What’s all this “____________”, anyhow? borrows from the late, great Robert Pease. He used this as a method of explaining complex topics to his readers. We’re attempting to carry on the tradition by explaining some of the specifications in component datasheets and why you should care about them when seeking out new parts.
An op-amp attempts to balance any difference in voltage potential at the inputs by raising or lowering the voltage at the output. So if you take an op-amp and hook up the rails (ie. to VCC and ground), then tie both inputs together, you would expect to have an output of 0V. When you put your multimeter on the output, though, you find a voltage! If you were to add a small voltage to one of the pins, eventually the output will be zero. That difference between the pins (when the output is 0V) is the input offset voltage (Vos).
Why does this happen?
It is caused by small imbalances created during manufacturing internal to the transistors that comprise the op-amp inputs. To make matters worse, this value will increase and fluctuate with temperature and over time.
The amount of Vos in your circuit can generate significant errors as the amount of op-amp gain increases. To adjust for this, you can either use various techniques such as AC-coupling or DC feedback; even better, look for op-amps that have the lowest Vos listed in their datasheet, on the order of a few µV. Some op-amps even have offset adjustment pins to make it even easier to compensate for the chips Vos. Others “chop” the input to account for this difference.
Here are some example components…
|OP177FPZ||Input Offset Voltage 40µV|
|MCP6V02-E/SN||Input Offset Voltage 2µV|
|ADA4528-1ACPZ-R2||Input Offset Voltage 4.3µV|
(graphic from Analog Devices Appnote MT-037)