In a three-op-amp instrumentation amplifier, how does mismatching resistors R3 and R4 affect the output?

In a three-op-amp instrumentation amplifier, how does mismatching resistors R3 and R4 affect the output?

• A. It has no effect if RG is adjusted correctly.
• B. It degrades CMRR and introduces gain errors.
• C. It doubles the output offset.
• D. It eliminates differential gain.

Answer: (B)

In a three-op-amp instrumentation amplifier, accuracy hinges on the balance of the resistors in the final differential stage. The first two op-amps buffer the inputs and set the overall gain, but the last op-amp acts as a differential subtractor using matched resistor ratios to cancel any common-mode signal. When R3 and R4 are mismatched, the two input paths to the final stage are no longer perfectly balanced. This means the common-mode voltage that is present on both inputs no longer cancels out completely, so part of it appears at the output. That is a degradation of CMRR, the measure of how well common-mode signals are rejected. Along with reduced CMRR, the imbalance also introduces gain errors because the intended relationship between the two input signals and the output relies on those resistor ratios being equal. Even if RG is adjusted to set the desired gain for the first stage, the mismatched R3 and R4 distort the differential path, leading to inaccuracies in the output that depend on the common-mode input. So mismatching R3 and R4 causes both poorer common-mode rejection and corresponding gain errors, rather than having no effect, simply offset changes, or eliminating differential gain.

In a three-op-amp instrumentation amplifier, accuracy hinges on the balance of the resistors in the final differential stage. The first two op-amps buffer the inputs and set the overall gain, but the last op-amp acts as a differential subtractor using matched resistor ratios to cancel any common-mode signal. When R3 and R4 are mismatched, the two input paths to the final stage are no longer perfectly balanced. This means the common-mode voltage that is present on both inputs no longer cancels out completely, so part of it appears at the output. That is a degradation of CMRR, the measure of how well common-mode signals are rejected.

Along with reduced CMRR, the imbalance also introduces gain errors because the intended relationship between the two input signals and the output relies on those resistor ratios being equal. Even if RG is adjusted to set the desired gain for the first stage, the mismatched R3 and R4 distort the differential path, leading to inaccuracies in the output that depend on the common-mode input.

So mismatching R3 and R4 causes both poorer common-mode rejection and corresponding gain errors, rather than having no effect, simply offset changes, or eliminating differential gain.