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How to cancel offset in OpAmp?

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joskin

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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?
 

Re: offset cancellation

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.
 

Re: offset cancellation

Chopped OPA
or Big W Big L of input device
 

offset cancellation

some papers use a clock switch at input to cancel the offset.
 

Re: offset cancellation

yes, offset is a problem.

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.
 

Re: offset cancellation

sunking said:
Chopped OPA
or Big W Big L of input device
Click to expand...

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.
 

Re: offset cancellation

mbright said:
yes, offset is a problem.

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.
Click to expand...

If construct a gain stage such as non-inverting amplifier with the OPAMP,the offset will be amplified.
 

Re: offset cancellation

Refer to Ch. 13 in Razavi's book.
Autozeroing,DC feedback loop,or Chopper stabilization.
 

Re: offset cancellation

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.
 

Re: offset cancellation

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.
 

Re: offset cancellation

also, how does the floating gate tech. help in this case?

floating gate in analog means u can adjust the charge stored on it therefore the Vth. normally for low-power application.

do u mean this can help to improve the Vth matching?


for cap feedback, do u mean auto-zero? or analog calibration?

digital-error-correction can help for sure.
 

Re: offset cancellation

there is some paper on how to cancel op-amp offset.

The normal method is automatic-zero. You can find it in Sansen's book.

Added after 1 minutes:


there is some paper on how to cancel op-amp offset.

The normal method is automatic-zero. You can find it in Sansen's book.

Added after 32 seconds:


there is some paper on how to cancel op-amp offset.

The normal method is automatic-zero. You can find it in Sansen's book.
 

Re: offset cancellation

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
 

offset cancellation

What are DEC and AnalogCalib.
I 'm a new.
 

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