Op-Amp Input Offset Error

Formula

Description

Op-amp input offset voltage is a DC error that appears as if a small voltage source exists between the two inputs. It is caused by mismatches in the input differential pair transistors. The offset drifts with temperature, adding to the initial room-temperature offset over the operating range. For precision applications like strain gauge amplifiers and current sense circuits, total offset error (initial + drift) must be much smaller than the signal being measured. Auto-zero and chopper-stabilized op-amps reduce offset to sub-microvolt levels.

Variables

• Vos_total — Total input offset voltage (V)
• Vos — Initial offset at room temperature (V)
• drift — Offset temperature drift (V/°C)
• ΔT — Temperature change from calibration (°C)

Practical Notes

Typical Vos values: general purpose op-amps 1-5 mV, precision op-amps 25-250 µV, chopper op-amps 1-10 µV. Offset drift ranges from 0.01 µV/°C (chopper) to 5-20 µV/°C (general purpose). The offset error is multiplied by the closed-loop gain, so a 1 mV offset with gain of 100 produces a 100 mV output error.