AD8420ARMZ-R7 datasheet(25/28 Pages) AD

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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 VREF pin of an analog-to-digital converter (ADC)

whose input range is (VREF ± VIN). With an available output swing

on the AD8420 of (−VS + 100 mV) to (+VS − 150 mV), the

maximum programmable gain is simply this output range divided

by the input range.

+VS

–VS

0.1µF

VOUT

VREF

VDIFF

AD8420

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.

AD627

LINE

IMPEDANCE

4mA TO 20mA

5Ω

4mA TO 20mA

TRANSDUCER

R2 = 97.6kΩ

R1 = 2.49kΩ

0.1µF

AD8420

G = 40

POWER

SUPPLY

–

0.8V TO 4.0V

Figure 71. 4 mA to 20 mA Receiver Circuit

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