Formula: Gain = 1 + 2R/R_gain — the standard 3-op-amp instrumentation amplifier topology: two matched input-buffer op-amps (each with feedback resistor R) feeding a unity-gain difference amplifier, with a single external resistor R_gain between the two input buffers' inverting nodes setting the overall gain. This differs from a plain op-amp difference amplifier by offering very high input impedance on both inputs and excellent common-mode rejection, which is why it's the standard front-end for bridge sensors, thermocouples, and other low-level differential signals. (Purpose-built INA ICs like the AD620/INA128 use this same topology internally with their own fixed R — check the specific IC's datasheet for its exact gain-resistor formula if you're using one of those instead of discrete op-amps.)