Due to the MOS mismatch,real OPAMP inevitably has a input-refered offset.If constructing a gain stage with this OPAMP,the offset will be amplified at the output.
How to cancel the offset?Anyone can help me?
If this offset is really problem for you (high gain or instrumentation) you can connect a (mutiturn)potentiometer trough high-ohm (1M or more) resistor to + or - input.
Because this potentiometer operates between +V and -V you will be able to adjust 0V (or other value) at the output.
but talking about the gain stage, generally the gain comes from the ratio of capacitor in a SC circuits, and i don't think the offset will be amplified in that way.
and normaly, the offset of a opamp is a constant value, thus not hard to cor.
unless used as comparator, where u may employ either DEC ot AnalogCalib. to solve the problem.
yes,Chopped OPA is a solution if the input signal is band-limited.But in this case,I am designing a VGA,its input is from mixer.So I think Chopped-OPA will result in alias.
but talking about the gain stage, generally the gain comes from the ratio of capacitor in a SC circuits, and i don't think the offset will be amplified in that way.
and normaly, the offset of a opamp is a constant value, thus not hard to cor.
unless used as comparator, where u may employ either DEC ot AnalogCalib. to solve the problem.
I think it mainly depends on your opamp operates continuously or clocked.
If it's clocked, you can used switch-cap feedback to cancel the offset successfully.
It it's continuous, it is much harder. Following are some methods I can think of,
1. better matching in layout and design of the two sides of the opamp input/output. Like larger transistor size, device trimming. The method talked about before (adding a large resistor at output) looks like a simple trimming method but it might not work in all cases.
2. Use floating-gate circuit to reduce the device mismatch.
3. digitally calibrate in post processing.
4. use fully differential strucrure to reduce it.
Does fully-differential structure reduce the effect of offset? Offset is mean the differential potential applied in opamp such that the differential output is zero, so by its nature offset is in differential mode error.
1 autozero (discrete but lead high write noise aliasing)
2 chopper (continuous but need good low-pass filter)
3 good layout and big input MOS
4 carefully design mOS switches