AD8221 Differential Sensor Amplifier

Application

This circuit shows the AD8221 used as a precision instrumentation amplifier for a small differential sensor signal. It is a practical starting point for bridge sensors, current-shunt monitors, and other low-level differential measurements that need a ground-referenced output.

Design Context

VINP and VINN model the two sensor terminals. VINN is held at 100 mV while VINP is swept around it, so the chart shows how the output follows a small differential input. The two input-bias sources are arranged in one row with a shared return rail below them, making the common reference easy to identify. RG sets the AD8221 gain, REF is tied to ground so the output is referenced to 0 V, and RLOAD provides a light 10 kOhm output load. A dual-rail supply provides +15 V and -15 V, with the rails routed above the signal area so the input source row and gain network stay readable.

Expected Behavior

As VINP rises above VINN, Vout rises above ground. As VINP falls below VINN, Vout moves negative. With the 49.9 kOhm gain resistor, the output change is roughly twice the input differential change, so the transfer curve should be linear and centered near 0 V when the two sensor inputs are equal.

Key Values

• VINP sweep: 90 mV to 110 mV.
• VINN reference: 100 mV.
• Gain resistor RG: 49.9K.
• Load resistor: 10K.
• Dual rails: +15 V and -15 V.

Suggested Experiments

• Change RG to see how the input-to-output slope changes.
• Change VINN to move the sensor common-mode voltage while keeping the differential sweep small.
• Drive REF with a small DC offset to shift the output baseline.

Featured Component

The AD8221 is the instrumentation amplifier under test. Its IN+/IN- pins receive the differential sensor signal, RG+/RG- set gain with one external resistor, REF establishes the output reference, and OUT drives the measured load.