AD629A AD [Analog Devices], AD629A Datasheet - Page 9

• Manufacturer Part Number: AD629A
• Description: High Common-Mode Voltage Difference Amplifier
• Manufacturer: AD [Analog Devices]

Corner Frequency
No Filter
50 kHz
5 kHz
500 Hz
50 Hz
Figure 31. Optimal Grounding Practice for a Bipolar Supply
Environment with Separate Analog and Digital Supplies
Figure 32. 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 32 shows how to
minimize interference between the digital and analog circuitry.
In this example, the ADC’s reference is used to drive the
AD629’s REF(+) and REF(–) pins. This means that the reference
must be capable of sourcing and sinking a current equal to V
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 be connected at the power supply’s ground pin.
Separate traces (or power planes) should be 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 shunt resistance across the input Pins 2 and 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
REV. A
+IN
–IN
+IN
–IN
–V
AD629
REF(–)
0.1 F 0.1 F
REF(–)
AD629
S
+V
S
0.1 F
REF(+)
REF(+)
–V
V
V
+V
–5V
S
OUT
OUT
ANALOG POWER
S
SUPPLY
+5V
R1
2.94 kΩ ± 1%
2.94 kΩ ± 1%
2.94 kΩ ± 1%
2.7 kΩ ± 10%
POWER SUPPLY
V
V
+5V
V
IN
REF
V
DD
V
V
GND
IN1
IN2
DD
0.1 F
0.1 F
AD7892-2
AGND DGND
AGND DGND
Table II. Recommended Values for 2-Pole Butterworth Filter
ADC
GND
12
SHUNT
R2
1.58 kΩ ± 1%
1.58 kΩ ± 1%
1.58 kΩ ± 1%
1.5 kΩ ± 10%
POWER SUPPLY
PROCESSOR
PROCESSOR
V
GND
GND
DD
DIGITAL
0.1 F
. Table I
0.1 F
GND
+5V
V
DD
CM
/
–9–
C1
2.2 nF ± 10%
22 nF ± 10%
220 nF ± 10%
2.2 µF ± 20%
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 has been selected to utilize the
full ± 10 V output swing of the AD629, the error voltage becomes
quite significant as R
R
20
1000
2000
If it is desired to measure low current or current near zero in a
high common-mode environment, an external resistor equal to
the shunt resistor value may be added to the low impedance side
of the shunt resistor as shown in Figure 33.
Output Filtering
A simple 2-pole low-pass Butterworth filter can be implemented
using the OP177 at the output of the AD629 to limit noise at
the output, as shown in Figure 34. Table II gives recommended
component values for various corner frequencies, along with the
peak-to-peak output noise for each case.
–V
–IN
+IN
S
0.1 F
S
( )
Figure 34. Filtering of Output Noise Using a 2-Pole
Butterworth Filter
Figure 33. Compensating for Large Sense Resistors
REF(–)
Table I. Error Resulting from Large Values of R
(Uncompensated Circuit)
I
–V
SHUNT
S
1
2
3
4
NC = NO CONNECT
380k
21.1k
380k
–V
R
R
SHUNT
S
COMP
Error V
0.01
0.498
1
AD629
380k
0.1 F
20k
REF(–)
SHUNT
C2
1 nF ± 10%
10 nF ± 10%
0.1 µF ± 10%
1 µF ± 20%
–V
–IN
+IN
S
8
7
6
5
OUT
1
2
3
4
+V
increases.
NC
REF(+)
NC = NO CONNECT
21.1k
380k
380k
S
+V
0.1 F
(V)
S
R1
AD629
380k
20k
C1
R2
Error Indicated (mA)
0.5
0.498
0.5
C2
8
7
6
5
Output Noise (p-p)
3.2 mV
1 mV
0.32 mV
100 µV
32 µV
NC
+V
REF(+)
OP177
S
+V
–V
+V
S
S
AD629
S
0.1 F
0.1 F
0.1 F
SHUNT
V
OUT
V
OUT