Adding a negative- resistance circuit (U2) to the REF pin allows the CMRR of a differential amplifier or instrumentation amplifier to be trimmed. This is done by applying a sine wave of 20Vp-p to the inputs of U1 and adjusting the potentiometer P1 for a minimum signal at the amplifier output. By using an AC source the DC input offset errors do not effect this trim. At 50% rotation of P1 the resistance of R8 cancels the resistance of R5, appearing as a virtual ground to the REF pin. As P1 is adjusted from end to end, R5 is “undercancelled”or “overcancelled” by R8 and the resistance presented to the REF pin goes from a real, positive resistance to a negative resistance. This allows the internal resistor network of U1to be trimmed for maximum CMRR. ( Circuit is created by Neil P. Albaugh TI – Tucson )
Differential Amplifier CMRR Trim Circuit
Online Simulation of the Differential Amplifier CMRR Trim Circuit
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