AD624 Selectable Gain Instrumentation Amplifier

Application

This circuit uses the AD624 as a precision instrumentation-amplifier front end for a small bridge or sensor imbalance.

Schematic

The amplifier is centered with differential inputs on the left, output and sense feedback on the right, a local RG resistor beside the gain pins, and all source returns tied into one lower ground bus.

Design Context

VIN+ is 110 mV and VIN- is 100 mV. RG is 49.9 kOhm for a low-gain operating point, REF is grounded, SENSE is tied to OUT, and the selectable gain terminals are parked at the reference bus.

Expected Behavior

The table should show a small positive output because VIN+ is higher than VIN-. Changing RG or the gain-select connections should change the output scaling.

Key Values

• VCC / VEE: +15 V and -15 V.
• Inputs: 110 mV and 100 mV.
• RG: 49.9 kOhm.
• REF, G100, G200, and G500: tied to return for this baseline setup.
• RLOAD: 10 kOhm.

Suggested Experiments

• Reduce RG to increase gain.
• Reconnect one gain-select terminal according to the intended gain option and compare Vout.
• Move REF to a small positive voltage to offset the output baseline.

Featured Component

U1 is the AD624 Instrumentation Amplifier. IN+ and IN- sense the differential signal, RG and gain-select pins set gain, SENSE closes local output feedback, REF sets the output baseline, and OUT drives RLOAD.