Electronic – Determining Operational Amplifier Offset Voltage

analogcircuit-designcmosintegrated-circuitoperational-amplifier

I am designing two stage operational amplifier in CMOS technology. After sizing all transistors and preeliminary checks I wanted to determine input offset voltage of my circuit, and browsing many pages with simillar questions, I actually did not find answer that would make thing clear for me.

First of all, most of the answers in that topic assume that by applying only common voltage to both inputs the Op-Amp output would be 0, which is basically not truth, as there will always be some Common Output Voltage. And this Common Output Voltage is already affected by Input Offset Voltage.

And here is where I am unable to handle this problem. I think applying any resistor divider etc could not work as any obtained result will be useless without knowledge about common output voltage. That seems as a vicious circle to me.

Should I just make spice .op to make input pair's VGS to be equal, or there's a better way to simulate the offset value?

enter image description here

Best Answer

When dealing with real world op-amps there are two major offset parameters: input offset error and output offset error. Generally, these are combined into one parameter because they are difficult or impossible to separate in practice. So, you will rarely read about output offset error, even though it is a real metric of the design. Instead, output offset and input offset errors are combined into one characteristic, and it is called "input offset error".

How to measure input offset error? (i.e. The total of input & output offset errors as explained above.) The easiest, most practical way is to configure the op-amp as a unity gain buffer ( connect output to the minus input). Apply a voltage, within the common mode range of the op-amp, to the positive (i.e. non-inverting) input. The difference between the applied test voltage and the output voltage is virtually totally attributable to the input offset error of the amplifier.

You can connect a DVM set to its lowest range between the output and the NI input and read the actual input offset voltage value directly, assuming you have a sensitive enough DVM. Vary the test voltage across the common mode input range to find the worst case.

I'm not a SPICE guy, but I'd expect you can do the same thing in a SPICE model.