AD622 Analog Devices, AD622 Datasheet - Page 10

• Manufacturer Part Number: AD622
• Description: Low Cost Instrumentation Amplifier
• Manufacturer: Analog Devices

AD622
GROUNDING
Since the AD622 output voltage is developed with respect to the
potential on the reference terminal, it can solve many grounding
problems by simply tying the REF pin to the appropriate “local
ground.” The REF pin should however be tied to a low imped-
ance point for optimal CMR.
The use of ground planes is recommended to minimize the
impedance of ground returns (and hence the size of dc errors).
In order to isolate low level analog signals from a noisy digital
environment, many data-acquisition components have separate
analog and digital ground returns (Figure 16). All ground pins
from mixed signal components such as analog to digital converters
should be returned through the “high quality” analog ground
plane. Maximum isolation between analog and digital is
achieved by connecting the ground planes back at the supplies.
The digital return currents from the ADC which flow in the
analog ground plane will in general have a negligible effect on
noise performance.
AD622
0.1 F
Figure 16. Basic Grounding Practice
+5V
ANALOG P.S.
–5V
V
V
V
DD
IN
IN
2
1
0.1 F
AGND
AD7892-2
C
DGND
12
DIGITAL P.S.
PROCESSOR
V
C
DD
0.1 F
+5V
GND
–10–
GROUND RETURNS FOR INPUT BIAS CURRENTS
Input bias currents are those currents necessary to bias the input
transistors of an amplifier. There must be a direct return path
for these currents; therefore when amplifying “floating” input
sources such as transformers, or ac-coupled sources, there must
be a dc path from each input to ground as shown in Figure 17.
Refer to the Instrumentation Amplifier Application Guide (free
from Analog Devices) for more information regarding in amp
applications.
Figure 17a. Ground Returns for Bias Currents with
Transformer Coupled Inputs
Figure 17b. Ground Returns for Bias Currents with
Thermocouple Inputs
Figure 17c. Ground Returns for Bias Currents with
AC Coupled Inputs
100k
–INPUT
+INPUT
–INPUT
+INPUT
–INPUT
+INPUT
100k
R
R
G
G
R
2
1
8
3
2
1
8
3
G
AD622
AD622
–V
AD622
–V
4
4
+V
S
+V
–V
S
7
7
S
+V
REFERENCE
S
S
REFERENCE
S
REFERENCE
5
5
6
6
LOAD
LOAD
LOAD
TO POWER
SUPPLY
GROUND
TO POWER
SUPPLY
GROUND
V
V
OUT
TO POWER
SUPPLY
GROUND
OUT
V
OUT
REV. C