calibration register.
REV.G
–9–
2
AD7711
PGA Gain
G2
Gl
G0
0
0
0
0
1
1
1
1
0
1
0
1
0
1
0
1
0
0
1
1
0
0
1
1
Gain
1
2
4
8
16
32
64
128
(Default Condition after the Internal Power-On Reset)
CHANNEL SELECTION
CH
0
1
Channel
AIN1
AIN2
(Default Condition after Internal Power-On Reset)
Power-Down
PD
0
1
Normal Operation
Power-Down
(Default Condition after Internal Power-On Reset)
Word Length
WL
Output Word Length
0
1
16-bit
24-bit
(Default Condition after Internal Power-On Reset)
RTD Excitation Current
IO
0
1
Off
On
(Default Condition after Internal Power-On Reset)
Burnout Current
BO
0
1
Off
On
(Default Condition after Internal Power-On Reset)
Bipolar/Unipolar Selection (Both Inputs)
B/U
0
1
Bipolar
Unipolar
(Default Condition after Internal Power-On Reset)
FILTER SELECTION (FS11–FS0)
The on-chip digital filter provides a sinc3(or (sinx/x)3) filter
response. The 12 bits of data programmed into these bits determine the filter cutoff frequency, the position of the first notch of
the filter and the data rate for the part. In association with the
gain selection, it also determines the output noise (and therefore
the effective resolution) of the device.
The first notch of the filter occurs at a frequency determined by
the relationship: filter first notch frequency = (fCLK IN/512)/code
where code is the decimal equivalent of the code in bits FS0 to
FS11 and is in the range 19 to 2,000. With the nominal fCLK IN
of 10 MHz, this results in a first notch frequency range from
9.76 Hz to 1.028 kHz. To ensure correct operation of the
AD7711, the value of the code loaded to these bits must be
within this range. Failure to do this will result in unspecified
operation of the device.
Changing the filter notch frequency, as well as the selected gain,
impacts resolution. Tables I and II and Figure 2 show the effect
of the filter notch frequency and gain on the effective resolution
of the AD7711. 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 rate
or every 20 ms. If the first notch is at 1 kHz, a new word is available every 1 ms.
The settling time of the filter to a full-scale step input change
is worst case 4 ¥ 1/(output data rate). This settling time is to
100% of the final value. For example, with the first filter notch
at 50 Hz, the settling time of the filter to a full-scale step input
change is 80 ms max. If the first notch is at 1 kHz, the settling
time of the filter to a full-scale input step is 4 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 SYNC low, the
settling time is 3 ¥ 1/(output data rate). If a change of channels
takes place, the settling time is 3 ¥ 1/(output data rate) regardless of the SYNC input.
The –3 dB frequency is determined by the programmed first
notch frequency according to the relationship:
filter –3 dB frequency = 0.262 ¥ first notch frequency.
–10–
REV. G
AD7711
Tables I and II show the output rms noise for some typical
notch and –3 dB frequencies. The numbers given are for the
bipolar input ranges with a VREF of 2.5 V. These numbers are
typical and are generated with an analog input voltage of 0 V.
The output noise from the part comes from two sources. The
first is the electrical noise in the semiconductor devices used in
the implementation of the modulator (device noise). The second
occurs when the analog input signal is converted into the digital
domain adding quantization noise. The device noise is at a low
level and is largely independent of frequency. The quantization
noise starts at an even lower level but rises rapidly with increasing frequency to become the dominant noise source. Consequently, lower filter notch settings (below 60 Hz approximately)
tend to be device-noise dominated while higher notch settings
are dominated by quantization noise. Changing the filter notch
and cutoff frequency in the quantization-noise dominated region
results in a more dramatic improvement in noise performance
than it does in the device-noise dominated region as shown in
Table I. Furthermore, quantization noise is added after the PGA,
so effective resolution is independent of gain for the higher filter
notch frequencies. Meanwhile, device noise is added in the PGA
and, therefore, effective resolution suffers a little at high gains
for lower notch frequencies.
At the lower filter notch settings (below 60 Hz), the no missing
codes performance of the device is at the 24-bit level. At the higher
settings, more codes will be missed until at the 1 kHz notch setting,
no missing codes performance is guaranteed only to the 12-bit
level. However, since the effective resolution of the part is 10.5 bits
for this filter notch setting, this no missing codes performance
should be more than adequate for all applications.
The effective resolution of the device is defined as the ratio of the
output rms noise to the input full scale. This does not remain
constant with increasing gain or with increasing bandwidth.
Table II is the same as Table I except that the output is expressed
in terms of effective resolution (the magnitude of the rms noise
with respect to 2 ¥ VREF/GAIN, or the input full scale). It is possible to do post filtering on the device to improve the output data
rate for a given –3 dB frequency and also to further reduce the
output noise (see the Digital Filtering section).
Table I. Output Noise vs. Gain and First Notch Frequency
First Notch of
Filter and O/P –3 dB
Data Rate1
Frequency Gain of 1
2
10 Hz
25 Hz2
30 Hz2
50 Hz2
60 Hz2
100 Hz3
250 Hz3
500 Hz3
1 kHz3
2.62 Hz
6.55 Hz
7.86 Hz
13.1 Hz
15.72 Hz
26.2 Hz
65.5 Hz
131 Hz
262 Hz
1.0
1.8
2.5
4.33
5.28
13
130
0.6 ¥ 103
3.1 ¥ 103
Typical Output RMS Noise (␮V)
Gain of 2
Gain of 4
Gain of 8
Gain of 16
Gain of 32
Gain of 64
Gain of 128
0.78
1.1
1.31
2.06
2.36
6.4
75
0.26 ¥ 103
1.6 ¥ 103
0.48
0.63
0.84
1.2
1.33
3.7
25
140
0.7 ¥ 103
0.33
0.50
0.57
0.64
0.87
1.8
12
70
0.29 ¥ 103
0.25
0.44
0.46
0.54
0.63
1.1
7.5
35
180
0.25
0.41
0.43
0.46
0.62
0.9
4
25
120
0.25
0.38
0.4
0.46
0.6
0.65
2.7
15
70
0.25
0.38
0.4
0.46
0.56
0.65
1.7
8
40
NOTES
1
The default condition (after the internal power-on reset) for the first notch of filter is 60 Hz.
2
For these filter notch frequencies, the output rms noise is primarily dominated by device noise, and, as a result, is independent of the value of the reference voltage.
Therefore, increasing the reference voltage will give an increase in the effective resolution of the device (that is, the ratio of the rms noise to the input full scale is
increased because the output rms noise remains constant as the input full scale increases).
3
For these filter notch frequencies, the output rms noise is dominated by quantization noise, and, as a result, is proportional to the value of the reference voltage.
Table II. Effective Resolution vs. Gain and First Notch Frequency
Effective Resolution* (Bits)
First Notch of
Filter and O/P –3 dB
Data Rate
Frequency Gain of 1
Gain of 2
Gain of 4
Gain of 8
Gain of 16
Gain of 32 Gain of 64 Gain of 128
10 Hz
25 Hz
30 Hz
50 Hz
60 Hz
100 Hz
250 Hz
500 Hz
1 kHz
21.5
21
21
20
20
18.5
15
13
10.5
21.5
21
20.5
20
20
18.5
15.5
13
11
21
20
20
20
19.5
18.5
15.5
13
11
20.5
19.5
19.5
19
19
18
15.5
13
11
19.5
18.5
18.5
18.5
18
17.5
15.5
12.5
10.5
2.62 Hz
6.55 Hz
7.86 Hz
13.1 Hz
15.72 Hz
26.2 Hz
65.5 Hz
131 Hz
262 Hz
22.5
21.5
21
20
20
18.5
15
13
10.5
18.5
17.5
17.5
17.5
17
17
15
12.5
10
17.5
16.5
16.5
16.5
16
16
14.5
12.5
10
*Effective resolution is defined as the magnitude of the output rms noise with respect to the input full scale (i.e., 2 ¥ VREF/GAIN). The above table applies for
a VREF of 2.5 V and resolution numbers are rounded to the nearest 0.5 LSB.
REV.G
–11–
2
AD7711
Figure 2 shows similar information to that outlined in Table I.
In these plots, however, the output rms noise is shown for the
full range of available cutoffs frequencies. The numbers given in
these plots are typical values at 25∞C.
10000
GAIN OF 1
GAIN OF 2
OUTPUT NOISE – ␮V
1000
GAIN OF 4
GAIN OF 8
100
10
The basic connection diagram for the part is shown in Figure 3.
This figure shows the AD7711 in the external clocking mode
with both the AVDD and DVDD pins of the AD7711 being driven
from the analog 5 V supply. Some applications have separate
supplies for both AVDD and DVDD, and in some cases, the analog supply exceeds the 5 V digital supply (see the Power Supplies and Grounding section).
1
0.1
10
100
1000
NOTCH FREQUENCY – Hz
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 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 therefore
its –3 dB frequency) can be programmed via an on-chip control register. The programmable range for this first notch
frequency is 9.76 Hz to 1.028 kHz, giving a programmable range
for the –3 dB frequency of 2.58 Hz to 269 Hz.
10000
Figure 2a. Output Noise vs. Gain and Notch
Frequency (Gains of 1 to 8)
ANALOG
+5V SUPPLY
10␮F
0.1␮F
0.1␮F
1000
AVDD
GAIN OF 16
GAIN OF 32
100
OUTPUT NOISE – ␮V
DIFFERENTIAL
ANALOG INPUT
SINGLE-ENDED
ANALOG INPUT
GAIN OF 64
GAIN OF 128
DRDY
AIN2
AD7711
RTD1
RTD2
10
ANALOG GROUND
DIGITAL GROUND
1
AGND
VSS
DGND
REF OUT
REF IN(+)
VBIAS
0.1
10
REF IN(–)
100
1000
NOTCH FREQUENCY – Hz
10000
DATA READY
TFS
TRANSMIT (WRITE)
RFS
RECEIVE (READ)
SDATA
SCLK
A0
SERIAL DATA
SERIAL CLOCK
ADDRESS INPUT
MODE
SYNC
+5V
MCLK OUT
MCLK IN
Figure 3. Basic Connection Diagram
Figure 2b. Output Noise vs. Gain and Notch
Frequency (Gains of 16 to 128)
CIRCUIT DESCRIPTION
The AD7711 is a sigma-delta A/D converter with on-chip digital
filtering for measuring wide dynamic range, low frequency signals
such as those in RTD applications, 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 contains two analog input channels, a programmable
gain differential analog input, and a programmable gain singleended input. The gain range is from 1 to 128 allowing the part
to accept unipolar signals of 0 mV to 20 mV and 0 V to 2.5 V
or bipolar signals in the range ± 20 mV to ± 2.5 V when the
reference input voltage equals 2.5 V. The input signal to the
selected analog input channel is continuously sampled at a
rate determined by the frequency of the master clock, MCLK
IN, and the selected gain (see Table III). A charge-balancing
DVDD
AIN1(+)
AIN1(–)
The AD7711 provides a number of calibration options that can
be programmed via the on-chip control register. A calibration
cycle may be initiated at any time by writing to this control
register. The part can perform self-calibration using the on-chip
calibration microcontroller and SRAM to store calibration
parameters. Other system components may also be included in
the calibration loop to remove offset and gain errors in the input
channel using the system calibration mode. Another option is a
background calibration mode where the part continuously
performs self-calibration and updates the calibration coefficients. Once the part is in this mode, the user does not have to
issue periodic calibration commands to the device or recalibrate
when there is a change in the ambient temperature or power
supply voltage.
The AD7711 gives the user access to the on-chip calibration
registers, allowing the microprocessor to read the device calibration coefficients and also to write its own calibration coefficients
to the part from prestored values in E2PROM. This gives the
–12–
REV. G
AD7711
microprocessor much greater control over the AD7711’s calibration procedure. It also means that the user can verify the
calibration is correct by comparing the coefficients after calibration with prestored values in E2PROM.
low-pass filter or integrator. It also illustrates the derivation of
the alternative name for these devices, charge-balancing ADCs.
DIFFERENTIAL
AMPLIFIER
The AD7711 can be operated in single-supply systems provided
that the analog input voltage does not go more negative than
–30 mV. For larger bipolar signals, a VSS of –5 V is required by
the part. For battery operation, the AD7711 also offers a programmable standby mode that reduces idle power consumption
to typically 7 mW.
–FS
The device consists of a differential amplifier (whose output is
the difference between the analog input and the output of a
1-bit DAC), an integrator, and a comparator. The term chargebalancing comes from the fact that this system is a negative
feedback loop that tries to keep the net charge on the integrator
capacitor at zero, by balancing charge injected by the input
voltage with charge injected by the 1-bit DAC. When the analog
input is zero, the only contribution to the integrator output
comes from the 1-bit DAC. For the net charge on the integrator
capacitor to be zero, the DAC output must spend half its time at
+FS and half its time at –FS. Assuming ideal components, the
duty cycle of the comparator will be 50%.
COMPARATOR
ANALOG
LOW-PASS
FILTER
DIGITAL
FILTER
DAC
DIGITAL
DATA
When a positive analog input is applied, the output of the 1-bit
DAC must spend a larger proportion of the time at +FS, so the
duty cycle of the comparator increases. When a negative input
voltage is applied, the duty cycle decreases.
Figure 4. General Sigma-Delta ADC
The AD7711 uses a second-order sigma-delta modulator and a
digital filter that provides a rolling average of the sampled output. After power-up, or if there is a step change in the input
voltage, there is a settling time that must elapse before valid
data is obtained.
In operation, the analog signal sample is fed to the subtracter,
along with the output of the 1-bit DAC. The filtered difference
signal is fed to the comparator, which samples the difference
signal at a frequency many times that of the analog signal sampling
frequency (oversampling).
Input Sample Rate
Oversampling is fundamental to the operation of sigma-delta
ADCs. Using the quantization noise formula for an ADC,
SNR = (6.02 ¥ number of bits + 1.76) dB,
a 1-bit ADC or comparator yields an SNR of 7.78 dB.
The AD7711 samples the input signal at a frequency of 39 kHz or
greater (see Table III). As a result, the quantization noise is
spread over a much wider frequency than that of the band of
interest. The noise in the band of interest is reduced still further
by analog filtering in the modulator loop, which shapes the
quantization noise spectrum to move most of the noise energy to
frequencies outside the bandwidth of interest. The noise performance is thus improved from this 1-bit level to the performance
outlined in Tables I and II and in Figure 2.
The output of the comparator provides the digital input for the
1-bit DAC, so that the system functions as a negative feedback
loop that tries to minimize the difference signal. The digital data
that represents the analog input voltage is contained in the duty
cycle of the pulse train appearing at the output of the comparator. It can be retrieved as a parallel binary data-word using a
digital filter.
Sigma-delta ADCs are generally described by the order of the
analog low-pass filter. A simple example of a first-order sigmadelta ADC is shown in Figure 5. This contains only a first-order
REV.G
2
DAC
Figure 5. Basic Charge-Balancing ADC
The general block diagram of a sigma-delta ADC is shown in
Figure 4. It contains the following elements:
∑ A sample-hold amplifier.
∑ A differential amplifier or subtracter.
∑ An analog low-pass filter.
∑ A 1-bit A/D converter (comparator).
∑ A 1-bit DAC.
∑ A digital low-pass filter.
–
兰
+FS
THEORY OF OPERATION
S/H AMP
+
INTEGRATOR
COMPARATOR
VIN
The modulator sample frequency for the device remains at
fCLK IN/512 (19.5 kHz @ fCLK IN = 10 MHz) regardless of the
selected gain. However, gains greater than ¥1 are achieved by a
combination of multiple input samples per modulator cycle and
scaling 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 III). The
effective input impedance is 1/C ¥ fS where C is the input sampling capacitance and fS is the input sample rate.
Table III. Input Sampling Frequency vs. Gain
Gain
Input Sampling Frequency (fS)
1
2
4
8
16
32
64
128
fCLK IN/256 (39 kHz @ fCLK IN = 10 MHz)
2 ¥ fCLK IN/256 (78 kHz @ fCLK IN = 10 MHz)
4 ¥ fCLK IN/256 (156 kHz @ fCLK IN = 10 MHz)
8 ¥ fCLK IN/256 (312 kHz @ fCLK IN = 10 MHz)
8 ¥ fCLK IN/256 (312 kHz @ fCLK IN = 10 MHz)
8 ¥ fCLK IN/256 (312 kHz @ fCLK IN = 10 MHz)
8 ¥ fCLK IN/256 (312 kHz @ fCLK IN = 10 MHz)
8 ¥ fCLK IN/256 (312 kHz @ fCLK IN = 10 MHz)
DIGITAL FILTERING
The AD7711’s digital filter behaves like a similar analog filter,
with a few minor differences.
–13–
AD7711
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.
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 AD7711 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%).
Filter Characteristics
The cutoff frequency of the digital filter is determined by the
value loaded to Bits FS0 to FS11 in the control register. At the
maximum clock frequency of 10 MHz, the minimum cutoff
frequency of the filter is 2.58 Hz while the maximum programmable cutoff frequency is 269 Hz.
Figure 6 shows the filter frequency response for a cutoff frequency of 2.62 Hz, which corresponds to a first filter notch frequency of 10 Hz. This is a (sinx/x)3 response (also called sinc3)
that provides >100 dB of 50 Hz and 60 Hz rejection. Programming a different cutoff frequency via FS0–FS11 does not alter
the profile of the filter response; it changes the frequency of the
notches as outlined in the Control Register section.
0
–20
–40
notch frequency of the filter. Since the output data rate exceeds the Nyquist criterion, the output rate for a given bandwidth will satisfy most application requirements. However,
there may be some applications that 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 AD7711.
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 AD7711
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 2.62 Hz. At a gain of 128, the
output rms noise is 250 nV. This is essentially device noise or
white noise, and since the input is chopped, the noise has a flat
frequency response. By reducing the bandwidth below 2.62 Hz,
the noise in the resultant pass band can be reduced. A reduction
in bandwidth by a factor of 2 results in a ÷2 reduction in the
output rms noise. This additional filtering will result in a longer
settling time.
Antialias Considerations
The digital filter does not provide any rejection at integer multiples of the modulator sample frequency (n ¥ 19.5 kHz, where
n = 1, 2, 3 . . . ). This means that there are frequency bands,
±f3 dB wide (f3 dB is the cutoff frequency selected by FS0 to FS11),
where noise passes unattenuated to the output. However, due to
the AD7711’s high oversampling ratio, these bands occupy only
a small fraction of the spectrum, and most broadband noise is
filtered. In any case, because of the high oversampling ratio a
simple, RC, single-pole filter is generally sufficient to attenuate
the signals in these bands on the analog input and thus provide
adequate antialiasing filtering.
–60
GAIN – dB
–80
–100
–120
–140
–160
–180
–200
–220
–240
0
10
20
30
40
FREQUENCY – Hz
50
60
Figure 6. Frequency Response of AD7711 Filter
Since the AD7711 contains this on-chip, low-pass filtering,
there is a settling time 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
notch frequency chosen for the filter. The output data rate
equates to this filter notch frequency and the settling time of the
filter to a full-scale step input is four times the output data
period. In applications using both input channels, the settling
time of the filter must be allowed to elapse before data from
the second channel is accessed.
If passive components are placed in front of the AD7711, care
must be taken to ensure that the source impedance is low enough
so as not to introduce gain errors in the system. The dc input
impedance for the AD7711 is over 1 GW. The input appears as
a dynamic load that varies with the clock frequency and with the
selected gain (see Figure 7). The input sample rate, as shown
in Table III, determines the time allowed for the analog input
capacitor, CIN, to be charged. External impedances result in a
longer charge time for this capacitor, which may result in gain
errors being introduced on the analog inputs. Table IV shows the
allowable external resistance/capacitance values such that no
gain error to the 16-bit level is introduced while Table V shows
the allowable external resistance/capacitance values such that no
gain error to the 20-bit level is introduced. Both inputs of the
differential input channel (AIN1) look into similar input circuitry.
Post Filtering
The on-chip modulator provides samples at a 19.5 kHz output
rate. The on-chip digital filter decimates these samples to provide
data at an output rate that corresponds to the programmed first
–14–
AD7711
RINT
7k⍀ TYP
AIN
CINT
11.5pF TYP
HIGH
IMPEDANCE
>1G⍀
VBIAS
SWITCHING FREQUENCY DEPENDS
ON fCLKIN AND SELECTED GAIN
Figure 7. Analog Input Impedance
REV. G
AD7711
Table IV. External Series Resistance That Will Not Introduce
16-Bit Gain Error
Gain
0
External Capacitance (pF)
50
100
500
1000
1
2
4
8–128
184 kW
88.6 kW
41.4 kW
17.6 kW
45.3 kW
22.1 kW
10.6 kW
4.8 kW
27.1 kW
13.2 kW
6.3 kW
2.9 kW
7.3 kW
3.6 kW
1.7 kW
790 W
4.1 kW
2.0 kW
970 W
440 W
5000
1.1 kW
560 W
270 W
120 W
Table V. External Series Resistance That Will Not Introduce
20-Bit Gain Error
Gain
0
External Capacitance (pF)
50
100
500
1000
5000
1
2
4
8–128
145 kW
70.5 kW
31.8 kW
13.4 kW
34.5 kW
16.9 kW
8.0 kW
3.6 kW
700 W
350 W
170 W
80 W
20.4 kW
10 kW
4.8 kW
2.2 kW
5.2 kW
2.5 kW
1.2 kW
550 W
2.8 kW
1.4 kW
670 W
300 W
The temperature coefficient of the RTD current sources is
typically 20 ppm/∞C with a typical matching between the temperature coefficients of both current sources of 3 ppm/∞C. For
applications where the absolute value of the temperature coefficient is too large, the following schemes can be used to remove
the drift error.
The conversion result from the AD7711 is ratiometric to the
VREF voltage. Therefore, if the VREF voltage varies with the RTD
temperature coefficient, the temperature drift from the current
source will be removed. For 4-wire RTD applications, the reference voltage can be made ratiometric to RTD current source
by using the second current with a low TC resistor to generate
the reference voltage for the part. In this case, if a 12.5 kW
resistor is used, the 200 mA current source generates 2.5 V across
the resistor. This 2.5 V can be applied to the REF IN(+) input
of the AD7711 and with the REF IN(–) input at ground, it will
supply a VREF of 2.5 V for the part. For 3-wire RTD configurations, the reference voltage for the part is generated by placing a
low TC resistor (12.5 kW for 2.5 V reference) in series with one
of the constant current sources. The RTD current sources can
be driven to within 2 V of AVDD. The reference input of the
AD7711 is differential so the REF IN(+) and REF IN(–) of the
AD7711 are driven from either side of the resistor. Both schemes
ensure that the reference voltage for the part tracks the RTD
current sources over temperature and, thereby, remove the
temperature drift error.
The numbers in Tables IV and V assume a full-scale change
on the analog input. In any case, the error introduced due to
longer charging times is a gain error that can be removed using
the system calibration capabilities of the AD7711, provided
the resultant span is within the limits of the system calibration
techniques.
ANALOG INPUT FUNCTIONS
Analog Input Ranges
Both analog inputs are programmable gain input channels that
can handle either unipolar or bipolar input signals. The AIN1
channel is a differential channel with a common-mode range
from VSS to AVDD, provided the absolute value of the analog
input voltage lies between VSS – 30 mV and AVDD + 30 mV.
The AIN2 input channel is a single-ended input that is referred
to as AGND.
Bipolar/Unipolar Inputs
The two analog inputs on the AD7711 can accept either unipolar or bipolar input voltage ranges. Bipolar or unipolar options
are chosen by programming the B/U bit of the control register.
This programs both channels for either unipolar or bipolar
operation. Programming the part for either type of operation
does not change any of the input signal conditioning; it simply
changes the data output coding, using binary for unipolar inputs
and offset binary for bipolar inputs.
The dc input leakage current is 10 pA maximum at 25∞C (±1 nA
over temperature). This results in a dc offset voltage developed
across the source impedance. However, this dc offset effect can
be compensated for by a combination of the differential input
capability of the part and its system calibration mode.
Burnout Current
For 4-wire RTD applications, one of these excitation currents is
used to provide the excitation current for the RTD; the second
current source can be left unconnected. For 3-wire RTD configurations, the second on-chip current source can be used to
eliminate errors due to voltage drops across lead resistances.
Figures 19 to 21 in the Applications section show some RTD
configurations with the AD7711.
The AIN1(+) input of the AD7711 contains a 4.5 mA current
source that can be turned on/off via the control register. This
current source can be used in checking that a transducer has not
burned out or gone open circuit before attempting to take measurements on that channel. If the current is turned on and
allowed to flow into the transducer and a measurement of the
input voltage on the AIN1 input is taken, it can indicate that the
transducer has burned out or gone open circuit. For normal
operation, this burnout current is turned off by writing a 0 to
the BO bit in the control register.
The AIN1 input channel is differential and, as a result, the
voltage to which the unipolar and bipolar signals are referenced
is the voltage on the AIN1(–) input. For example, if AIN1(–) is
1.25 V and the AD7711 is configured for unipolar operation
with a gain of 1 and a VREF of 2.5 V, the input voltage range on
the AIN1(+) input is 1.25 V to 3.75 V. If AIN1(–) is 1.25 V,
and the AD7711 is configured for bipolar mode with a gain of
1 and a VREF of 2.5 V, the analog input range on the AIN1(+)
input is –1.25 V to +3.75 V. For the AIN2 input, the input
signals are referenced to AGND.
RTD Excitation Current
REFERENCE INPUT/OUTPUT
The AD7711 also contains two matched 200 mA constant current sources that are provided at the RTD1 and RTD2 pins of
the device. These currents can be turned on/off via the control
register. Writing a 1 to the RO bit of the control register enables
these excitation currents.
REV.G
The AD7711 contains a temperature compensated 2.5 V reference
that has an initial tolerance of ± 1%. This reference voltage is
provided at the REF OUT pin, and it can be used as the reference
voltage for the part by connecting the REF OUT pin to the REF
–15–
2
AD7711
IN(+) pin. This REF OUT pin is a single-ended output, referenced to AGND, which is capable of providing up to 1 mA to
an external load. In applications where REF OUT is connected
directly to REF IN(+), REF IN(–) should be tied to AGND to
provide the nominal 2.5 V reference for the AD7711.
The reference inputs of the AD7711, REF IN(+) and REF IN(–),
provide a differential reference input capability. The commonmode range for these differential inputs is from VSS to AVDD.
The nominal differential voltage, VREF (REF IN(+) – REF IN(–)),
is 2.5 V for specified operation, but the reference voltage can go
to 5 V with no degradation in performance if the absolute value
of REF IN(+) and REF IN(–) does not exceed its AVDD and
VSS limits and the VBIAS input voltage range limits are obeyed.
The part is also functional with VREF voltages down to 1 V but with
degraded performance because 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 AD7711.
Both reference inputs provide a high impedance, dynamic load
similar to the analog inputs. The maximum dc input leakage
current is 10 pA (± 1 nA over temperature), and source resistance may result in gain errors on the part. The reference inputs
look like the analog input (see Figure 7). In this case, RINT is
5 kW typ and CINT varies with gain. The input sample rate is
fCLK IN/256 and does not vary with gain. For gains of 1 to 8, CINT
is 20 pF; for a gain of 16, it is 10 pF; for a gain of 32, it is 5 pF; for
a gain of 64, it is 2.5 pF; and for a gain of 128, it is 1.25 pF.
The digital filter of the AD7711 removes noise from the reference input just as it does with the analog input, and the same
limitations apply regarding lack of noise rejection at integer
multiples of the sampling frequency. The output noise performance outlined in Tables I and II assumes a clean reference. If
the reference noise in the bandwidth of interest is excessive, it
can degrade the performance of the AD7711. Using the on-chip
reference as the reference source for the part (connecting
REF OUT to REF IN) results in degraded output noise performance from the AD7711 for portions of the noise table that are
dominated by the device noise. The on-chip reference noise
effect is eliminated in ratiometric applications where the reference is used to provide the excitation voltage for the analog
front end. The connection shown in Figure 8 is recommended
when using the on-chip reference. Recommended reference
voltage sources for the AD7711 include the AD580 and AD680
2.5 V references.
REF OUT
REF IN(+)
For maximum internal headroom, the VBIAS voltage should be
set halfway between AVDD and VSS. The difference between
AVDD and (VBIAS + 0.85 ¥ VREF) determines the amount of
headroom the circuit has at the upper end, while the difference
between VSS and (VBIAS – 0.85 ¥ VREF) determines the amount
of headroom the circuit has at the lower end. When choosing a
VBIAS voltage, ensure that it stays within prescribed limits. For
single 5 V operation, the selected VBIAS voltage must ensure that
VBIAS ± 0.85 ¥ VREF does not exceed AVDD or VSS or that the
VBIAS voltage itself is greater than VSS + 2.1 V and less than
AVDD – 2.1 V. For single 10 V operation or dual ± 5 V operation, the selected VBIAS voltage must ensure that VBIAS ¥ 0.85 ¥
VREF does not exceed AVDD or VSS or that the VBIAS voltage
itself is greater than VSS + 3 V or less than AVDD – 3 V. For
example, with AVDD = 4.75 V, VSS = 0 V, and VREF = 2.5 V, the
allowable range for the VBIAS voltage is 2.125 V to 2.625 V. With
AVDD = 9.5 V, VSS = 0 V, and VREF = 5 V, the range for VBIAS
is 4.25 V to 5.25 V. With AVDD = +4.75 V, VSS = –4.75 V,
and VREF = +2.5 V, the VBIAS range is –2.625 V to +2.625 V.
The VBIAS voltage does have an effect on the AVDD power supply rejection performance of the AD7711. If the VBIAS voltage
tracks the AVDD supply, it improves the power supply rejection
from the AVDD supply line from 80 dB to 95 dB. Using an
external Zener diode connected between the AVDD line and
VBIAS as the source for the VBIAS voltage gives the improvement
in AVDD power supply rejection performance.
USING THE AD7711
SYSTEM DESIGN CONSIDERATIONS
The AD7711 operates differently from successive approximation ADCs or integrating ADCs. Because it samples the signal
continuously, like a tracking ADC, there is no need for a start
convert command. The output register is updated at a rate
determined by the first notch of the filter, and the output can
be read at any time, either synchronously or asynchronously.
Clocking
The AD7711 requires a master clock input, which may be an
external TTL/CMOS compatible clock signal applied to the
MCLK IN pin with the MCLK OUT pin left unconnected.
Alternatively, a crystal of the correct frequency can be connected
between MCLK IN and MCLK OUT, in which case the clock
circuit will function as a crystal-controlled oscillator. For lower
clock frequencies, a ceramic resonator may be used instead of
the crystal. For these lower frequency oscillators, external
capacitors may be required on either the ceramic resonator or
on the crystal.
VBIAS Input
The input sampling frequency, the modulator sampling frequency,
the –3 dB frequency, the output update rate, and the calibration
time are all directly related to the master clock frequency,
fCLK IN. Reducing the master clock frequency by a factor of 2 will
halve the above frequencies and update rate and will double the
calibration time.
The VBIAS input determines at what voltage the internal analog
circuitry is biased. It essentially provides the return path for
analog currents flowing in the modulator and, as such, it should
be driven from a low impedance point to minimize errors.
The current drawn from the DVDD power supply is also directly
related to fCLK IN. Reducing fCLK IN by a factor of 2 will halve the
DVDD current but will not affect the current drawn from the
AVDD power supply.
AD7711
REF IN(–)
Figure 8. REF OUT/REF IN Connection
System Synchronization
If multiple AD7711s are operated from a common master clock,
they can be synchronized to update their output registers simultaneously. A falling edge on the SYNC input resets the filter and
–16–
REV. G
AD7711
places the AD7711 into a consistent, known state. A common
signal to the AD7711s’ SYNC inputs will synchronize their
operation. This would typically be done after each AD7711 has
performed its own calibration or has had calibration coefficients
loaded to it.
The SYNC input can also be used to reset the digital filter in
systems where the turn-on time of the digital power supply
(DVDD) is very long. In such cases, the AD7711 starts operating internally before the DVDD line has reached its minimum
operating level, 4.75 V. With a low DVDD voltage, the
AD7711’s internal digital filter logic does not operate correctly.
Thus, the AD7711 may have clocked itself into an incorrect
operating condition by the time DVDD has reached its correct
level. The digital filter is reset upon issue of a calibration command (whether it is self-calibration, system calibration, or background calibration) to the AD7711. This ensures correct
operation of the AD7711. In systems where the power-on default conditions of the AD7711 are acceptable, and no calibration is performed after power-on, issuing a SYNC pulse to the
AD7711 resets the AD7711’s digital filter logic. An R, C on the
SYNC line, with R, C time constant longer than the DVDD
power-on time, performs the SYNC function.
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 AD7711 achieves excellent
linearity by the use of high quality, on-chip silicon dioxide capacitors, which have a very low capacitance/voltage coefficient. The
device also achieves low input drift through the use of chopper
stabilized techniques in its input stage. To ensure excellent
performance over time and temperature, the AD7711 uses digital
calibration techniques that minimize offset and gain error.
Self-Calibration
In the self-calibration mode with a unipolar input range, the
zero-scale point used in determining the calibration coefficients
is with both inputs shorted (that is, AIN1(+) = AIN1(–) =
VBIAS for AIN1 and AIN2 = VBIAS for AIN2), and the full-scale
point is VREF. The zero-scale coefficient is determined by converting an internal shorted input node. The full-scale coefficient
is determined from the span between this shorted input conversion and a conversion on an internal VREF node. The selfcalibration mode is invoked by writing the appropriate values
(0, 0, 1) to the MD2, MD1, and MD0 bits of the control register. In this calibration mode, the shorted input node is switched
into the modulator first and a conversion is performed; the VREF
node is then switched in and another conversion is performed.
When the calibration sequence is complete, the calibration coefficients updated, and the filter resettled to the analog input voltage,
the DRDY output goes low. The self-calibration procedure takes
into account the selected gain on the PGA.
For bipolar input ranges in the self-calibrating mode, the sequence
is very similar to that just outlined. In this case, the two points
that the AD7711 calibrates are midscale (bipolar zero) and
positive full scale.
System Calibration
Autocalibration
Autocalibration on the AD7711 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. However, if the AD7711 is in background calibration mode, these changes are taken care of automatically (after
the settling time of the filter has been allowed for).
The AD7711 offers self-calibration, system calibration, and
background calibration facilities. For calibration to occur on
the selected channel, the on-chip microcontroller must record
the modulator output for two different input conditions. These
are zero-scale and full-scale points. With these readings, the
microcontroller can calculate the gain slope for the input-tooutput transfer function of the converter. Internally, the part
works with a resolution of 33 bits to determine its conversion
result of either 16 bits or 24 bits.
The AD7711 also provides the facility to write to the on-chip
calibration registers, and in this manner, the span and offset for
the part can be adjusted by the user. The offset calibration register contains a value that is subtracted from all conversion
results, while the full-scale calibration register contains a value
that is multiplied by all conversion results. The offset calibration
coefficient is subtracted from the result prior to the multiplication by the full-scale coefficient. In the first three modes outlined
here, the DRDY line indicates that calibration is complete by
REV.G
going low. If DRDY is low before (or goes low during) the calibration command, it may take up to one modulator cycle 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 of the calibration command is written to
the control register.
System calibration allows the AD7711 to compensate for system
gain and offset errors as well as its own internal errors. System
calibration performs the same slope factor calculations as selfcalibration but uses voltage values presented by the system to
the AIN inputs for the zero- and full-scale points. System
calibration is a two-step process. 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 must
remain stable until the step is complete. System calibration is
initiated by writing the appropriate values (0, 1, 0) to the MD2,
MD1, and MD0 bits of the control register. The DRDY output
from the device signals when the step is complete by going low.
After the zero-scale point is calibrated, the full-scale point is
applied and the second step of the calibration process is initiated
by again writing the appropriate values (0, 1, 1) to MD2, 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. DRDY goes low at the end of this second step to
indicate that the system calibration is complete. 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 and positive full scale.
This two-step system calibration mode offers another feature.
After the sequence has been completed, additional offset or gain
calibrations can be performed by themselves to adjust the zero
reference point or the system gain. This is achieved by performing the first step of the system calibration sequence (by writing
0, 1, 0 to MD2, MD1, MD0). This adjusts the zero-scale or
offset point but does not change the slope factor from that set
during a full system calibration sequence.
–17–
2
AD7711
System calibration can also be used to remove any errors from
an antialiasing filter on the analog input. 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.
System Offset Calibration
System offset calibration is a variation of both the system calibration and self-calibration. In this case, the zero-scale point
for the system is presented to the AIN input of the converter.
System offset calibration is initiated by writing 1, 0, 0 to MD2,
MD1, MD0. The system zero-scale coefficient is determined by
converting the voltage applied to the AIN input, while the fullscale coefficient is determined from the span between this AIN
conversion and a conversion on VREF. The zero-scale point
should be applied to the AIN input for the duration of the calibration sequence. This is a one-step calibration sequence with
DRDY going low when the sequence is completed. In unipolar
mode, the system offset calibration is performed between the
two endpoints of the transfer function; in bipolar mode, it is
performed between midscale and positive full scale.
Background Calibration
The AD7711 also offers a background calibration mode where
the part interleaves its calibration procedure with its normal
conversion sequence. In the background calibration mode, the
same voltages are used as the calibration points that are used in
the self-calibration mode, that is, shorted inputs and VREF. The
background calibration mode is invoked by writing 1, 0, 1 to
MD2, MD1, MD0 of the control register. When invoked, the
background calibration mode reduces the output data rate of the
AD7711 by a factor of 6 while the –3 dB bandwidth remains
unchanged. The advantage is that the part is continually performing calibration and automatically updating its calibration
coefficients. As a result, the effects of temperature drift, supply sensitivity, and time drift on zero- and full-scale errors are
automatically removed. When the background calibration mode
is turned on, the part will remain in this mode until Bits MD2,
MD1, and MD0 of the control register are changed. With background calibration mode on, the first result from the AD7711
will be incorrect because the full-scale calibration will not have
been performed. For a step change on the input, the second
output update will have settled to 100% of the final value.
Table VI summarizes the calibration modes and the calibration
points associated with them. It also gives the duration from
when the calibration is invoked to when valid data is available
to the user.
Span and Offset Limits
Whenever a system calibration mode is used, there are limits on
the amount of offset and span that can be accommodated. 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.
The amount of offset that can be accommodated depends on
whether the unipolar or bipolar mode is being used. This offset
range is limited by the requirement that the positive full-scale
calibration limit is £ 1.05 ¥ VREF/GAIN. Therefore, the offset
range plus the span range cannot exceed 1.05 ¥ VREF/GAIN. If
the span is at its minimum (0.8 ¥ VREF/GAIN), the maximum
the offset can be is (0.25 ¥ VREF/GAIN).
In bipolar mode, the system offset calibration range is again
restricted by the span range. The span range of the converter in
bipolar mode is equidistant around the voltage used for the zeroscale point; thus the offset range plus half the span range cannot
exceed (1.05 ¥ VREF/GAIN). If the span is set to 2 ¥ VREF/GAIN,
the offset span cannot move more than ± (0.05 ¥ VREF/GAIN)
before the endpoints of the transfer function exceed the input
overrange limits ± (1.05 ¥ VREF/GAIN). If the span range is set to
the minimum ± (0.4 ¥ VREF/GAIN), the maximum allowable
offset range is ± (0.65 ¥ VREF/GAIN).
POWER-UP AND CALIBRATION
On power-up, the AD7711 performs an internal reset that sets
the contents of the control register to a known state. 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 AD7711
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 calibration is initiated.
Drift Considerations
The AD7711 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.
Table VI. Calibration Truth Table
Cal Type
MD2, MD1, MD0
Zero-Scale Cal
Full-Scale Cal
Sequence
Duration
Self-Cal
System Cal
System Cal
System Offset Cal
Background Cal
0, 0, 1
0, 1, 0
0, 1, 1
1, 0, 0
1, 0, 1
Shorted Inputs
AIN
VREF
One-Step
Two-Step
Two-Step
One-Step
One-Step
9 ¥ 1/Output Rate
4 ¥ 1/Output Rate
4 ¥ 1/Output Rate
9 ¥ 1/Output Rate
6 ¥ 1/Output Rate
AIN
Shorted Inputs
–18–
AIN
VREF
VREF
REV. G
AD7711
Measurement errors due to offset drift or gain drift can be eliminated at any time by recalibrating the converter or by operating
the part in the background calibration mode. 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.
It is also important that power is applied to the AD7711 before
signals at REF IN, AIN, or the logic input pins in order to avoid
latch-up. If separate supplies are used for the AD7711 and the
system digital circuitry, the AD7711 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.
ANALOG
SUPPLY
POWER SUPPLIES AND GROUNDING
Because the analog inputs and reference input are differential,
most of the voltages in the analog modulator are common-mode
voltages. VBIAS provides the return path for most of the analog
currents flowing in the analog modulator. As a result, the VBIAS
input should be driven from a low impedance to minimize errors
due to charging/discharging impedances on this line. When the
internal reference is used as the reference source for the part,
AGND is the ground return for this reference voltage.
The analog and digital supplies to the AD7711 are independent
and separately pinned out to minimize coupling between the
analog and digital sections of the device. The digital filter will
provide rejection of broadband noise on the power supplies,
except at integer multiples of the modulator sampling frequency.
The digital supply (DVDD) must not exceed the analog positive
supply (AVDD) by more than 0.3 V in normal operation. If separate
analog and digital supplies are used, the recommended decoupling
scheme is shown in Figure 9. In systems where AVDD = 5 V and
DVDD = 5 V, it is recommended that AVDD and DVDD are
driven from the same 5 V supply, although each supply should be
decoupled separately as shown in Figure 9. It is preferable that
the common supply is the system’s analog 5 V supply.
REV.G
10␮F
DIGITAL 5V
SUPPLY
0.1␮F
0.1␮F
AVDD
DVDD
AD7711
Figure 9. Recommended Decoupling Scheme
DIGITAL INTERFACE
The AD7711’s serial communications port provides a flexible
arrangement to allow easy interfacing to industry-standard
microprocessors, microcontrollers, and digital signal processors.
A serial read to the AD7711 can access data from the output
register, the control register, or the calibration registers. A serial
write to the AD7711 can write data to the control register or the
calibration registers.
Two different modes of operation are available, optimized for
different types of interfaces where the AD7711 can act either as
master in the system (it provides the serial clock) or as slave (an
external serial clock can be provided to the AD7711). These
two modes, labelled self-clocking mode and external clocking
mode, are discussed in detail in the following sections.
–19–
2
AD7711
Self-Clocking Mode
The AD7711 is configured for its self-clocking mode by tying
the MODE pin high. In this mode, the AD7711 provides the
serial clock signal used for the transfer of data to and from the
AD7711. This self-clocking mode can be used with processors
that allow an external device to clock their serial port, including
most digital signal processors and microcontrollers such as the
68HC11 and 68HC05. It also allows easy interfacing to serialparallel conversion circuits in systems with parallel data communication, allowing interfacing to 74XX299 universal shift
registers without any additional decoding. In the case of shift
registers, the serial clock line should have a pull-down resistor
instead of the pull-up resistor shown in Figures 10 and 11.
Read Operation
Data can be read from either the output register, the control
register, or the calibration registers. A0 determines whether the
data read accesses data from the control register or from the
output/calibration registers. This A0 signal must remain valid
for the duration of the serial read operation. With A0 high, data
is accessed from either the output register or the calibration
registers. With A0 low, data is accessed from the control register.
The function of the DRDY line is dependent only on the output
update rate of the device and the reading of the output data
register. DRDY goes low when a new data-word is available in
the output data register. It is reset high when the last bit of data
(either 16th bit or 24th bit) is read from the output register. If
data is not read from the output register, the DRDY line remains low. The output register continues to be updated at the
output update rate but DRDY will not indicate this. A read
from the device in this circumstance accesses the most recent
word in the output register. If a new data-word becomes available
to the output register while data is being read from the output
register, DRDY will not indicate this and the new data-word
will be lost to the user. DRDY is not affected by reading from
the control register or the calibration registers.
Data can be accessed from the output data register only when
DRDY is low. If RFS goes low with DRDY high, no data transfer takes place. DRDY does not have any effect on reading data
from the control register or from the calibration registers.
Figure 10 shows a timing diagram for reading from the AD7711
in the self-clocking mode. The read operation shows a read from
the AD7711’s output data register. A read from the control
register or calibration registers is similar, but, in these cases, the
DRDY line is not related to the read function. Depending on
the output update rate, it can go low at any stage in the control/
calibration register read cycle without affecting the read, and its
status should be ignored. A read operation from either the control or calibration registers must always read 24 bits of data
from the respective register.
Figure 10 shows a read operation from the AD7711. For the
timing diagram shown, it is assumed that there is a pull-up
resistor on the SCLK output. With DRDY low, the RFS
input is brought low. RFS going low enables the serial clock of
the AD7711 and also places the MSB of the word on the serial
data line. All subsequent data bits are clocked out on a high to
low transition of the serial clock and are valid prior to the following rising edge of this clock. The final active falling edge of
SCLK clocks out the LSB, and this LSB is valid prior to the final
active rising edge of SCLK. Coincident with the next falling
edge of SCLK, DRDY is reset high. DRDY going high turns off
the SCLK and the SDATA outputs, which means the data hold
time for the LSB is slightly shorter than for all other bits.
DRDY (O)
t3
t2
A0 (I)
t5
t4
RFS (I)
t9
t6
SCLK (O)
t7
SDATA (O)
t8
t10
MSB
LSB
THREE-STATE
Figure 10. Self-Clocking Mode, Output Data Read Operation
–20–
REV. G
AD7711
Write Operation
Read Operation
Data can be written to either the control register or the calibration registers. In either case, the write operation is not affected
by the DRDY line and does not have any effect on the status of
DRDY. A write operation to the control register or the calibration register must always write 24 bits.
As with self-clocking mode, data can be read from either the
output register, the control register, or the calibration registers.
A0 determines whether the data read accesses data from the
control register or from the output/calibration registers. This A0
signal must remain valid for the duration of the serial read
operation. With A0 high, data is accessed from either the output
register or from the calibration registers. With A0 low, data is
accessed from the control register.
Figure 11 shows a write operation to the AD7711. A0 determines whether a write operation transfers data to the control
register or to the calibration registers. This A0 signal must
remain valid for the duration of the serial write operation. The
falling edge of TFS enables the internally generated SCLK
output. The serial data to be loaded to the AD7711 must be
valid on the rising edge of this SCLK signal. Data is clocked
into the AD7711 on the rising edge of the SCLK signal with the
MSB transferred first. On the last active high time of SCLK, the
LSB is loaded to the AD7711. Subsequent to the next falling
edge of SCLK, the SCLK output is turned off. (The timing diagram in Figure 11 assumes a pull-up resistor on the SCLK line.)
The function of the DRDY line is dependent only on the output
update rate of the device and the reading of the output data
register. DRDY goes low when a new data-word is available in
the output data register. It is reset high when the last bit of data
(either the 16th bit or 24th bit) is read from the output register.
If data is not read from the output register, the DRDY line
remains low. The output register continues to be updated at the
output update rate, but DRDY will not indicate this. A read
from the device in this circumstance accesses the most recent
word in the output register. If a new data-word becomes available to the output register while data is being read from the
output register, DRDY will not indicate this and the new dataword will be lost to the user. DRDY is not affected by reading
from the control register or the calibration register.
External Clocking Mode
The AD7711 is configured for external clocking mode by tying
the MODE pin low. In this mode, SCLK of the AD7711 is configured as an input, and an external serial clock must be provided to
this SCLK pin. This external clocking mode is designed for direct
interface to systems that provide a serial clock output that is synchronized to the serial data output, including microcontrollers
such as the 80C51, 87C51, 68HC11, 68HC05, and most digital
signal processors.
Data can be accessed from the output data register only when
DRDY is low. If RFS goes low while DRDY is high, no data
transfer will take place. DRDY does not have any effect on reading
data from the control register or from the calibration registers.
A0 (I)
t14
t15
TFS (I)
t17
t16
t9
SCLK (O)
t18
SDATA (I)
t19
t10
MSB
LSB
Figure 11. Self-Clocking Mode, Control/Calibration Register Write Operation
REV.G
–21–
2
AD7711
Figures 12a and 12b show timing diagrams for reading from the
AD7711 in external clocking mode. In Figure 12a, all the data is
read from the AD7711 in one operation. In Figure 12b, the data
is read from the AD7711 over a number of read operations. Both
read operations show a read from the AD7711’s output data
register. A read from the control register or calibration registers is
similar, but, in these cases, the DRDY line is not related to the
read function. Depending on the output update rate, it can go
low at any stage in the control/calibration register read cycle
without affecting the read, and its status should be ignored. A
read operation from either the control or calibration registers
must always read 24 bits of data.
Figure 12a shows a read operation from the AD7711 where
RFS remains low for the duration of the data-word transmission.
With DRDY low, the RFS input is brought low. The input
SCLK signal should be low between read and write operations.
RFS going low places the MSB of the word to be read on the
serial data line. All subsequent data bits are clocked out on a
high to low transition of the serial clock and are valid prior to
the following rising edge of this clock. The penultimate falling
edge of SCLK clocks out the LSB and the final falling edge
resets the DRDY line high. This rising edge of DRDY turns off
the serial data output.
Figure 12b shows a timing diagram for a read operation where
RFS returns high during the transmission of the word and
returns low again to access the rest of the data-word. Timing
parameters and functions are very similar to that outlined for
Figure 12a, but Figure 12b has a number of additional times to
show timing relationships when RFS returns high in the middle
of transferring a word.
RFS should return high during a low time of SCLK. On the
rising edge of RFS, the SDATA output is turned off. DRDY
remains low and will remain low until all bits of the data-word
are read from the AD7711, regardless of the number of times
RFS changes state during the read operation. Depending on the
time between the falling edge of SCLK and the rising edge of
RFS, the next bit (BIT N+1) may appear on the data bus before
RFS goes high. When RFS returns low again, it activates the
SDATA output. When the entire word is transmitted, the
DRDY line will go high, turning off the SDATA output as shown
in Figure 12a.
DRDY (O)
t21
t20
A0 (I)
t22
t23
RFS (I)
t26
t28
SCLK (I)
t24
SDATA (O)
t27
t25
MSB
t29
LSB
THREE-STATE
Figure 12a. External Clocking Mode, Output Data Read Operation
DRDY (O)
t20
A0 (I)
t22
RFS (I)
t26
t30
SCLK (I)
t24
t25
SDATA (O)
t24
t27
MSB
t31
BIT N
THREE-STATE
t25
BIT N+1
Figure 12b. External Clocking Mode, Output Data Read Operation (RFS Returns High during Read Operation)
–22–
REV. G
AD7711
MSB transferred first. On the last active high time of SCLK, the
LSB is loaded to the AD7711.
Write Operation
Data can be written to either the control register or calibration
registers. In either case, the write operation is not affected by
the DRDY line and does not have any effect on the status of
DRDY. A write operation to the control register or the calibration register must always write 24 bits.
Figure 13b shows a timing diagram for a write operation to the
AD7711 with TFS returning high during the operation and
returning low again to write the rest of the data-word. Timing
parameters and functions are very similar to that outlined for
Figure 13a, but Figure 13b has a number of additional times to
show timing relationships when TFS returns high in the middle
of transferring a word.
Figure 13a shows a write operation to the AD7711 with TFS
remaining low for the duration of the operation. A0 determines
whether a write operation transfers data to the control register
or to the calibration registers. This A0 signal must remain valid
for the duration of the serial write operation. As before, the serial
clock line should be low between read and write operations. The
serial data to be loaded to the AD7711 must be valid on the
high level of the externally applied SCLK signal. Data is clocked
into the AD7711 on the high level of this SCLK signal with the
Data to be loaded to the AD7711 must be valid prior to the
rising edge of the SCLK signal. TFS should return high during
the low time of SCLK. After TFS returns low again, the next bit
of the data-word to be loaded to the AD7711 is clocked in on
next high level of the SCLK input. On the last active high time
of the SCLK input, the LSB is loaded to the AD7711.
A0 (I)
t32
t33
TFS (I)
t26
t34
SCLK (I)
SDATA (I)
t27
t36
t35
MSB
LSB
Figure 13a. External Clocking Mode, Control/Calibration Register Write Operation
A0 (I)
t32
TFS (I)
t26
t30
SCLK (I)
t35
t27
t36
SDATA (I)
MSB
BIT N
t35
t36
BIT N+1
Figure 13b. External Clocking Mode, Control/Calibration Register Write Operation
(TFS Returns High during Write Operation)
REV.G
–23–
2
AD7711
SIMPLIFYING THE EXTERNAL CLOCKING MODE
INTERFACE
START
In many applications, the user may not need to write to the
on-chip calibration registers. In this case, the serial interface to
the AD7711 in external clocking mode can be simplified by
connecting the TFS line to the A0 input of the AD7711 (see
Figure 14). This means that any write to the device will load
data to the control register (because A0 is low while TFS is
low), and any read to the device will access data from the output
data register or from the calibration registers (because A0 is high
while RFS is low). Note that in this arrangement, the user does
not have the capability of reading from the control register.
CONFIGURE AND
INITIALIZE ␮C/␮P
SERIAL PORT
BRING
RFS, TFS HIGH
POLL DRDY
RFS
FOUR
INTERFACE
LINES
SDATA
SCLK
AD7711
DRDY
LOW?
TFS
NO
A0
YES
Figure 14. Simplified Interface with TFS Connected to A0
BRING
RFS LOW
Another method of simplifying the interface is to generate the
TFS signal from an inverted RFS signal. However, generating
the signals the opposite way around (RFS from an inverted
TFS) will cause writing errors.
ⴛ3
READ
SERIAL BUFFER
MICROCOMPUTER/MICROPROCESSOR INTERFACING
The AD7711’s flexible serial interface allows easy interface to
most microcomputers and microprocessors. Figure 15 shows a
flowchart for a typical programming sequence for reading data
from the AD7711 to a microcomputer, while Figure 16 shows a
flowchart for writing data to the AD7711. Figures 17, 18, and
19 show some typical interface circuits.
BRING
RFS HIGH
Figure 15 shows continuous read operations from the AD7711
output register where the DRDY line is continuously polled.
Depending on the microprocessor configuration, the DRDY line
may come to an interrupt input, in which case DRDY will automatically generate an interrupt without being polled. The reading of the serial buffer could be anything from one read
operation up to three read operations (where 24 bits of data
are read into an 8-bit serial register). A read operation to the
control/calibration registers is similar, but, in this case, the status
of DRDY can be ignored. The A0 line is brought low when the
RFS line is brought low during a read from the control register.
The flowchart also shows the bits being reversed after they have
been read in from the serial port. This depends on whether the
microprocessor expects the MSB of the word first or the LSB of
the word first. The AD7711 outputs the MSB first.
REVERSE
ORDER OF BITS
Figure 15. Flowchart for Continuous Read Operations
to the AD7711
Figure 16 shows a single 24-bit write operation to the AD7711
control or calibration registers. This shows data being transferred
from data memory to the accumulator before being written to
the serial buffer. Some microprocessor systems allow data to be
written directly to the serial buffer from data memory. Writing
data to the serial buffer from the accumulator generally consists
of either two or three write operations, depending on the size of
the serial buffer.
Figure 16 also shows the option of the bits being reversed before
being written to the serial buffer, which depends on whether the
first bit transmitted by the microprocessor is the MSB or the
LSB. The AD7711 expects the MSB as the first bit in the data
stream. In cases where the data is being read or being written in
bytes and the data has to be reversed, the bits have to be reversed
for every byte.
–24–
REV. G
AD7711
Table VII shows some typical 8XC51 code used for a single 24-bit
read from the output register of the AD7711. Table VIII shows
some typical code for a single write operation to the control
register of the AD7711. The 8XC51 outputs the LSB first in a
write operation, while the AD7711 expects the MSB first so the
data to be transmitted has to be rearranged before being written
to the output serial register. Similarly, the AD7711 outputs the
MSB first during a read operation that the 8XC51 expects the
LSB first. Therefore, the data that is read into the serial buffer
needs to be rearranged before the correct data-word from the
AD7711 is available in the accumulator.
START
CONFIGURE AND
INITIALIZE ␮C/␮P
SERIAL PORT
BRING
RFS, TFS AND
A0 HIGH
LOAD DATA FROM
ADDRESS TO
ACCUMULATOR
Table VII. 8XC51 Code for Reading from the AD7711
MOV SCON,#00010001B;
MOV IE,#00010000B;
SETB 90H;
SETB 91H;
SETB 93H;
MOV R1,#003H;
REVERSE
ORDER OF
BITS
BRING
TFS AND A0 LOW
MOV R0,#030H;
ⴛ3
WRITE DATA FROM
ACCUMULATOR TO
SERIAL BUFFER
BRING
TFS AND A0 HIGH
END
Figure 16. Flowchart for Single Write Operation
to the AD7711
AD7711 to 8051 Interface
Figure 17 shows an interface between the AD7711 and the 8XC51
microcontroller. The AD7711 is configured for external clocking
mode, while the 8XC51 is configured in its Mode 0 serial interface
mode. The DRDY line from the AD7711 is connected to the Port
P1.2 input of the 8XC51, so the DRDY line is polled by the
8XC51. The DRDY line can be connected to the INT1 input of
the 8XC51 if an interrupt driven system is preferred.
DVDD
SYNC
8XC51
P1.0
RFS
P1.1
TFS
P1.2
DRDY
P1.3
A0
P3.0
SDATA
P3.1
AD7711
MOV R6,#004H;
WAIT:
NOP;
MOV A,P1;
Read Port 1
ANL A,R6;
Mask Out All Bits Except DRDY
JZ READ;
If Zero Read
SJMP WAIT;
Otherwise Keep Polling
READ:
CLR 90H;
Bring RFS Low
CLR 98H;
Clear Receive Flag
POLL:
JB 98H, READ1
Tests Receive Interrupt Flag
SJMP POLL
READ 1:
MOV A,SBUF;
Read Buffer
RLC A;
Rearrange Data
MOV B.0,C;
Reverse Order of Bits
RLC A; MOV B.1,C; RLC A; MOV B.2,C;
RLC A; MOV B.3,C; RLC A; MOV B.4,C;
RLC A; MOV B.5,C; RLC A; MOV B.6,C;
RLC A; MOV B.7,C;
MOV A,B;
MOV @R0,A;
Write Data to Memory
INC R0;
Increment Memory Location
DEC R1
Decrement Byte Counter
MOV A,R1
JZ END
Jump if Zero
JMP WAIT
Fetch Next Byte
END:
SETB 90H
Bring RFS High
FIN:
SJMP FIN
SCLK
MODE
Figure 17. AD7711 to 8XC51 Interface
REV.G
Configure 8051 for MODE 0
Disable All Interrupts
Set P1.0, Used as RFS
Set P1.1, Used as TFS
Set P1.3, Used as A0
Sets Number of Bytes to Be Read in
Read Operation
Start Address for where Bytes Will
Be Loaded
Use P1.2 as DRDY
–25–
2
AD7711
Table VIII. 8XC51 Code for Writing to the AD7711
MOV SCON,#00000000B;
MOV IE,#10010000B;
MOV IP,#00010000B;
SETB 91H;
SETB 90H;
MOV R1,#003H;
MOV R0,#030H;
MOV A,#00H;
MOV SBUF,A;
WAIT:
JMP WAIT;
INT ROUTINE:
NOP;
MOV A,R1;
JZ FIN;
DEC R1;
MOV A,@R;
INC R0;
RLC A;
DVDD
DVDD
Configure 8051 for MODE 0
and Enable Serial Reception
Enable Transmit Interrupt
Prioritize the Transmit Interrupt
Bring TFS High
Bring RFS High
Sets Number of Bytes to Be Written
in a Write Operation
Start Address in RAM for Bytes
Clear Accumulator
Initialize the Serial Port
SYNC
SS
68HC11
PC0
RFS
PC1
TFS
PC2
DRDY
PC3
A0
SCK
SCLK
MISO
SDATA
MOSI
MODE
AD7711
Figure 18. AD7711 to 68HC11 Interface
Wait for Interrupt
APPLICATIONS
4-Wire RTD Configurations
Interrupt Subroutine
Load R1 to Accumulator
If Zero Jump to FIN
Decrement R1 Byte Counter
Move Byte into the Accumulator
Increment Address
Rearrange Data from LSB First
to MSB First
MOV B.0,C; RLC A; MOV B.1,C; RLC A;
MOV B.2,C; RLC A; MOV B.3,C; RLC A;
MOV B.4,C; RLC A; MOV B.5,C; RLC A;
MOV B.6,C; RLC A; MOV B.7,C; MOV A,B;
CLR 93H;
Bring A0 Low
CLR 91H;
Bring TFS Low
MOV SBUF,A;
Write to Serial Port
RETI;
Return from Subroutine
FIN:
SETB 91H;
Set TFS High
SETB 93H;
Set A0 High
RETI;
Return from Interrupt Subroutine
Figure 19 shows a 4-wire RTD application where the RTD
transducer is interfaced directly to the AD7711. In the 4-wire
configuration, there are no errors associated with lead resistances
because no current flows in the measurement leads connected to
AIN1(+) and AIN1(–). One of the RTD current sources is used
to provide the excitation current for the RTD. A common nominal
resistance value for the RTD is 100 W and, therefore, the RTD
will generate a 20 mV signal that can be handled directly by the
analog input of the AD7711. In the circuit shown, the second
RTD excitation current is used to generate the reference voltage
for the AD7711. This reference voltage is developed across RREF
and applied to the differential reference inputs. For the nominal
reference voltage of 2.5 V, RREF is 12.5 kW. This scheme ensures
that the analog input voltage span remains ratiometric to the
reference voltage. Any errors in the analog input voltage due to
the temperature drift of the RTD current source is compensated
for by the variation in the reference voltage. The typical matching
between the RTD current sources is less than 3 ppm/∞C.
5V
AD7711 to 68HC11 Interface
AVDD
Figure 18 shows an interface between the AD7711 and the
68HC11 microcontroller. The AD7711 is configured for its
external clocking mode, while the SPI port is used on the
68HC11 is in single-chip mode. The DRDY line from the
AD7711 is connected to the Port PC2 input of the 68HC11, so
the DRDY line is polled by the 68HC11. The DRDY line can
be connected to the IRQ input of the 68HC11 if an interrupt
driven system is preferred. The 68HC11 MOSI and MISO lines
should be configured for wire-OR operation. Depending on the
interface configuration, it may be necessary to provide bidirectional buffers between the 68HC11 MOSI and MISO lines.
The 68HC11 is configured in the master mode with its CPOL
Logic 0 bit set to a Logic 0 and its CPHA bit set to a Logic 1.
With a 10 MHz master clock on the AD7711, the interface will
operate with all four serial clock rates of the 68HC11.
DVDD
200␮A
RTD2
REF IN(+)
RREF
INTERNAL
CIRCUITRY
REF IN(–)
200␮A
RTD1
AIN1(+)
RTD
PGA
AIN1(–)
A = 1–128
AGND
AD7711
VSS
DGND
Figure 19. 4-Wire RTD Application with the AD7711
–26–
REV. G
AD7711
3-Wire RTD Configurations
One possible 3-wire configuration using the AD7711 is outlined
in Figure 20. In the 3-wire configuration, the lead resistances
will result in errors if only one current source is used because
the 200 mA will flow through RL1, developing a voltage error
between AIN1(+) and AIN1(–). In the scheme outlined below,
the second RTD current source is used to compensate for the
error introduced by the 200 mA flowing through RL1. The second
RTD current flows through RL2. Assuming RL1 and RL2 are
equal (the leads would normally be of the same material and of
equal length) and RTD1 and RTD2 match, then the error voltage
across RL2 equals the error voltage across RL1 and no error voltage
is developed between AIN1(+) and AIN1(–). Twice the voltage
is developed across RL3 but because this is a common-mode
voltage, it will not introduce any errors. The circuit in Figure 20
shows the reference voltage for the AD7711 derived from the
part’s own internal reference.
The circuit in Figure 21 shows an alternate 3-wire configuration. In this case, the circuit has the same benefits in terms of
eliminating lead resistance errors as outlined in Figure 20, but it
has the additional benefit that the reference voltage is derived
from one of the current sources. This gives all the benefits of
eliminating RTD tempco errors as outlined in Figure 19. The
voltage on either RTD input can go to within 2 V of the AVDD
supply. The circuit is shown for a 2.5 V reference.
AVDD DVDD
REF IN(–)
RTD1
INTERNAL
CIRCUITRY
12.5k⍀
200␮A
RL1
AIN1(+)
RTD
AIN1(–)
PGA
A = 1–128
ANALOG 5V SUPPLY
AVDD
DVDD
REF IN(+)
RL2
REF OUT
RTD2
REF IN(–)
200␮A
RL3
2.5V
REFERENCE
200␮A
DGND
AIN1(+)
RTD
A = 1–128
RL2
RTD2
200␮A
AD7711
AGND
DGND
VSS
Figure 20. 3-Wire RTD Application with the AD7711
OUTLINE DIMENSIONS
24-Lead Plastic Dual In-Line Package [PDIP]
(N-24)
Dimensions shown in inches and (millimeters)
1.185 (30.01)
1.165 (29.59)
1.145 (29.08)
0.150 (3.81)
0.130 (3.30)
0.110 (2.79)
0.295 (7.49)
0.285 (7.24)
0.275 (6.99)
24
13
1
12
0.180
(4.57)
MAX
0.015 (0.38) MIN
0.022 (0.56)
0.018 (0.46)
0.014 (0.36)
0.100
(2.54)
BSC
0.060 (1.52) SEATING
0.050 (1.27) PLANE
0.045 (1.14)
0.325 (8.26)
0.310 (7.87)
0.300 (7.62)
0.150 (3.81)
0.135 (3.43)
0.120 (3.05)
0.015 (0.38)
0.010 (0.25)
0.008 (0.20)
COMPLIANT TO JEDEC STANDARDS MO-095AG
CONTROLLING DIMENSIONS ARE IN INCHES; MILLIMETER DIMENSIONS
(IN PARENTHESES) ARE ROUNDED-OFF INCH EQUIVALENTS FOR
REFERENCE ONLY AND ARE NOT APPROPRIATE FOR USE IN DESIGN
REV.G
VSS
Figure 21. Alternate 3-Wire Configuration
INTERNAL
CIRCUITRY
PGA
AIN1(–)
RL3
AD7711
AGND
RTD1
RL1
REF IN(+)
–27–
2
AD7711
OUTLINE DIMENSIONS
24-Lead Ceramic Dual In-Line Package [CERDIP]
(Q-24)
0.098 (2.49)
MAX
0.005 (0.13)
MIN
24
C01170–0–3/04(G)
Dimensions shown in inches and (millimeters)
0.310 (7.87)
0.220 (5.59)
13
PIN 1
1
12
0.200 (5.08)
MAX
0.060 (1.52)
0.015 (0.38)
1.280 (32.51) MAX
0.320 (8.13)
0.290 (7.37)
0.150 (3.81)
MIN
0.200 (5.08)
0.125 (3.18)
0.100
(2.54)
BSC
0.023 (0.58)
0.014 (0.36)
0.015 (0.38)
0.008 (0.20)
15
0
0.070 (1.78) SEATING
PLANE
0.030 (0.76)
CONTROLLING DIMENSIONS ARE IN INCHES; MILLIMETER DIMENSIONS
(IN PARENTHESES) ARE ROUNDED-OFF INCH EQUIVALENTS FOR
REFERENCE ONLY AND ARE NOT APPROPRIATE FOR USE IN DESIGN
24-Lead Standard Small Outline Package [SOIC]
Wide Body
(R-24)
Dimensions shown in millimeters and (inches)
15.60 (0.6142)
15.20 (0.5984)
24
13
7.60 (0.2992)
7.40 (0.2913)
1
12
2.65 (0.1043)
2.35 (0.0925)
10.65 (0.4193)
10.00 (0.3937)
0.75 (0.0295)
ⴛ 45ⴗ
0.25 (0.0098)
0.30 (0.0118)
0.10 (0.0039)
COPLANARITY
0.10
1.27 (0.0500)
BSC
0.51 (0.0201)
0.31 (0.0122)
8ⴗ
0ⴗ
SEATING
0.33 (0.0130)
PLANE
0.20 (0.0079)
1.27 (0.0500)
0.40 (0.0157)
COMPLIANT TO JEDEC STANDARDS MS-013AD
CONTROLLING DIMENSIONS ARE IN MILLIMETERS; INCH DIMENSIONS
(IN PARENTHESES) ARE ROUNDED-OFF MILLIMETER EQUIVALENTS FOR
REFERENCE ONLY AND ARE NOT APPROPRIATE FOR USE IN DESIGN
Revision History
Location
Page
3/04—Data Sheet changed from REV. F to REV. G.
Changes to SPECIFICATIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
Changes to ORDERING GUIDE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
Deleted AD7711 to ADSP-2105 Interface section . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
Changes to AD7711 to 68HC11 Interface section . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
Updated OUTLINE DIMENSIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
–28–
REV. G