
AD629
Rev. B | Page 11 of 16 
ANALOG POWER
SUPPLY
+5V
–5V
DIGITAL
POWER SUPPLY
GND
0.1μF
0.1μF
0.1μF
0.1μF
+IN
–IN
–V
S
V
IN1
V
IN2
V
DD
V
DD
OUTPUT
AGND
GND
MICROPROCESSOR
DGND
+V
S
AD629
AD7892-2
REF(–) REF(+)
1
6
7
14
4
1
3
3
2
6
4
5
12
GND
+5V
0
Figure 32. Optimal Grounding Practice for a Bipolar Supply Environment 
with Separate Analog and Digital Supplies 
POWER SUPPLY
+5V
V
IN1
V
IN2
V
DD
AGND DGND
ADC
0.1μF
0.1μF
+IN
–IN
+V
S
OUTPUT
–V
S
AD629
REF(–) REF(+)
1
4
7
3
2
6
5
V
DD
GND
MICROPROCESSOR
GND
0.1μF
0
Figure 33. Optimal Ground Practice in a Single-Supply Environment 
If there is only a single power supply available, it must be shared 
by both digital and analog circuitry. Figure 33 shows how to 
minimize interference between the digital and analog circuitry. 
In this example, the ADC’s reference is used to drive Pin REF(+) 
and Pin REF(–). This means that the reference must be capable 
of sourcing and sinking a current equal to V
CM
/200 kΩ. As in 
the previous case, separate analog and digital ground planes 
should be used (reasonably thick traces can be used as an 
alternative to a digital ground plane). These ground planes 
should connect at the power supply’s ground pin. Separate 
traces (or power planes) should run from the power supply to 
the supply pins of the digital and analog circuits. Ideally, each 
device should have its own power supply trace, but these can be 
shared by a number of devices, as long as a single trace is not 
used to route current to both digital and analog circuitry.  
USING A LARGE SENSE RESISTOR 
Insertion of a large value shunt resistance across the input pins, 
Pin 2 and Pin 3, will imbalance the input resistor network, 
introducing a common-mode error. The magnitude of the error 
will depend on the common-mode voltage and the magnitude 
of R
SHUNT
.  
Table 3 shows some sample error voltages generated by a 
common-mode voltage of 200 V dc with shunt resistors from 
20 Ω to 2000 Ω. Assuming that the shunt resistor is selected to 
use the full ±10 V output swing of the AD629, the error voltage 
becomes quite significant as R
SHUNT
 increases. 
Table 3. Error Resulting from Large Values of R
SHUNT
(Uncompensated Circuit) 
R
S
 (Ω)  
Error V
OUT
 (V)  
20  
0.01  
1000  
0.498  
2000  
1  
Error Indicated (mA)  
0.5  
0.498  
0.5  
To measure low current or current near zero in a high common-
mode environment, an external resistor equal to the shunt 
resistor value can be added to the low impedance side of the 
shunt resistor, as shown in Figure 34. 
REF (–)
REF (+)
–V
S
–V
S
+V
S
+V
S
V
OUT
NC
–IN
+IN
R
SHUNT
R
COMP
I
SHUNT
0.1μF
0.1μF
NC = NO CONNECT
21.1k
380k
380k
20k
380k
AD629
1
2
3
4
8
7
6
5
0
Figure 34. Compensating for Large Sense Resistors  
OUTPUT FILTERING 
A simple 2-pole, low-pass Butterworth filter can be implemented 
using the OP177 after the AD629 to limit noise at the output, as 
shown in Figure 35. Table 4 gives recommended component 
values for various corner frequencies, along with the peak-to-
peak output noise for each case. 
REF (–)
REF (+)
–V
S
–V
S
+V
S
+V
S
+V
S
V
OUT
NC
–IN
+IN
0.1μF
0.1μF
0.1μF
0.1μF
NC = NO CONNECT
21.1k
380k
380k
20k
380k
AD629
1
2
3
4
8
7
6
5
0
R1
R2
C1
C2
OP177
Figure 35. Filtering of Output Noise Using a 2-Pole Butterworth Filter 
Table 4. Recommended Values for 2-Pole Butterworth Filter 
Corner Frequency  
R1  
No Filter  
50 kHz  
2.94 kΩ ± 1%  
5 kHz  
2.94 kΩ ± 1%  
500 Hz  
2.94 kΩ ± 1%  
50 Hz  
2.7 kΩ ± 10%  
R2  
1.58 kΩ ± 1%  
1.58 kΩ ± 1%  
1.58 kΩ ± 1%  
1.5 kΩ ± 10%  
C1  
2.2 nF ± 10%  
22 nF ± 10%  
220 nF ± 10%  
2.2 μF ± 20%  
C2  
1 nF ± 10%  
10 nF ± 10%  
0.1 μF ± 10%  
1 μF ± 20%  
Output Noise (p-p)  
3.2 mV  
1 mV  
0.32 mV  
100 μV  
32 μV