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How to Calculate Input Offset Voltage of OP Amps

Operational amplifiers, or op-amps, are a useful type of integrated circuit that amplify a signal's voltage. If, for example, a signal of one volt entered an op-amp with a gain of 10, then the output voltage would be 10 volts strong. An ideal op-amp should output zero volts when zero volts connects to its input, but in practice that's rarely the case. Imbalances in the op-amp's internal resistors and transistors create a small, measurable current that adds or subtracts to the output voltage. A circuit designer must calculate and then account for this "input offset voltage" when using the circuit.

Things You'll Need

Voltmeter

Instructions

- 1
  Use the voltmeter to measure the output voltage of the op-amp at its output terminal. Make sure there are zero volts applied to the input. For example, it may be 0.2 volts.
- 2
  Calculate the op-amp's gain, which is the factor by which the device amplifies the input signal. Divide the resistance of the resistor that bridges the input and output terminals by the resistance of the resistor that attaches only to the input terminal, then add 1. For example, if a 10 kilo-ohm resistor bridges the terminals and a 10-ohm resistor attaches to the input, divide 10,000 by 10 and then add 1 to yield a gain of 1001.
- 3
  Divide the output voltage from the first step by the gain from the previous step to calculate the input offset voltage. Using the above example, divide 0.2 volts by 1001 to yield an approximately input offset voltage of 0.2 mV.

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