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A four-resistor difference amplifier gives Vout = (R2/R1)(V+ - V-) when the ratios match. Resistor mismatch, not the op-amp, usually limits common-mode rejection: with 1% resistors CMRR is capped near 50 dB, which is why matched networks or instrumentation amplifiers are used for small differential signals.
Difference amplifier
Vout = (R2/R1) x (V+ - V-) when R1/R2 = R3/R4
Electrical results are planning estimates only. Verify against the applicable wiring rules (IEC 60364, BS 7671, NEC or local code) and have all installation work designed and checked by a qualified electrician.
A ratio mismatch converts common-mode input into output. The achievable CMRR is roughly (1 + gain) divided by four times the fractional tolerance.
Whenever the source impedance is significant or CMRR above about 80 dB is needed — the three-op-amp topology gives high input impedance and trimmed matching.