Checking Resistor-Ratio Matching in a Difference Amplifier

A difference amplifier relies on matching between two resistor ratios to reject a shared input voltage. Precise individual resistor values do not automatically establish precise ratio matching. Review the complete resistor network before attributing common-mode error to the op amp.

Identify the paired ratios

TI’s resistor-matching application note analyzes common-mode rejection in difference-amplifier circuits and shows the importance of the relationship between divider ratios. Its analysis provides a basis for comparing discrete resistors with appropriately matched networks.

Label the input and feedback resistors on both sides of the circuit. Calculate the two relevant ratios using their tolerance limits or supported matching specification. Keep absolute resistance tolerance separate from ratio tolerance and tracking over temperature.

Translate common mode into output error

Define the maximum common-mode voltage and the allowed output error attributable to it. Evaluate the circuit with equal input voltages while retaining the real source impedances. In the ideal matched case, that shared voltage should not create the intended differential output signal.

For an illustrative unity-gain measurement, a small unwanted common-mode gain can create a significant error when the shared input voltage is much larger than the differential signal. Use the actual network equations to calculate the error rather than assuming a resistor’s percentage tolerance directly equals the circuit’s voltage error percentage.

Include temperature and external resistance

Review how the ratios change with temperature and self-heating. A matched network’s tracking specification may be more relevant than four separate temperature-coefficient values. Check whether all resistors experience similar temperatures in the assembled layout.

Include sensor, cable, protection and switch resistance in the effective input paths. A carefully matched four-resistor network can lose its intended balance when unequal external series resistance is added. Record these contributions explicitly in the schematic-level calculation.

Verify DC and frequency behavior

Apply a controlled common-mode signal with minimal differential component and measure the resulting output. Repeat across the common-mode range and relevant temperatures. At higher frequencies, include the effect of parasitic capacitance and input-network imbalance rather than assuming the DC result remains unchanged.

Retain the ratio assumptions, common-mode test conditions and measured output error with the design. Separate the amplifier’s own limitations from resistor-network and source effects where practical. Recheck the result after changing protection resistors or component technology. A justified common-mode rejection claim describes the assembled network and its environment, rather than copying the op amp’s data-sheet CMRR as the performance of a circuit whose resistor ratios were never evaluated.