Bridge Sensor Instrumentation Amplifier

Application

This circuit shows the AD620 used as a bridge-sensor front end. Two low-level bridge sense nodes feed the differential inputs, and the amplifier converts the small differential signal into a single-ended output that can be read by the operating-point table.

Schematic

VCC, VIN+, VIN-, and VEE are arranged as a left-to-right source row. All source returns, the AD620 REF pin, and RLOAD return join the same bottom ground rail so the reference point is visible and unambiguous.

Design Context

VIN+ is set to 110 mV and VIN- is set to 100 mV, representing a 10 mV bridge imbalance with about 105 mV common-mode voltage. The split +/-15 V rails give the macro-model ample headroom for this small-signal operating point.

RG is 49.9 kOhm. For an AD620-style instrumentation amplifier, the nominal gain is approximately 1 + 49.4 kOhm / RG, or about 2 V/V here. With REF tied to ground, the expected output is therefore roughly 20 mV for the 10 mV differential input.

Expected Behavior

The table should show Vin+ slightly higher than Vin-, and Vout should be a small positive voltage near twice the differential input. Because this is a DC operating-point demo, the result is a steady-state value rather than a waveform.

Key Values

• VCC / VEE: +15 V and -15 V.
• Bridge inputs: 110 mV and 100 mV.
• RG: 49.9 kOhm, setting about 2 V/V gain.
• RLOAD: 10 kOhm.

Suggested Experiments

• Increase VIN+ by a few millivolts and confirm that Vout rises by roughly twice that input change.
• Change RG to 10 kOhm for a higher-gain bridge interface.
• Move REF to a nonzero reference voltage to see how the output baseline shifts without changing the input difference.

Featured Component

U1 is the AD620 instrumentation amplifier. IN+ and IN- sense the bridge differential voltage, RG sets the gain, REF defines the output baseline, and OUT drives the measured load.