Gain programming in an instrumentation amplifier: The single external resistor typically provided is intended to set the closed-loop gain of the INA—what parameter does this resistor primarily control?

Introduction / Context:
Modern instrumentation amplifiers often expose a single external resistor pin pair, allowing designers to set the amplifier’s closed-loop gain with one component. This simplifies design while the device internally preserves precision resistor ratios for high common-mode rejection.

Given Data / Assumptions:

• An INA with a dedicated RG (gain) resistor connection.
• Internal precision resistors define the gain formula and CMRR.
• Application involves small differential signals with large common-mode voltage.
• We assume proper supply rails and compliance for the selected gain.

Concept / Approach:
The external resistor alters the transresistance of the input stage (or the effective feedback ratio), thereby setting the differential gain. The device datasheet gives an equation such as Gain = 1 + K / RG, where K is an internal constant derived from matched resistors. This keeps gain accuracy high and leaves CMRR largely determined by on-chip matching, not by the external resistor’s absolute tolerance, although RG tolerance still affects absolute gain.

Step-by-Step Solution:

Verification / Alternative check:
Simulate with a SPICE model using different RG values to confirm that changing RG scales the closed-loop gain while leaving CMRR nearly constant (within device limits).

Why Other Options Are Wrong:

Common Pitfalls:
Choosing RG without checking bandwidth/noise trade-offs; neglecting input filtering symmetry which degrades CMRR; forgetting output headroom and input common-mode range at high gains.

Final Answer:
gain