AD629 查看數據表（PDF） - Analog Devices
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AD629 Datasheet PDF : 16 Pages
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AD629
ANALOG POWER
SUPPLY
–5V
+5V
GND
DIGITAL
POWER SUPPLY
GND +5V
0.1µF
0.1µF 0.1µF
4
7
–VS
+VS
+IN 3
AD629 OUTPUT 6
–IN 2
REF(–) REF(+)
1
5
0.1µF
1
6
14
VDD AGND DGND 12
GND VDD
4 VIN1 AD7892-2
MICROPROCESSOR
3 VIN2
Figure 32. Optimal Grounding Practice for a Bipolar Supply Environment
with Separate Analog and Digital Supplies
POWER SUPPLY
+5V GND
0.1µF
7
4
+IN 3 +VS
–VS
AD629 OUTPUT 6
–IN 2
REF(–) REF(+)
1
5
0.1µF
VDD
VIN1
VIN2
AGND DGND
ADC
0.1µF
VDD GND
MICROPROCESSOR
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 VCM/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 RSHUNT.
Table 4. Recommended Values for 2-Pole Butterworth Filter
Corner Frequency R1
R2
No Filter
50 kHz
2.94 kΩ ± 1%
1.58 kΩ ± 1%
5 kHz
2.94 kΩ ± 1%
1.58 kΩ ± 1%
500 Hz
2.94 kΩ ± 1%
1.58 kΩ ± 1%
50 Hz
2.7 kΩ ± 10%
1.5 kΩ ± 10%
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 RSHUNT increases.
Table 3. Error Resulting from Large Values of RSHUNT
(Uncompensated Circuit)
RS (Ω)
20
1000
2000
Error VOUT (V)
0.01
0.498
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 (–) 21.1kΩ AD629
+VS
1
8 NC
ISHUNT
RCOMP
RSHUNT
–IN 380kΩ 380kΩ
2
+IN 380kΩ
3
–VS 4
20kΩ
7
+VS
0.1µF
6
VOUT
REF (+)
5
–VS
0.1µF
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 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 (–) 21.1kΩ AD629
+VS
1
8 NC
380kΩ 380kΩ
0.1µF
–IN
2
7
+VS
380kΩ
+IN
3
R1
6
–VS
0.1µF
20kΩ
REF (+)
4
5
NC = NO CONNECT
+VS
C1
0.1µF
R2
OP177
0.1µF
C2
–VS
VOUT
Figure 35. Filtering of Output Noise Using a 2-Pole Butterworth Filter
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
Rev. B | Page 11 of 16
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