Datasheet

Data Sheet AD8420
Rev. 0 | Page 25 of 28
CLASSIC BRIDGE CIRCUIT
Figure 70 shows the AD8420 configured to amplify the signal from
a classic resistive bridge. This circuit works in dual-supply mode or
single-supply mode. Typically, the same voltage that powers the
instrumentation amplifier excites the bridge. Connecting the
bottom of the bridge to the negative supply of the instrumentation
amplifier sets up an input common-mode voltage that is located
midway between the supply voltages. The voltage on the REF pin
can be varied to suit the application. For example, the REF pin
is tied to the V
REF
pin of an analog-to-digital converter (ADC)
whose input range is (V
REF
± V
IN
). With an available output swing
on the AD8420 of (−V
S
+ 100 mV) to (+V
S
− 150 mV), the
maximum programmable gain is simply this output range divided
by the input range.
+
V
S
–V
S
0.1µF
0.1µF
V
OUT
V
REF
V
DIFF
AD8420
09945-069
Figure 70. Classic Bridge Circuit
4 mA TO 20 mA SINGLE-SUPPLY RECEIVER
The 80 µA maximum supply current, input range that goes
below ground, and low drift characteristics make the AD8420 a
very good candidate for use in a 4 mA to 20 mA loop. Figure 71
shows how a signal from a 4 mA to 20 mA transducer can be
interfaced to the AD8420. The signal from a 4 mA to 20 mA
transducer is single-ended, which initially suggests the need for
a simple shunt resistor to ground to convert the current to a voltage.
However, any line resistance in the return path (to the transducer)
adds a current-dependent offset error; therefore, the current must
be sensed differentially.
In this example, a 5 Ω shunt resistor generates a differential voltage
at the inputs of the AD8420 between 20 mV (for 4 mA in) and
100 mV (for 20 mA in) with a very low common-mode value.
With the gain resistors shown, the AD8420 amplifies the 100 mV
input voltage by a factor of 40 to 4.0 V.
09945-073
AD627
LINE
IMPEDANCE
4mA TO 20mA 5
4mA TO 20mA
TRANSDUCER
R2 = 97.6k
R1 = 2.49k
0.1µF
5
V
AD8420
R2R1
G = 40
POWER
SUPPLY
+
–
+
–
0.8V TO 4.0V
Figure 71. 4 mA to 20 mA Receiver Circuit