AD629
ANALOG POWER
SUPPLY
DIGITAL
POWER SUPPLY
GND +5V
Table 3 shows some sample error voltages generated by a
common-mode voltage of 277 ꢁ dc with shunt resistors from
27 Ω to 2777 Ω. Assuming that the shunt resistor is selected to
use the full ±17 ꢁ output swing of the AD629, the error voltage
becomes quite significant as RSHUNT increases.
–5V
+5V
GND
0.1µF
0.1µF
0.1µF 0.1µF
1
6
14
AGND DGND
Table 3. Error Resulting from Large Values of RSHUNT
(Uncompensated Circuit)
4
7
V
DD
V
DD
GND
12
–V
+V
S
S
3
2
+IN
–IN
MICROPROCESSOR
V
V
6
4
3
OUTPUT
AD629
IN1 AD7892-2
RS (Ω)
2ꢀ
Error VOUT (V)
Error Indicated (mA)
REF(–) REF(+)
IN2
ꢀ.ꢀ1
ꢀ.498
1
ꢀ.5
1
5
1ꢀꢀꢀ
2ꢀꢀꢀ
ꢀ.498
ꢀ.5
Figure 32. Optimal Grounding Practice for a Bipolar Supply Environment
with Separate Analog and Digital Supplies
POWER SUPPLY
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.
+5V
GND
0.1µF
0.1µF
0.1µF
+V
S
7
4
AD629
REF (–)
V
AGND DGND
21.1kΩ
DD
+V
–V
S
NC
1
2
3
4
8
7
6
5
S
V
GND
DD
3
2
+IN
–IN
V
V
6
OUTPUT
IN1
AD629
MICROPROCESSOR
R
R
380kΩ 380kΩ
COMP
–IN
+IN
ADC
IN2
REF(–) REF(+)
0.1µF
+V
S
I
1
5
SHUNT
SHUNT
380kΩ
V
OUT
Figure 33. Optimal Ground Practice in a Single-Supply Environment
20kΩ
–V
REF (+)
S
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 ꢁCM/277 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.
0.1µF
–V
S
NC = NO CONNECT
Figure 34. Compensating for Large Sense Resistors
OUTPUT FILTERING
A simple 2-pole, low-pass Butterworth filter can be implemented
using the OP1ꢀꢀ after the AD629 to limit noise at the output, as
shown in Figure 3±. Table 4 gives recommended component
values for various corner frequencies, along with the peak-to-
peak output noise for each case.
+V
S
AD629
REF (–)
21.1kΩ
NC
1
2
3
4
8
7
6
5
+V
S
C1
0.1µF
R1
0.1µF
0.1µF
380kΩ 380kΩ
–IN
+IN
+V
S
V
OP177
OUT
R2
C2
380kΩ
USING A LARGE SENSE RESISTOR
20kΩ
REF (+)
–V
S
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
–V
S
0.1µF
NC = NO CONNECT
Figure 35. Filtering of Output Noise Using a 2-Pole Butterworth Filter
of RSHUNT
.
Table 4. Recommended Values for 2-Pole Butterworth Filter
Corner Frequency
R1
R2
C1
C2
Output Noise (p-p)
No Filter
5ꢀ kHz
5 kHz
5ꢀꢀ Hz
5ꢀ Hz
3.2 mV
1 mV
ꢀ.32 mV
1ꢀꢀ μV
32 μV
2.94 kΩ 1%
2.94 kΩ 1%
2.94 kΩ 1%
2.7 kΩ 1ꢀ%
1.58 kΩ 1%
1.58 kΩ 1%
1.58 kΩ 1%
1.5 kΩ 1ꢀ%
2.2 nF 1ꢀ%
22 nF 1ꢀ%
22ꢀ nF 1ꢀ%
2.2 μF 2ꢀ%
1 nF 1ꢀ%
1ꢀ nF 1ꢀ%
ꢀ.1 μF 1ꢀ%
1 μF 2ꢀ%
Rev. B | Page 11 of 16
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