Datasheet

Data Sheet AD8422
Rev. 0 | Page 23 of 24
APPLICATIONS INFORMATION
PRECISION BRIDGE CONDITIONING
With its high CMRR, low drift, and rail-to-rail output, the
AD8422 is an excellent choice for conditioning a signal from a
Wheatstone bridge. With appropriate supply voltages, the gain
and reference pin voltage can be adjusted to match the full-scale
bridge output to any desired output range, such as 0 V to 5 V.
Figure 65 shows a circuit to convert a bridge signal into a 4 mA
to 20 mA output using the AD8276 low power, precision difference
amplifier, and the ADA4096-2 low power, rail-to-rail input and
output, overvoltage protected op amp. With high precision bridge
circuits, care must be taken to compensate offsets and temperature
errors. For example, if the voltage at the REF pin is used to
compensate for the bridge offset, ensure that the AD8422 is within
its operating range for the maximum expected offset. If the zero-
adjust potentiometer is excluded, connect the positive op amp
input to the center of the 24.9 kΩ, 10.7 kΩ divider, which is at
1.5 V. If lower supply voltages are used for the AD8276 and the
ADA4096-2, ensure that the desired output voltage of the AD8276
is within its output range, and V
L
is within the input and output
range of the ADA4096-2. The transistor must have sufficient
breakdown voltage and I
C
. Low cost transistors, such as the BC847
or 2N5210, are recommended.
PROCESS CONTROL ANALOG INPUT
In process control systems such as programmable logic controllers
(PLC) and distributed control systems (DCS), analog variables
typically occur in just a few standard voltage or current ranges,
including 4 mA to 20 mA and ±10 V. Variables within these input
ranges must often be gained or attenuated and level shifted to
match a specific ADC input range such as 0 V to 5 V. The circuit in
Figure 66 shows one way this can be done with a single AD8422.
Low power, overvoltage protection, and high precision make the
AD8422 a good match for process control applications, and high
input impedance, low bias current, and low current noise allow
significant source resistance with minimum additional errors.
+5
V
+5V
+5V
REF
REF
SENSE
+24V
+24V
+24V
+24V
+IN
10.7kΩ
24.9kΩ
124Ω
1
1
OPTIONAL
ZERO
ADJUST
–IN
+IN
–IN
V
OUT_FS
= ±15mV
I
OUT
= 4mA TO 20mA
V = 0.5V TO 2.5V
R
L
V
L
R
G
= 301Ω
G = 66.8V/V
R
G
V
OUT
AD8422
ADA4096-2
AD8276
ADA4096-2
11197-066
Figure 65. Bridge Circuit with 4 mA to 20 mA Output
+15V
–15V
2.5V
REF
+IN
42.2kΩ
0V TO 10V, ±10V
TERMIN
A
L
BLOCK
0V TO 5V, ±5V
0V TO 1V, ±1V
4mA TO 20mA,
0mA TO 20mA
±20mA
34kΩ
8.45kΩ
1kΩ
1kΩ
49.9kΩ
–IN
R
G
AD8422
R
G
= 13.2kΩ
G = 2.5V/V
V
OUT
= 2.5V ±2.5V
11197-067
Figure 66. Process Control Analog Input