Basic instrumentation amplifier topology: How many inputs does the classic instrumentation amplifier accept, and what is their nature?

Introduction / Context:
Instrumentation amplifiers measure the difference between two signals while rejecting any voltage common to both. This is the essence of differential measurement used in bridge sensors, ECG/EEG electrodes, and industrial transducers.

Given Data / Assumptions:

• Classic 3-op-amp instrumentation amplifier architecture.
• Two sensor nodes supply signals that include both desired differential content and unwanted common-mode terms.
• High input impedance is provided at both input pins.

Concept / Approach:
The instrumentation amplifier has two inputs (noninverting terminals of input buffers), accepting a pair of voltages. The device outputs Vout proportional to (V2 − V1) times a programmable gain, while rejecting the average (common-mode) component through symmetry and high CMRR.

Step-by-Step Solution:

Verification / Alternative check:
Reference schematics show two input buffers feeding a differential stage. Datasheets specify two input pins and a single gain-setting resistor.

Why Other Options Are Wrong:
1: would be a single-ended amplifier, not differential.

“Capacitive” or “resistive”: these describe coupling elements, not the number of inputs.

Common Pitfalls:
Confusing instrumentation amplifiers with differential op-amp stages lacking the input buffer pair. Assuming a single-ended front end can provide high CMRR like an INA.

Final Answer:
2