What does a high input impedance in an instrumentation amplifier help with during sensor interfacing?

• A. It reduces loading on the sensor, preserving signal integrity.
• B. It increases noise susceptibility.
• C. It reduces common-mode rejection.
• D. It lowers gain accuracy.

Answer: (A)

High input impedance means the amplifier draws very little current from the sensor, so it doesn’t load the sensor. This keeps the sensor’s output voltage from being pulled down or altered by the amplifier’s input, preserving the true signal your sensor is producing. In practice, sensors—especially high-impedance ones like many voltage sources, bridge sensors, or piezo elements—can have their voltage reduced or distorted if the next stage presents a low impedance. With a high input impedance, the sensor’s signal sees almost no loading, so the measured signal stays accurate and the initial signal strength is maintained, which is crucial for reliable sensing and precise amplification.

The other statements aren’t the goal here. Higher input impedance doesn’t inherently make the system more susceptible to noise; noise performance depends on the overall front-end design, including input-referred voltage noise and shielding. Reducing common-mode rejection is not a benefit of high input impedance; in fact, good instrumentation amplifiers strive to maintain strong common-mode rejection, and this is governed by the differential design and matching rather than input impedance alone. And gain accuracy is set by the feedback network and resistor ratios, not by how large the input impedance is.