AD AD7713SQ Lc2mos loop-powered signal conditioning adc Datasheet

a
LC2MOS
Loop-Powered Signal Conditioning ADC
AD7713*
FEATURES
Charge Balancing ADC
24 Bits No Missing Codes
60.0015% Nonlinearity
Three-Channel Programmable Gain Front End
Gains from 1 to 128
Two Differential Inputs
One Single Ended High Voltage Input
Low-Pass Filter with Programmable Filter Cutoffs
Ability to Read/Write Calibration Coefficients
Bidirectional Microcontroller Serial Interface
Single Supply Operation
Low Power (3.5 mW typ) with Power-Down Mode
(150 mW typ)
APPLICATIONS
Loop Powered (Smart) Transmitters
RTD Transducers
Process Control
Portable Industrial Instruments
GENERAL DESCRIPTION
The AD7713 is a complete analog front end for low frequency
measurement applications. The device accepts low level signals
directly from a transducer or high level signals (4 × VREF) and
outputs a serial digital word. It employs a sigma-delta 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 two differential analog inputs and one singleended high level analog input as well as a differential reference
input. It can be operated from a single supply (AVDD and DVDD
at +5 V). The part provides two current sources which can be
used to provide excitation in three-wire and four-wire RTD configurations. The AD7713 thus performs all signal conditioning
and conversion for a single, dual or three-channel system.
The AD7713 is ideal for use in smart, microcontroller-based
systems. Gain settings, signal polarity and RTD current control
can be configured in software using the bidirectional serial port.
The AD7713 contains self-calibration, system calibration and
background calibration options and also allows the user to read
and to write the on-chip calibration registers.
FUNCTIONAL BLOCK DIAGRAM
REF REF
IN(–) IN(+)
AVDD DVDD
VBIAS
STANDBY
AVDD
AD7713
1µA
CHARGING BALANCING A/D
CONVERTER
AIN1(+)
AIN1(–)
M
U
X
AIN2(+)
AIN2(–)
AIN3
AUTO-ZEROED
∑−∆
MODULATOR
PGA
A = 1 – 128
CLOCK
GENERATION
INPUT
SCALING
200µA
AVDD
DIGITAL
FILTER
SYNC
MCLK
IN
MCLK
OUT
SERIAL INTERFACE
CONTROL
REGISTER
RTD1
200µA
OUTPUT
REGISTER
RTD2
AGND
DGND
RFS TFS MODE SDATA SCLK DRDY A0
CMOS construction ensures low power dissipation and a hardware programmable power-down mode reduces the standby
power consumption to only 150 µW typical. The part is available in a 24-pin, 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 AD7713 consumes less than 1 mA in total supply current, making it ideal for use in loop-powered systems.
2. The two programmable gain channels allow the AD7713 to
accept input signals directly from a transducer removing a
considerable amount of signal conditioning. To maximize the
flexibility of the part, the high level analog input accepts
4 × VREF signals. On-chip current sources provide excitation
for three-wire and four-wire RTD configurations.
3. No Missing Codes ensures true, usable, 24-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.
4. The AD7713 is ideal for microcontroller or DSP processor
applications with an on-chip control register which allows
control over filter cutoff, input gain, signal polarity and calibration modes. The AD7713 allows the user to read and
write the on-chip calibration registers.
*Protected by U.S. Patent No. 5,134,401.
REV. C
Information furnished by Analog Devices is believed to be accurate and
reliable. However, no responsibility is assumed by Analog Devices for its
use, nor for any infringements of patents or other rights of third parties
which may result from its use. No license is granted by implication or
otherwise under any patent or patent rights of Analog Devices.
© Analog Devices, Inc., 1995
One Technology Way, P.O. Box 9106, Norwood. MA 02062-9106, U.S.A.
Tel: 617/329-4700
Fax: 617/326-8703
(AV = +5 V 6 5%; DV = +5 V 6 5%; REF IN(+) = +2.5 V; REF IN(–) = AGND;
AD7713–SPECIFICATIONS
MCLK IN = 2 MHz unless otherwise noted. All specifications T to T unless otherwise noted.)
DD
DD
MIN
Parameter
STATIC PERFORMANCE
No Missing Codes
Output Noise
Integral Nonlinearity
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
Bipolar Negative Full-Scale Drift5
ANALOG INPUTS
Input Sampling Rate, fS
Normal-Mode 50 Hz Rejection6
Normal-Mode 60 Hz Rejection6
AIN1, AIN27
Input Voltage Range8
Common-Mode Rejection (CMR)
Common-Mode 50 Hz Rejection6
Common-Mode 60 Hz Rejection6
Common-Mode Voltage Range10
DC Input Leakage Current @ +25°C
TMIN to TMAX
Sampling Capacitance6
AIN3
Input Voltage Range
Gain Error11
Gain Drift
Offset Error11
Input Impedance
MAX
A, S Versions1
Units
Conditions/Comments
24
22
18
15
12
See Tables I & II
± 0.0015
See Note 4
1
0.3
See Note 4
0.5
0.25
See Note 4
0.5
0.25
2
± 0.004
1
0.3
Bits min
Bits min
Bits min
Bits min
Bits min
% of FSR max
Guaranteed by Design. For Filter Notches ≤ 12 Hz
For Filter Notch = 20 Hz
For Filter Notch = 50 Hz
For Filter Notch = 100 Hz
For Filter Notch = 200 Hz
Depends on Filter Cutoffs and Selected Gain
Filter Notches ≤ 12 Hz; Typically ± 0.0003%
µV/°C typ
µV/°C typ
For Gains of 1, 2
For Gains of 4, 8, 16, 32, 64, 128
µV/°C typ
µV/°C typ
For Gains of 1, 2
For Gains of 4, 8, 16, 32, 64, 128
µV/°C typ
µV/°C typ
ppm/°C typ
% of FSR max
µV/°C typ
µV/°C typ
For Gains of 1, 2
For Gains of 4, 8, 16, 32, 64, 128
Typically ± 0.0006%
For Gains of 1, 2
For Gains of 4, 8, 16, 32, 64, 128
See Table III
100
100
dB min
dB min
For Filter Notches of 2 Hz, 5 Hz, 10 Hz, 25 Hz, 50 Hz, ± 0.02 × fNOTCH
For Filter Notches of 2 Hz, 6 Hz, 10 Hz, 30 Hz, 60 Hz, ±0.02 × fNOTCH
0 to +VREF9
± VREF
100
150
150
AGND to AVDD
10
1
20
V max
V max
dB min
dB min
dB min
V min to V max
pA max
nA max
pF max
0 to + 4 × VREF
± 0.05
1
4
30
V max
% typ
ppm/°C typ
mV max
kΩ min
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)
At DC
For Filter Notches of 2 Hz, 5 Hz, 10 Hz, 25 Hz, 50 Hz, ±0.02 × fNOTCH
For Filter Notches of 2 Hz, 6 Hz, 10 Hz, 30 Hz, 60 Hz, ±0.02 × fNOTCH
For Normal Operation. Depends on Gain Selected
Additional Error Contributed by Resistor Attenuator
Additional Drift Contributed by Resistor Attenuator
Additional Error Contributed by Resistor Attenuator
NOTES
1
Temperature range is as follows: A Version, –40°C to +85°C; S Version, –55°C to +125°C.
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 µV 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
The AIN1 and AIN2 analog inputs presents a very high impedance dynamic load which varies with clock frequency and input sample rate. The maximum
recommended source resistance depends on the selected gain.
8
The analog input voltage range on the AIN1(+) and AIN2(+) inputs is given here with respect to the voltage on the AIN1(–) and AIN2 (–) inputs. The input
voltage range on the AIN3 input is with respect to AGND. The absolute voltage on the AIN1 and AIN2 inputs should not go more positive than A VDD + 30 mV or
more negative than AGND – 30 mV.
9
VREF = REF IN(+) – REF IN(–).
10
This common-mode voltage range is allowed provided that the input voltage on AIN(+) and AIN(–) does not exceed A VDD + 30 mV and AGND – 30 mV.
11
This error can be removed using the system calibration capabilities of the AD7713. This error is not removed by the AD7713’s self-calibration feature. The offset
drift on the AIN3 input is four times the value given in the Static Performance section.
–2–
REV. C
AD7713
Parameter
A, S Versions1
Units
Conditions/Comments
REFERENCE INPUT
REF IN(+) – REF IN(–) Voltage
+2.5 to AVDD/1.8
V min to V max
For Specified Performance. Part Is Functional with Lower
VREF Voltages
fCLK IN/512
100
100
100
150
150
AGND to AVDD
10
1
dB min
dB min
dB min
dB min
dB min
V min to V max
pA max
nA max
For Filter Notches of
For Filter Notches of
At DC
For Filter Notches of
For Filter Notches of
± 10
µA 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 Capacitance12
0.4
4.0
± 10
9
V max
V min
µA max
pF typ
TRANSDUCER BURN-OUT
Current
Initial Tolerance @ +25°C
Drift
1
± 10
0.1
µA 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
200
± 20
20
±1
3
200
200
µA nom
% max
ppm/°C typ
% max
ppm/°C typ
nA/V max
nA/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
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)
+(4.2 × VREF)/GAIN
0 to VREF/GAIN
+3.2 × VREF/GAIN
+(4.2 × VREF)/GAIN
V max
V max
V min
V max
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)
Input Sampling Rate, fS
Normal-Mode 50 Hz Rejection6
Normal-Mode 60 Hz Rejection6
Common-Mode Rejection (CMR)
Common-Mode 50 Hz Rejection6
Common-Mode 60 Hz Rejection6
Common-Mode Voltage Range10
DC Input Leakage Current @ +25°C
TMIN to TMAX
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
AIN1, AIN2
Positive Full-Scale Calibration Limit13
Negative Full-Scale Calibration Limit13
Offset Calibration Limit14, 15
Input Span14
AIN3
Positive Full-Scale Calibration Limit13
Offset Calibration Limit15
Input Span
2 Hz, 5 Hz, 10 Hz, 25 Hz, 50 Hz, ±0.02 × f NOTCH
2 Hz, 6 Hz, 10 Hz, 30 Hz, 60 Hz, ±0.02 × f NOTCH
2 Hz, 5 Hz, 10 Hz, 25 Hz, 50 Hz, ±0.02 × f NOTCH
2 Hz, 6 Hz, 10 Hz, 30 Hz, 60 Hz, ±0.02 × f NOTCH
ISINK = 1.6 mA
ISOURCE = 100 µA
Matching Between RTD1 and RTD2 Currents
Matching Between RTD1 and RTD2 Current Drift
AVDD = +5 V
NOTES
12
Guaranteed by design, not production tested.
13
After calibration, if the analog input exceeds positive full scale, the converter will output all 1s. If the analog input is less than negative full scale, then the device will
output all 0s.
14
These calibration and span limits apply provided the absolute voltage on the AIN1 and AIN2 analog inputs does not exceed AV DD + 30 mV or go more negative
than AGND – 30 mV.
15
The offset calibration limit applies to both the unipolar zero point and the bipolar zero point.
REV. C
–3–
AD7713–SPECIFICATIONS
Parameter
A, S Versions1
Units
Conditions/Comments
POWER REQUIREMENTS
Power Supply Voltages
AVDD Voltage
DVDD Voltage16
Power Supply Currents
AVDD Current
+5 to +10
+5
V nom
V nom
± 5% for Specified Performance
± 5% for Specified Performance
0.6
0.7
0.5
1
mA max
mA max
mA max
mA max
AVDD = +5 V
AVDD = +10 V
fCLK IN = 1 MHz. Digital Inputs 0 V to DVDD
fCLK IN = 2 MHz. Digital Inputs 0 V to DVDD
Rejection w.r.t. AGND
See Note 18
dB typ
5.5
300
mW max
µW max
DVDD Current
Power Supply Rejection17
(AVDD and DVDD)
Power Dissipation
Normal Mode
Standby (Power-Down) Mode
AVDD = DVDD = +5 V, fCLK IN = 1 MHz; Typically 3.5 mW
AVDD = DVDD = +5 V, Typically 150 µW
NOTES
16
The ± 5% tolerance on the DV DD input is allowed provided that DV DD does not exceed AVDD by more than 0.3 V.
17
Measured at dc and applies in the selected passband. PSRR at 50 Hz will exceed 120 dB with filter notches of 2 Hz, 5 Hz, 10 Hz, 25 Hz or 50 Hz. PSRR at 60 Hz
will exceed 120 dB with filter notches of 2 Hz, 6 Hz, 10 Hz, 30 Hz or 60 Hz.
18
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.
Specifications subject to change without notice.
(DVDD = +5 V ± 5%; AVDD = +5 V or +10 V ± 5%; AGND = DGND = 0 V; fCLKIN =2 MHz;
DD unless otherwise noted.)
TIMING CHARACTERISTICS1, 2 Input Logic 0 = 0 V, Logic 1 = DV
Parameter
fCLK IN3, 4
tCLK IN LO
tCLK IN HI
tr5
tf 5
t1
Self-Clocking Mode
t2
t3
t4
t5
t6
t 76
t 86
t9
t10
t14
t15
t16
t17
t18
t19
Limit at TMIN, TMAX
(A, S Versions)
Units
Conditions/Comments
400
2
0.4 × tCLK IN
0.4 × tCLK IN
50
50
1000
kHz min
MHz max
ns min
ns min
ns max
ns max
ns min
Master Clock Frequency: Crystal Oscillator or
Externally Supplied for Specified Performance
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
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
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
–4–
REV. C
AD7713
Parameter
External-Clocking Mode
fSCLK
t20
t21
t22
t23
t246
t256
t26
t27
t28
t297
t30
t317
t32
t33
t34
t35
t36
Limit at TMIN, TMAX
(A, S Versions)
Units
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
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
CLK IN duty cycle range is 45% to 55%. CLK IN must be supplied whenever the AD7713 is not in STANDBY mode. If no clock is present in this case, the
device can draw higher current than specified and possibly become uncalibrated.
4
The AD7713 is production tested with f CLK IN at 2 MHz. It is guaranteed by characterization to operate at 400 kHz.
5
Specified using 10% and 90% points on waveform of interest.
6
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.
7
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.
1.6mA
TO OUTPUT
PIN
+2.1V
100pF
200µA
Figure 1. Load Circuit for Access Time and Bus Relinquish Time
REV. C
–5–
2
AD7713
ABSOLUTE MAXIMUM RATINGS*
(TA = +25°C, unless otherwise noted)
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
AGND to DGND . . . . . . . . . . . . . . . . . . . . . . –0.3 V to +6 V
AIN1, AIN2 Input Voltage
to AGND . . . . . . . . . . . . . . . . . . . . . –0.3 V to AVDD + 0.3 V
AIN3 Input Voltage to AGND . . . . . . . . . . . . –0.3 V to +22 V
Reference Input Voltage to AGND . . – 0.3 V to AVDD + 0.3 V
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
Junction Temperature . . . . . . . . . . . . . . . . . . . . . . . . . . +150°C
Plastic DIP Package, Power Dissipation . . . . . . . . . . . 450 mW
θJA Thermal Impedance . . . . . . . . . . . . . . . . . . . . . . 105°C/W
Lead Temperature, Soldering (10 sec) . . . . . . . . . . . . +260°C
Cerdip Package, Power Dissipation . . . . . . . . . . . . . . . 450 mW
θJA Thermal Impedance . . . . . . . . . . . . . . . . . . . . . . . 70°C/W
Lead Temperature, Soldering . . . . . . . . . . . . . . . . . . . +300°C
SOIC Package, Power Dissipation . . . . . . . . . . . . . . . . 450 mW
θJA Thermal Impedance . . . . . . . . . . . . . . . . . . . . . . . 75°C/W
Lead Temperature, Soldering
Vapor Phase (60 sec) . . . . . . . . . . . . . . . . . . . . . . . +215°C
Infrared (15 secs) . . . . . . . . . . . . . . . . . . . . . . . . . . +220°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 and 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.
CAUTION
ESD (electrostatic discharge) sensitive device. Electrostatic charges as high as 4000 V, which readily
accumulate on the human body and on test equipment, can discharge without detection. Although
devices feature proprietary ESD protection circuitry, permanent damage may still occur on these
devices if they are subjected to high energy electrostatic discharges. Therefore, proper precautions are
recommended to avoid any performance degradation or loss of functionality.
WARNING!
ESD SENSITIVE DEVICE
ORDERING GUIDE
Model
Temperature Range
Package Option*
AD7713AN
AD7713AR
AD7713AQ
AD7713SQ
EVAL-AD7713EB
–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
Q-24
Q-24
*N = Plastic DIP; Q = Cerdip; R = SOIC.
PIN CONFIGURATION
DIP AND SOIC
SCLK 1
24 DGND
MCLK IN 2
23 DVDD
MCLK OUT 3
22 SDATA
A0 4
SYNC 5
21 DRDY
AD7713
20 RFS
MODE 6
TOP VIEW 19 TFS
AIN1(+) 7 (Not to Scale) 18 AGND
AIN1(–) 8
17 AIN3
AIN2(+) 9
16 RTD2
AIN2(–) 10
15 REF IN(+)
STANDBY 11
14 REF IN(–)
AVDD 12
13 RTD1
–6–
REV. C
AD7713
PIN FUNCTION DESCRIPTION
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 it 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 AD7713 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 2 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 thisinput
high, access is to either the data register or the calibration registers.
5
SYNC
Logic Input which allows for synchronization of the digital filters when using a number of AD7713s. 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 which can be used to check that an external transducer has burnt 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
AIN2(+)
Analog Input Channel 2. Positive input of the programmable gain differential analog input.
10
AIN2(–)
Analog Input Channel 2. Negative input of the programmable gain differential analog input.
11
STANDBY
Logic Input. Taking this pin low shuts down the internal analog and digital circuitry, reducing power
consumption to less than 50 µW.
12
AVDD
Analog Positive Supply Voltage, +5 V to +10 V.
13
RTD1
Constant Current Output. A nominal 200 µA constant current is provided at this pin and this can be used
as the excitation current for RTDs. This, current can be turned on or off via the control register.
14
REF IN(–)
Reference Input. The REF IN(–) can lie anywhere between AVDD and AGND provided REF IN(+) is
greater than REF IN(–).
15
REF IN(+)
Reference Input. The reference input is differential providing that REF IN(+) is greater than REF IN(–).
REF IN(+) can lie anywhere between AVDD and AGND.
16
RTD2
Constant Current Output. A nominal 200 µA constant current is provided at this pin and this can be used
as the excitation current for RTDs. This, current can be turned on or off via the control register. This
second current can be used to eliminate lead resistanced errors in three-wire RTD configurations.
17
AIN3
Analog Input Channel 3. High level analog input which accepts an analog input voltage range of
4 × VREF/GAIN. At the nominal VREF of +2.5 V and a gain of 1, the AIN3 input voltage range is
0 to ± 10 V.
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. In the external clocking mode, TFS must go low before the first bit of the data word
is written to the part.
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.
REV. C
–7–
2
AD7713
Pin Mnemonic
Function
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 AD7713 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
INTEGRAL NONLINEARITY
POSITIVE FULL-SCALE OVERRANGE
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.
Positive full-scale overrange is the amount of overhead available
to handle input voltages on AIN1(+) and AIN2(+) inputs
greater than (AIN1(–) + VREF/GAIN) or on AIN3 of greater
than +4 × VREF /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.
POSITIVE FULL-SCALE ERROR
NEGATIVE FULL-SCALE OVERRANGE
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(+) and AIN2(+), the ideal full-scale input
voltage is (AIN1(–) + VREF/GAIN – 3/2 LSBs) where AIN(–) is
either AIN1(–) or AIN2(–) as appropriate; for AIN3, the ideal
full-scale voltage is +4 × VREF/GAIN – 3/2 LSBs. Positive fullscale error applies to both unipolar and bipolar analog input
ranges.
This is the amount of overhead available to handle voltages on
AIN1(+) and AIN2(+) below (AIN1(–) – VREF/GAIN) without
overloading the analog modulator or overflowing the digital filter.
OFFSET CALIBRATION RANGE
In the system calibration modes, the AD7713 calibrates its offset
with respect to the analog input. The offset calibration range
specification defines the range of voltages that the AD7713 can
accept and still calibrate offset accurately.
UNIPOLAR OFFSET ERROR
FULL-SCALE CALIBRATION RANGE
Unipolar offset error is the deviation of the first code transition
from the ideal voltage. For AIN1(+) and AIN2(+), the ideal
input voltage is (AIN1(–) + 0.5 LSB); for AIN3, the ideal input
is 0.5 LSB when operating in the Unipolar Mode.
This is the range of voltages that the AD7713 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 AD7713’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 AD7713 can
accept and still calibrate gain accurately.
BIPOLAR ZERO ERROR
This is the deviation of the midscale transition (0111 . . . 111
to 1000 . . . 000) from the ideal input voltage. For AIN1(+) and
AIN2(+), the ideal input voltage is (AIN1(–) – 0.5 LSB); AIN3
can only accommodate unipolar input ranges.
BIPOLAR NEGATIVE FULL-SCALE ERROR
This is the deviation of the first code transition from the ideal
input voltage. For AIN1(+) and AIN2(+), the ideal input voltage is (AIN1(–) – VREF/GAIN + 0.5 LSB); AIN3 can only accommodate unipolar input ranges.
–8–
REV. C
AD7713
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 and when writing to the register 24 bits of data must be written
otherwise the data will not be loaded to the control register. 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
CH1
CH0
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 of the device whereby 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 CH0 and CH1. This
is a one-step calibration sequence, and when complete, the part returns to Normal Mode (with MD2,
MD1, MD0 of the control registers returning to 0, 0, 0). The DRDY output indicates when this selfcalibration is complete. For this calibration type, the zero-scale calibration is done internally on shorted
(zeroed) inputs and the full-scale calibration is done on VREF.
0
1
0
Activate System Calibration. This activates system calibration on the channel selected by CH0 and CH1.
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
CH0 and CH1. 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 CH0
and CH1. If the background calibration mode is on, then the AD7713 provides continuous selfcalibration 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 six. Its major
advantage is that the user does not have to worry about recalibrating 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 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 CH0 and CH1. A write to the device
with A0 high writes data to the zero-scale calibration coefficients of the channel selected by CH0 and
CH1. 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, when writing to the calibration register, 24 bits of data must be
written, otherwise 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 CH0 and CH1. A write to the device
with A0 high writes data to the full-scale calibration coefficients of the channel selected by CH0 and
CH1. 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, when writing to the calibration register, 24 bits of data must be
written, otherwise the new data will not be transferred to the calibration register.
REV. C
–9–
2
AD7713
PGA Gain
G2
Gl
G0
Gain
0
0
0
0
1
1
1
1
0
0
1
1
0
0
1
1
0
1
0
1
0
1
0
1
1
2
4
8
16
32
64
128
(Default Condition After the Internal Power-On Reset)
Channel Selection
CH1
0
0
1
CH0
0
1
0
Channel
AIN1
AIN2
AIN3
(Default Condition After the 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 Currents
RO
0
1
Off
On
(Default Condition After Internal Power-On Reset)
Burn-Out 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 hence 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
2 MHz, this results in a first notch frequency range from 1.952 Hz to 205.59 kHz. To ensure correct operation of the AD7713, 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 AD7713. 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
10 Hz, then a new word is available at a 10 Hz rate or every 100 ms. If the first notch is at 200 Hz, a new word is available every 5 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 100 Hz, the settling time of the filter to a full-scale step input change is
400 ms max. If the first notch is at 200 Hz, the settling time of the filter to a full-scale input step is 20 ms max. This settling time
can be reduced to 3 × l/(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 will be 3 × l/(output data rate). If a change of channels takes place, the settling time is 3 × l/(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. C
AD7713
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. First, there is the electrical noise in the semiconductor devices used in the implementation of the modulator (device noise). Secondly, when the analog input signal is converted into the digital domain, quantization noise
is added. 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 12 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 12 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 200 Hz notch setting, no missing codes performance is only guaranteed 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 shows the same table as Table I except that the output is now expressed in terms of effective resolution (the magnitude of the rms noise with respect to 2 × VREF/GAIN, i.e., 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 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
Gain of
2
Typical Output RMS Noise (µV)
Gain of
Gain of
Gain of
4
8
16
Gain of
32
Gain of
64
Gain of
128
2 Hz2
5 Hz2
6 Hz2
10 Hz2
12 Hz2
20 Hz3
50 Hz3
100 Hz3
200 Hz3
1.0
1.8
2.5
4.33
5.28
13
130
0.6 × 103
3.1 × 103
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
0.25
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
0.52 Hz
1.31 Hz
1.57 Hz
2.62 Hz
3.14 Hz
5.24 Hz
13.1 Hz
26.2 Hz
52.4 Hz
0.33
0.5
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
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 (i.e., the ratio of the rms noise to the input full scale is
increased since 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
First Notch of
Filter and O/P –3 dB
Data Rate
Frequency
Gain of
1
2 Hz
5 Hz
6 Hz
10 Hz
12 Hz
20 Hz
50 Hz
100 Hz
200 Hz
22.5
21.5
21
20
20
18.5
15
13
10.5
0.52 Hz
1.31 Hz
1.57 Hz
2.62 Hz
3.14 Hz
5.24 Hz
13.1 Hz
26.2 Hz
52.4 Hz
Gain of
2
Effective Resolution1 (Bits)
Gain of
Gain of
Gain of
4
8
16
Gain of
32
Gain of
64
Gain of
128
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
19.5
18.5
18.5
18.5
18
17.5
15.5
12.5
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
21
20
20
19.5
19.5
18.5
15.5
13
11
20.5
19.5
19.5
19
19
18
15.5
13
11
NOTE
1
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. C
–11–
2
AD7713
Figure 2 gives similar information to that outlined in Table I. In this plot, the output rms noise is shown for the full range of available cutoff frequencies rather than for some typical cutoff frequencies as in Tables I and II. The numbers given in these plots are
typical values at 25°C.
1000
10000
GAIN OF 1
100
GAIN OF 4
GAIN OF 8
0UTPUT NOISE — µV
OUTPUT NOISE — µV
GAIN OF 16
GAIN OF 2
1000
100
10
GAIN OF 32
GAIN OF 64
GAIN OF 128
10
1
1
0.1
10
100
1000
0.1
10
10000
100
10000
Figure 2b. Plot of Output Noise vs. Gain and Notch
Frequency (Gain of 16 to 128)
Figure 2a. Plot of Output Noise vs. Gain and Notch
Frequency (Gains of 1 to 8)
from the analog +5 V supply. Some applications will have separate supplies for both AVDD and DVDD and in some of these
cases the analog supply will exceed the +5 V digital supply (see
Power Supplies and Grounding section).
CIRCUIT DESCRIPTION
The AD7713 is a sigma-delta A/D converter with on-chip digital filtering, intended for the measurement of wide dynamic
range, low frequency signals such as those in industrial control
or process control applications. It contains a sigma-delta (or
charge balancing) ADC, a calibration microcontroller with onchip static RAM, a clock oscillator, a digital filter and a bidirectional serial communications port.
ANALOG +5V
SUPPLY
The part contains three analog input channels, two programmable gain differential input and one programmable gain highlevel single-ended input. The gain range on both inputs is from
1 to 128. For the AIN1 and AIN2 inputs, this means that the
input can accept unipolar signals of between 0 mV to +20 mV
and 0 V to +2.5 V or bipolar signals in the range from ± 20 mV
to ± 2.5 V when the reference input voltage equals +2.5 V. The
input voltage range for the AIN3 input is +4 × VREF/GAIN and
is 0 V to + 10 V with the nominal reference of +2.5 V and a
gain of 1. 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 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
hence its –3 dB frequency) can be programmed via an on-chip
control register. The programmable range for this first notch
frequency is from 1.952 Hz to 205.59 Hz, giving a programmable range for the –3 dB frequency of 0.52 Hz to 53.9 Hz.
The basic connection diagram for the part is shown in Figure 3.
This shows the AD7713 in the external clocking mode with
both the AVDD and DVDD pins of the AD7713 being driven
1000
NOTCH FREQUENCY — Hz
NOTCH FREQUENCY — Hz
10µF
0.1µF
0.1µF
AVDD
DIFFERENTIAL
ANALOG
INPUT
DIFFERENTIAL
ANALOG INPUT
{
AIN1(+)
{
AIN2(+)
SINGLE–ENDED
ANALOG INPUT
DVDD
DRDY
AIN1(–)
AIN2(–)
AIN3
TFS
TRANSMIT
(WRITE)
RFS
RECEIVE
(READ)
AD7713
SDATA
SERIAL
DATA
SCLK
SERIAL
CLOCK
DVDD
STANDBY
DATA
READY
ANALOG
GROUND
AGND
A0
DIGITAL
GROUND
DGND
MODE
ADDRESS
INPUT
DVDD
SYNC
+2.5V
REFERENCE
REF IN(+)
MCLK IN
REF IN(–)
MCLK OUT
Figure 3. Basic Connection Diagram
The AD7713 provides a number of calibration options which
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 worry
about issuing periodic calibration commands to the device or
asking the device to recalibrate when there is a change in the
ambient temperature or power supply voltage.
–12–
REV. C
AD7713
The AD7713 gives the user access to the on-chip calibration
registers allowing the microprocessor to read the device’s calibration coefficients and also to write its own calibration coefficients to the part from prestored values in E2PROM. This gives
the microprocessor much greater control over the AD7713’s
calibration procedure. It also means that the user can verify that
the device has performed its calibration correctly by comparing the
coefficients after calibration with prestored values in E2PROM.
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
low-pass filter or integrator. It also illustrates the derivation of
the alternative name for these devices: Charge Balancing ADCs
DIFFERENTIAL
AMPLIFIER
INTEGRATOR
∫
WIN
For battery operation or low power systems, the AD7713 offers
a standby mode (controlled by the STANDBY pin) that reduces
idle power consumption to typically 150 µW.
COMPARATOR
FS
THEORY OF OPERATION
DAC
FS
The general block diagram of a sigma-delta ADC is shown in
Figure 4. It contains the following elements:
1.
2.
3.
4.
5.
6.
Figure 5. Basic Charge-Balancing ADC
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.
S/H AMP
It 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 charge balancing,
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
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.
DAC
DIGITAL DATA
Figure 4. General Sigma-Delta ADC
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, whose output samples the difference signal at a frequency many times that of the analog signal
sampling frequency (oversampling).
Oversampling is fundamental to the operation of sigma-delta
ADCs. Using the quantization noise formula for an ADC:
Input Sample Rate
SNR = (6.02 × number of bits + 1.76) dB,
a 1-bit ADC or comparator yields an SNR of 7.78 dB.
The AD7713 samples the input signal at a frequency of 7.8 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.
REV. C
The AD7713 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.
The modulator sample frequency for the device remains at
fCLK IN/512 (3.9 kHz @ fCLK IN = 2 MHz) regardless of the
selected gain. However, gains greater than ×1 are achieved by a
combination of multiple input samples per modulator cycle and
a scaling of the ratio of reference capacitor to input capacitor.
As a result of the multiple sampling, the input sample rate of
the device varies with the selected gain (see Table 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 (7.8 kHz @ fCLK IN = 2 MHz)
2 × fCLK IN/256 (15.6 kHz @ fCLK IN = 2 MHz)
4 × fCLK IN/256 (31.2 kHz @ fCLK IN = 2 MHz)
8 × fCLK IN/256 (62.4 kHz @ fCLK IN = 2 MHz)
8 × fCLK IN/256 (62.4 kHz @ fCLK IN = 2 MHz)
8 × fCLK IN/256 (62.4 kHz @ fCLK IN = 2 MHz)
8 × fCLK IN/256 (62.4 kHz @ fCLK IN = 2 MHz)
8 × fCLK IN/256 (62.4 kHz @ fCLK IN = 2 MHz)
–13–
2
AD7713
DIGITAL FILTERING
Post Filtering
The AD7713’s digital filter behaves like a similar analog filter,
with a few minor differences.
The on-chip modulator provides samples at a 3.9 kHz output
rate. The on-chip digital filter decimates these samples to provide data at an output rate which corresponds to the programmed first 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 which require a
higher data rate for a given bandwidth and noise performance.
Applications that need this higher data rate will require some
post filtering following the digital filter of the AD7713.
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 AD7713 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 2 MHz, the minimum cutoff frequency of the filter is 0.52 Hz while the maximum programmable cutoff frequency is 53.9 Hz.
Figure 6 shows the filter frequency response for a cutoff frequency of 0.52 Hz which corresponds to a first filter notch frequency of 2 Hz. This is a (sinx/x)3 response (also called sinc3)
0
–20
–40
–60
GAIN – dBs
–80
–100
–120
–140
–160
–180
–200
–220
–240
0
2
4
6
8
FREQUENCY – Hz
10
12
Figure 6. Frequency Response of AD7713 Filter
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.
For example, if the required bandwidth is 1.57 Hz but the required update rate is 20 Hz, the data can be taken from the
AD7713 at the 20 Hz rate giving a –3 dB bandwidth of
5.24 Hz. Post filtering can be applied to this to reduce the bandwidth and output noise, to the 1.57 Hz bandwidth level, while
maintaining an output rate of 20 Hz.
Post filtering can also be used to reduce the output noise from
the device for bandwidths below 0.52 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 0.52 Hz,
the noise in the resultant passband can be reduced. A reduction
in bandwidth by a factor of two 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 × 3.9 kHz, where
n = 1, 2, 3 . . . ). This means that there are frequency bands,
± f3 dB wide (f3 dB is cutoff frequency selected by FS0 to FS11)
where noise passes unattenuated to the output. However, due to
the AD7713’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.
If passive components are placed in front of the AIN1 and AIN2
inputs of the AD7713, 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 AIN1 and
AIN2 inputs is over 1 GΩ. 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, and this may result in gain errors being
RINT (7kΩ typ)
AIN
Since the AD7713 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.
HIGH
IMPEDANCE
> 1GΩ
CINT
(11.5pF typ)
VBIAS
SWITCHING FREQ DEPENDS ON
fCLK IN AND SELECTED GAIN
Figure 7. AIN1, AIN2 Input Impedance
–14–
REV. C
AD7713
introduced on the analog inputs. Both inputs of the differential
input channels look into similar input circuitry.
In any case, the error introduced due to longer charging times is
a gain error which can be removed using the system calibration
capabilities of the AD7713 provided that the resultant span is
within the span limits of the system calibration techniques for
the AD7713.
The AIN3 input contains a resistive attenuation network as outlined in Figure 8. The typical input impedance on this input is
44 kΩ. As a result, the AIN3 input should be driven from a low
impedance source.
33kΩ
AIN3
11kΩ
MODULATOR
CIRCUIT
VBIAS
Figure 8. AIN3 Input Impedance
ANALOG INPUT FUNCTIONS
Analog Input Ranges
The analog inputs on the AD7713 provide the user with considerable flexibility in terms of analog input voltage ranges. Two of
the inputs are differential, programmable-gain, input channels
which can handle either unipolar or bipolar input signals. The
common-mode range of these inputs is from AGND to AVDD
provided that the absolute value of the analog input voltage lies
between AGND – 30 mV and AVDD + 30 mV. The third analog
input is a single-ended, programmable gain high-level input
which accepts analog input ranges of 0 to +4 × VREF/GAIN.
The dc input leakage current on the AIN1 and AIN2 inputs 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. The dc input current on the AIN3 input depends on the input voltage. For the nominal input voltage
range of +10 V, the input current is 225 µA typ.
Burn Out Current
The AIN1(+) input of the AD7713 contains a 1 µA current
source which can be turned on/off via the control register. This
current source can be used in checking that a transducer has not
burnt out or gone open circuit before attempting to take measurements on that channel. If the current is turned on and is allowed flow into the transducer and a measurement of the input
voltage on the AIN1 input is taken, it can indicate that the
transducer is not functioning correctly. For normal operation,
this burn out current is turned off by writing a 0 to the BO bit in
the control register.
For four-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 three-wire
RTD configurations, the second on-chip current source can be
used to eliminate errors due to voltage drops across lead resistances. Figures 20 and 21 in the Application section show some
RTD configurations with the AD7713.
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 AD7713 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 four-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 kΩ resistor is used, the 200 µA current source generates
+2.5 V across the resistor. This +2.5 V can be applied to the
REF IN(+) input of the AD7713 and the REF IN(–) input at
ground it will supply a VREF of 2.5 V for the part. For three-wire
RTD configurations, the reference voltage for the part is generated by placing a low TC resistor (12.5 kΩ 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 AD7713 is differential so the REF IN(+) and
REF IN(–) of the AD7713 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, removes the temperature drift error.
Bipolar/Unipolar Inputs
Two analog inputs on the AD7713 can accept either unipolar or
bipolar input voltage ranges while the third channel accepts only
unipolar signals. Bipolar or unipolar options for AIN1 and
AIN2 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 unipolar or
bipolar operation does not change any of the input signal conditioning; it simply changes the data output coding. The data coding is binary for unipolar inputs and offset binary for bipolar
inputs.
The AIN1 and AIN2 input channels are differential, and as a
result, the voltage to which the unipolar and bipolar signals are
referenced is the voltage on the AIN1(–) and AIN2(–) inputs.
For example, if AIN1(–) is +1.25 V and the AD7713 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. For the AIN3 input, the input signals are
referenced to AGND.
RTD Excitation Currents
The AD7713 also contains two matched 200 µA constant current sources which 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. C
REFERENCE INPUT
The reference inputs of the AD7713, REF IN(+) and
REF IN(–) provide a differential reference input capability. The
common-mode 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
–15–
2
AD7713
voltage can go to +5 V with no degradation in performance
provided that the absolute value of REF IN(+) and REF IN(–)
does not exceed its AVDD and AGND limits. The part is also
functional with VREF voltages down to 1 V but with degraded
performance as the output noise will, in terms of LSB size, be
larger. REF IN(+) must always be greater than REF IN(–) for
correct operation of the AD7713.
System Synchronization
If multiple AD7713s 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 places the AD7713 into a consistent, known state. A common signal to the AD7713s’ SYNC inputs will synchronize their
operation. This would normally be done after each AD7713 has
performed its own calibration or has had calibration coefficients
loaded to it.
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 AIN1 analog input (see Figure 7). In this case,
RINT is 5 kΩ 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 AD7713 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 AD7713. A recommended reference source for the AD7713 is the AD680, a 2.5 V reference.
USING THE AD7713
SYSTEM DESIGN CONSIDERATIONS
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 AD7713 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 AD7713 uses digital calibration techniques that minimize offset and gain error.
The AD7713 operates differently from successive approximation
ADCs or integrating ADCs. Since 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 AD7713 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.
AUTOCALIBRATION
The input sampling frequency, the modulator sampling frequency, the –3 dB frequency, output update rate and calibration
time are all directly related to the master clock frequency,
fCLK IN. Reducing the master clock frequency by a factor of two
will halve the above frequencies and update rate and will double
the calibration time.
The current drawn from the DVDD power supply is also directly
related to fCLK IN. Reducing fCLK IN by a factor of two will halve
the DVDD current but will not affect the current drawn from the
AVDD power supply.
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 AD7713 will start operating internally before the DVDD line has reached its minimum
operating level, +4.75 V. With a low DVDD voltage, the
AD7713’s internal digital filter logic does not operate correctly.
Thus, the AD7713 may have clocked itself into an incorrect
operating condition by the time that DVDD has reached its correct level. The digital filter will be reset upon issue of a calibration command (whether it is self-calibration, system calibration
or background calibration) to the AD7713. This ensures correct
operation of the AD7713. In systems where the power-on default conditions of the AD7713 are acceptable, and no calibration is performed after power-on, issuing a SYNC pulse to the
AD7713 will reset the AD7713’s digital filter logic. An R, C on
the SYNC line, with R, C time constant longer than the DVDD
power-on time, will perform the SYNC function.
Autocalibration on the AD7713 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 AD7713 is in its background calibration
mode, the above changes are all automatically taken care of
(after the settling time of the filter has been allowed for).
The AD7713 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 to output 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.
–16–
REV. C
AD7713
The AD7713 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 which is subtracted from all conversion
results, while the full-scale calibration register contains a value
which 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 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, the DRDY line should be ignored for up
to one modulator cycle after the last bit of the calibration command is written to the control register.
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 (i.e., AIN1(+) = AIN1(–) =
VBIAS for AIN1 and AIN2 and AIN3 = VBIAS for AIN3 ) and the
full-scale point is VREF. The zero-scale coefficient is determined
by converting an internal shorted inputs node. The full-scale coefficient is determined from the span between this shorted inputs conversion and a conversion on an internal VREF node. The
self-calibration 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 inputs node is
switched in to 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 AD7713 calibrates are midscale (bipolar
zero) and positive full scale.
System Calibration
System calibration allows the AD7713 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 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 will signal 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
REV. C
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 will adjust the zero-scale or
offset point but will not change the slope factor from what was
set during a full system calibration sequence.
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 the unipolar mode, the system offset calibration is performed between
the two endpoints of the transfer function; in the bipolar mode,
it is performed between midscale and positive full scale.
Background Calibration
The AD7713 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 as are used in
the self-calibration mode, i.e., 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
AD7713 by a factor of six while the –3 dB bandwidth remains
unchanged. Its 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 AD7713 will be incorrect as 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 IV 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.
–17–
2
AD7713
Table IV. 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
VREF
VREF
AIN
Shorted Inputs
Span and Offset Limits
rent 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.
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 for
AIN1 and AIN2 has a minimum value of 0.8 × VREF/GAIN and
a maximum value of 2.1 × VREF/GAIN. For AIN3, the minimum value is 3.2 × VREF/GAIN while the maximum value is
4.2 × VREF/GAIN.
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.
The amount of offset which 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 for AIN1 and AIN2.
Therefore, the offset range plus the span range cannot exceed
1.05 × VREF/GAIN for AIN1 and AIN2. If the span is at its
minimum (0.8 × VREF/GAIN) the maximum the offset can be is
(0.25 × VREF/GAIN) for AIN1 and AIN2. For AIN3, both
ranges are multiplied by a factor of four.
POWER SUPPLIES AND GROUNDING
In the 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
zero-scale point, thus the offset range plus half the span range
cannot exceed (1.05 × VREF/GAIN) for AIN1 and AIN2. 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) for AIN1. If the span range is set to the minimum
± (0.4 × VREF/GAIN), the maximum allowable offset range is
± (0.65 × VREF/GAIN) for AIN1 and AIN2. The AIN3 input can
only be used in the unipolar mode..
POWER-UP AND CALIBRATION
On power-up, the AD7713 performs an internal reset which 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 analog and digital supplies to the AD7713 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. 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.
It is also important that power is applied to the AD7713 before
signals at REF IN, AIN or the logic input pins in order to avoid
excessive current. If separate supplies are used for the AD7713
and the system digital circuitry, then the AD7713 should be
powered up first. If it is not possible to guarantee this, then current limiting resistors should be placed in series with the logic
inputs.
ANALOG
SUPPLY
10µF
The power dissipation and temperature drift of the AD7713 are
low and no warm-up time is required before the initial calibration is performed. However, the external reference must have
stabilized before calibration is initiated.
DIGITAL +5V
SUPPLY
0.1µF
0.1µF
AVDD
DVDD
AD7713
Drift Considerations
The AD7713 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 cur-
–18–
Figure 9. Recommended Decoupling Scheme
REV. C
AD7713
DIGITAL INTERFACE
The AD7713’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 AD7713 can access data from the output
register, the control register or from the calibration registers. A
serial write to the AD7713 can write data to the control register
or the calibration registers.
Two different modes of operation are available, optimized for
different types of interface where the AD7713 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 AD7713). These
two modes, labelled self-clocking mode and external clocking
mode, are discussed in detail in the following sections.
Self-Clocking Mode
The AD7713 is configured for its self-clocking mode by tying
the MODE pin high. In this mode, the AD7713 provides the
serial clock signal used for the transfer of data to and from the
AD7713. 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 serial
parallel 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 Figure 10 and Figure 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 from 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 will
remain low. The output register will continue to be updated at
the output update rate but DRDY will not indicate this. A read
from the device in this circumstance will access 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 only be accessed from the output data register when
DRDY is low. If RFS goes low with DRDY 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.
Figure 10 shows a timing diagram for reading from the AD7713
in the self-clocking mode. This read operation shows a read
from the AD7713’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 AD7713. 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
AD7713 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. This means that the data hold
time for the LSB is slightly shorter than for all other bits.
DRDY (O)
t3
t2
A0 (I)
t4
t5
RFS (I)
t6
t9
SCLK (O)
t8
t7
SDATA (O)
MSB
t 10
LSB
Figure 10. Self-Clocking Mode, Output Data Read Operation
REV. C
–19–
3-STATE
2
AD7713
A0 (I)
t 14
t 15
TFS (I)
t 16
t9
t 17
SCLK (O)
t 18
t 10
t 19
SDATA (I)
MSB
LSB
Figure 11. Self-Clocking Mode, Control/Calibration Register Write Operation
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 the write operation 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 to the
respective register.
Figure 11 shows a write operation to the AD7713. 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 AD7713 must be valid
on the rising edge of this SCLK signal. Data is clocked into the
AD7713 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 AD7713. Subsequent to the next falling edge of
SCLK, the SCLK output is turned off. (The timing diagram of
Figure 11 assumes a pull-up resistor on the SCLK line.)
External Clocking Mode
The AD7713 is configured for its external clocking mode by
tying the MODE pin low. In this mode, SCLK of the AD7713
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 which provide a serial
clock output which is synchronized to the serial data output,
including microcontrollers such as the 80C51, 87C51, 68HC11
and 68HC05 and most digital signal processors.
Read Operation
As with the 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.
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 will
remain low. The output register will continue to be updated at
the output update rate, but DRDY will not indicate this. A read
from the device in this circumstance will access the most recent
word in the output register. If a new data word becomes avail-
able 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 register.
Data can only be accessed from the output data register 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.
Figures 12a and 12b show timing diagrams for reading from the
AD7713 in the external clocking mode. Figure 12a shows a
situation where all the data is read from the AD7713 in one
read operation. Figure 12b shows a situation where the data is
read from the AD7713 over a number of read operations. Both
read operations show a read from the AD7713’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 12a shows a read operation from the AD7713 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 AD7713, 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
–20–
REV. C
AD7713
DRDY (O)
t 21
t 20
A0 (I)
t 22
t 23
RFS (I)
2
t 28
t 26
SCLK (I)
t 24
t 29
t 27
t 25
3-STATE
SDATA (O)
LSB
MSB
Figure 12a. External Clocking Mode, Output Data Read Operation
DRDY (O)
t20
A0 (I)
t22
RFS (I)
t26
t30
SCLK (I)
t24
t27
t25
BIT N
MSB
SDATA (O)
t24
t31
3-STATE
t25
BIT N+1
Figure 12b. External Clocking Mode, Output Data Read Operation (RFS Returns High During Read Operation)
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 AD7713
must be valid on the high level of the externally applied SCLK
signal. Data is clocked into the AD7713 on the high level of this
SCLK signal with the MSB transferred first. On the last active
high time of SCLK, the LSB is loaded to the AD7713.
RFS, the next bit (BIT N + 1) may appear on the databus 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 per
Figure 12a.
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 the write operation 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 to the respective register.
Figure 13b shows a timing diagram for a write operation to the
AD7713 with TFS returning high during the write 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 AD7713 with TFS
remaining low for the duration of the write operation. A0 determines whether a write operation transfers data to the control
A0 (I)
t 32
t 33
TFS (I)
t 26
t 34
SCLK (I)
t 35
t 27
t 36
SDATA (I)
LSB
MSB
Figure 13a. External Clocking Mode, Control/Calibration Register Write Operation
REV. C
–21–
AD7713
A0 (I)
t 32
TFS (I)
t 26
t 30
SCLK (I)
t 27
SDATA (I)
t 36
t 36
t 35
t 35
BIT N
MSB
BIT N+1
Figure 13b. External Clocking Mode, Control/Calibration Register Write Operation (TFS Returns High During
Write Operation)
Data to be loaded to the AD7713 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 AD7713 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 AD7713.
The flowchart of Figure 15 is for continuous read operations
from the AD7713 output register. In the example shown, the
DRDY line is continuously polled. Depending on the microprocessor configuration, the DRDY line may come to an interrupt
input in which case the 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
SIMPLIFYING THE EXTERNAL CLOCKING MODE
INTERFACE
START
In many applications, the user may not require the facility of
writing to the on-chip calibration registers. In this case, the
serial interface to the AD7713 in external clocking modecan be
simplified by connecting the TFS line to the A0 input of the
AD7713 (see Figure 14). This means that any write to the device will load data to the control register (since 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 (since A0 is
high while RFS is low). It should be noted that in this arrangement the user does not have the capability of reading from the
control register.
CONFIGURE &
INITIALIZE µC/µP
SERIAL PORT
BRING RFS, TFS
HIGH
POLL DRDY
RFS
FOUR
INTERFACE
LINES
SDATA
SCLK
DRDY
LOW?
AD7713
NO
YES
TFS
A0
BRING RFS LOW
Figure 14. Simplified Interface with TFS Connected to A0
x3
READ SERIAL BUFFER
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.
BRING RFS HIGH
MICROCOMPUTER/MICROPROCESSOR INTERFACING
The AD7713’s flexible serial interface allows for easy interface
to most microcomputers and microprocessors. Figure 15 shows
a flowchart diagram for a typical programming sequence for
reading data from the AD7713 to a microcomputer while Figure
16 shows a flowchart diagram for writing data to the AD7713.
Figures 17, 18 and 19 show some typical interface circuits.
–22–
REVERSE ORDER OF
BITS
Figure 15. Flowchart for Continuous Read Operations to
the AD7713
REV. C
AD7713
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 when
reading 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 AD7713 outputs the MSB first.
AD7713 to 8051 Interface
Figure 17 shows an interface between the AD7713 and the
8XC51 microcontroller. The AD7713 is configured for its external clocking mode while the 8XC51 is configured in its Mode 0
serial interface mode. The DRDY line from the AD7713 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.
The flowchart for Figure 16 is for a single 24-bit write operation
to the AD7713 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
will allow data to be written directly to the serial buffer from
data memory. The writing of data to the serial buffer from the
accumulator will generally consist of either two or three write
operations, depending on the size of the serial buffer.
The flowchart also shows the option of the bits being reversed
before being written to the serial buffer. This depends on
whether the first bit transmitted by the microprocessor is the
MSB or the LSB. The AD7713 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 will
have to be reversed for every byte.
START
CONFIGURE &
INITIALIZE µC/µP
SERIAL PORT
BRING RFS, TFS &
A0 HIGH
DVDD
SYNC
8XC51
P1.0
RFS
P1.1
TFS
P1.2
DRDY
P1.3
A0
P3.0
SDATA
P3.1
SCLK
AD7713
MODE
Figure 17. AD7713 to 8XC51 Interface
Table V shows some typical 8XC51 code used for a single
24-bit read from the output register of the AD7713. Table V
shows some typical code for a single write operation to the control register of the AD7713. The 8XC51 outputs the LSB first
in a write operation while the AD7713 expects the MSB first, so
the data to be transmitted has to be rearranged before being
written to the output serial register. Similarly, the AD7713 outputs the MSB first during a read operation while the 8XC51
expects the LSB first. Therefore, the data which is read into the
serial buffer needs to be rearranged before the correct data word
from the AD7713 is available in the accumulator.
Table V. 8XC51 Code for Reading from the AD7713
LOAD DATA FROM
ADDRESS TO
ACCUMULATOR
MOV SCON,#00010001B;
MOV IE,#00010000B;
SETB 90H;
SETB 91H;
SETB 93H;
MOV R1,#003H;
REVERSE ORDER OF
BITS
BRING RFS & A0 LOW
MOV R0,#030H;
x3
WRITE DATA FROM
ACCUMULATOR TO
SERIAL BUFFER
BRING TFS & A0 HIGH
END
Figure 16. Flowchart for Single Write Operation to the
AD7713
MOV R6,#004H;
WAIT:
NOP;
MOV A,P1;
ANL A,R6;
JZ READ;
SJMP WAIT;
READ:
CLR 90H;
CLR 98H;
POLL:
JB 98H, READ1
SJMP POLL
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
A Read Operation
Start Address for Where Bytes Will
Be Loaded
Use P1.2 as DRDY
Read Port 1
Mask Out All Bits Except DRDY
If Zero Read
Otherwise Keep Polling
Bring RFS Low
Clear Receive Flag
Tests Receive Interrupt Flag
continued on next page
REV. C
–23–
2
AD7713
READ 1:
MOV A,SBUF;
RLC A;
Read Buffer
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
which is in its single chip mode. The DRDY line from the
AD7713 is connected to the Port PC0 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 wired-or operation. Depending on the
interface configuration, it may be necessary to provide bidirectional buffers between the 68HC11’s MOSI and MISO lines.
The 68HC11 is configured in the master mode with its CPOL
bit set to a logic zero and its CPHA bit set to a logic one.
DVDD
SS
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;
Configure 8051 for MODE 0
Operation & 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
Wait for Interrupt
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
SYNC
PC0
68HC11
Table VI. 8XC51 Code for Writing to the AD7713
MOV SCON,#00000000B;
DVDD
RFS
PC1
TFS
PC2
DRDY
PC3
A0
SCK
SCLK
MISO
SDATA
MOSI
MODE
AD7713
Figure 18. AD7713 to 68HC11 Interface
AD7713 to ADSP-2105 Interface
An interface circuit between the AD7713 and the ADSP-2105
microprocessor is shown in Figure 19. In this interface, the
AD7713 is configured for its self-clocking mode while the RFS
and TFS pins of the ADSP-2105 are configured as inputs and
the ADSP-2105 serial clock line is also configured as an input.
When the ADSP-2105’s serial clock is configured as an input it
needs a couple of clock pulses to initialize itself correctly before
accepting data. Therefore, the first read from the AD7713 may
not read correct data. In the interface shown, a read operation
to the AD7713 accesses either the output register or the calibration registers. Data cannot be read from the control register. A
write operation always writes to the control or calibration
registers.
DRDY is used as the frame synchronization pulse for read operations from the output register and it is decoded with A0 to
drive the RFS inputs of both the AD7713 and the ADSP-2105.
The latched A0 line drives the TFS inputs of both the AD7713
and the ADSP-2105 as well as the AD7713 A0 input.
DVDD
MODE
RFS
RFS
ADSP-2105
DRDY
TFS
A0
AD7713
D
Q
A0
74HC74
DMWR
DR
AD7713 to 68HC11 Interface
Q
TFS
SDATA
DT
Figure 18 shows an interface between the AD7713 and the
68HC11 microcontroller. The AD7713 is configured for its external clocking mode while the SPI port is used on the 68HC11
SCLK
SCLK
Figure 19. AD7713 to ADSP-2105 Interface
–24–
REV. C
AD7713
APPLICATIONS
Four-Wire RTD Configurations
Figure 20 shows a four-wire RTD application where the RTD
transducer is interfaced directly to the AD7713. In the four-wire
configuration, there are no errors associated with lead resistances as 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 Ω and, therefore,
the RTD will generate a 20 mV signal which can be handled directly by the analog input of the AD7713. In the circuit shown,
the second RTD excitation current is used to generate the reference voltage for the AD7713. 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 kΩ. 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 two RTD current sources is less than 3 ppm/°C.
voltage is developed across RL3 but since this is a common-mode
voltage it will not introduce any errors. 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 20.
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(–)
REF IN(+)
200µA
RTD1
12.5kΩ
RL1
INTERNAL
CIRCUITRY
AIN(+)
RTD
PGA
AIN(–)
A = 1 – 128
RL2
RTD2
RL3
AGND
AD7713
200µA
DGND
+5V
AVDD
Figure 21. Three-Wire RTD Application with the AD7713
DVDD
4–20 mA Loop
200µA
The AD7713’s high level input can be used to measure the current in 4–20 mA loop applications as shown in Figure 22. In this
case, the system calibration capabilities of the AD7713 can be
used to remove the offset caused by the 4 mA flowing through
the 500 Ω resistor. The AD7713 can handle an input span as
low as 3.2 × VREF (= 8 V with a VREF of +2.5 V) even though the
nominal input voltage range for the input is 10 V. Therefore, the
full span of the A/D converter can be used for measuring the
current between 4 mA and 20 mA.
RTD2
REF IN(+)
RREF
INTERNAL
CIRCUITRY
REF IN(–)
200µA
RTD1
AD7713
ANALOG +5V SUPPLY
AIN1(+)
AVDD
DVDD
REF IN(–) REF IN(+)
PGA
RTD
AIN1(–)
AVDD
A = 1 – 128
AGND
1µA
INTERNAL
CIRCUITRY
DGND
AIN1(+)
M
U
X
AIN1(–)
Figure 20. Four-Wire RTD Application with the AD7713
AIN3
Three-Wire RTD Configurations
Figure 21 shows a three-wire RTD configuration using the
AD7713. In the three-wire configuration, the lead resistances
will result in errors if only one current source is used as the
200 µA 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 µA 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
REV. C
–25–
4–20mA
LOOP
PGA
VOLTAGE
ATTENUATION
A = 1 – 128
AD7713
500Ω
AGND
DGND
Figure 22. 4–20 mA Measurement Using the AD7713
2
AD7713
OTHER 24-BIT SIGNAL CONDITIONING ADCS AVAILABLE
FROM ANALOG DEVICES
AD7710
FUNCTIONAL BLOCK DIAGRAM
FEATURES
Charge Balancing ADC
24 Bits No Missing Codes
60.0015% Nonlinearity
Two-Channel Programmable Gain Front End
Gains from 1 to 128
Differential Inputs
Low-Pass Filter with Programmable Filter Cutoffs
Ability to Read/Write Calibration Coefficients
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}
REF
IN(+)
REF
IN(–)
AVDD DVDD
VBIAS
REF OUT
AVDD
2.5V REFERENCE
4.5µA
CHARGING BALANCING A/D
CONVERTER
AIN1(+)
M
U
X
AIN1(–)
AIN2(+)
AUTO-ZEROED
∑−∆
MODULATOR
PGA
AIN2(–)
A = 1 – 128
CLOCK
GENERATION
AVDD
20µA
SYNC
MCLK
IN
MCLK
OUT
SERIAL INTERFACE
CONTROL
REGISTER
RTD
CURRENT
APPLICATIONS
Weigh Scales
Thermocouples
Process Control
Smart Transmitters
Chromatography
DIGITAL
FILTER
OUTPUT
REGISTER
AD7710
AGND DGND VSS RFS TFS MODE SDATA SCLK DRDY A0
AD7711
FUNCTIONAL BLOCK DIAGRAM
FEATURES
Charge Balancing ADC
24 Bits No Missing Codes
60.0015% Nonlinearity
Two-Channel Programmable Gain Front End
Gains from 1 to 128
One Differential Input
One 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)
REF
IN(–)
AVDD DVDD
REF
IN(+)
VBIAS
REF OUT
AVDD
2.5V REFERENCE
4.5µA
CHARGING BALANCING A/D
CONVERTER
AIN1(+)
M
U
X
AIN1(–)
AIN2
AUTO-ZEROED
∑−∆
MODULATOR
PGA
DIGITAL
FILTER
A = 1 – 128
200µA
CLOCK
GENERATION
AVDD
RTD1
200µA
MCLK
IN
MCLK
OUT
SERIAL INTERFACE
CONTROL
REGISTER
RTD2
SYNC
OUTPUT
REGISTER
AD7711
APPLICATIONS
RTD Transducers
Process Control
Smart Transmitters
Portable Industrial Instruments
AGND DGND VSS RFS TFS MODE SDATA SCLK DRDY A0
–26–
REV. C
AD7713
FUNCTIONAL BLOCK DIAGRAM
AD7712
FEATURES
Charge Balancing ADC
24 Bits No Missing Codes
60.0015% Nonlinearity
High Level and Low Level Analog Input Channels
Programmable Gain for Both Inputs
Gains from 1 to 128
Differential Input for Low Level Channel
Low-Pass Filter with Programmable Filter Cutoffs
Ability to Read/Write Calibration Coefficients
Bidirectional Microcontroller Serial Interface
Internal/External Reference Option
Single or Dual Supply Operation
Low Power (25 mW typ) with Power-Down Mode
(100 mW typ}
AVDD DVDD
REF
IN(+)
VBIAS
REF OUT
AVDD
2.5V REFERENCE
4.5µA
CHARGING BALANCING A/D
CONVERTER
SYNC
AIN1(+)
AIN1(–)
M
U
X
AUTO-ZEROED
∑−∆
MODULATOR
PGA
DIGITAL
FILTER
A = 1 – 128
AIN2
CLOCK
GENERATION
VOLTAGE
ATTENUATION
SERIAL INTERFACE
TP
CONTROL
REGISTER
OUTPUT
REGISTER
AD7712
APPLICATIONS
Process Control
Smart Transmitters
Portable Industrial Instruments
REV. C
REF
IN(–)
AGND DGND VSS RFS TFS MODE SDATA SCLK DRDY A0
–27–
STANDBY
MCLK
IN
MCLK
OUT
2
AD7713
OUTLINE DIMENSIONS
Dimensions are shown in inches and (mm).
Plastic DIP (N-24)
13
C1657b–5–7/95
24
0.280 (7.11)
0.240 (6.10)
1
12
0.325 (8.25)
0.300 (7.62)
1.275 (32.30)
1.125 (28.60)
0.060 (1.52)
0.015 (0.38)
0.210
(5.33) MAX
0.195 (4.95)
0.115 (2.93)
SEATING PLANE
0.015 (0.381)
0.008 (0.204)
0.200 (5.05)
0.125 (3.18)
0.022 (0.558)
0.014 (0.356)
0.100
(2.54) BSC
0.150
(3.81) MIN
0.070 (1.77)
0.045 (1.15)
0 - 15
Cerdip (Q-24)
0.005 (0.13) MIN
0.098 (2.49) MAX
24
13
1
12
0.320 (8.13)
0.290 (7.37)
0.310 (7.87)
0.220 (5.59
1.280 (32.51) MAX
0.200
(5.08)
MAX
0.060 (1.52)
0.015 (0.38)
0.150
(3.81)
MIN
0.200 (5.08)
0.125 (3.18)
0.023 (0.58)
0.014 (0.36)
0.110 (2.79)
0.090 (2.29)
0.070 (1.78)
0.030 (0.76)
SEATING PLANE
0° – 15°
0.015 (0.38)
0.008 (0.20)
SOIC (R-24)
15.6 (0.614)
15.2 (0.598)
24
13
PRINTED IN U.S.A.
0.299 (7.6)
0.291 (7.4)
0.419 (10.65)
0.394 (10.00)
1
0.050 (1.27) BSC
12
0.019 (0.49)
0.014 (0.35)
0.104 (2.65)
0.093 (2.35)
0.012 (0.3)
0.004 (0.1)
–28–
0.013 (0.32)
0.009 (0.23)
0.050 (1.27)
0.016 (0.40)
REV. C