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OPA227 Offset Voltage adjustment

Alex Dunfield50166
Prodigy 80 points

The datasheet for the OPA227U precision op amp indicates that the offset voltage can be trimmed using the circuit shown on page 11 ( doc# SBOS110A).

Can you tell me which direction the offset changes when the resistance between pin 1 and V+ is increased?

For example, if the resistance from Pin1 to V+ is 5K, and the resitance from pin 8 to V+ is 15K, will the offset voltage be higher than when the wiper is in the middle of the trim resistor?

 

  • 0
    TI__Guru 71711 points

    Alex,

    Pin 1 controls the passive resistors on Vin+ (pin 3) positive side and pin 8 controls the passive resistor on Vin- (pin 2) negative side of the input differential pair.  However, you ask your question in two contradictory ways: first you imply that "resistance between pin1 and V+ is increased'  (and thus is greater than between pin 8 and V+)  but then you say an opposite thing:  "if the resistance from pin 1 to V+ is 5k and the resistance from pin 8 to V+ is 15k, will the offset be higher than when the wiper is in the middle of the trim resistor?"

    For the case that the resistance from pin 1 to V+ is 5K and the resistance from pin 8 to V+ is 15K, you will increase the current on the positive side of the input differential pair and thus the Vin+ (pin 3) will go higher with respect to Vin- (pin 2).  Therefore, if your input offset voltage reference point is with respect to negative input (pin 2), the offset voltage will increase under above conditions.