Author Topic: offset voltage compensation  (Read 1790 times)

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Offline nabardwellTopic starter

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offset voltage compensation
« on: April 21, 2020, 11:55:30 pm »
I have been looking for an op amp that is stable with av=2.25 ish frequency to 100mhz the op amp also has to have very very good large signal response 2VPP at av~2.25 100mhz. It is appropriate to assume RL~1Mohm so 1k AV=2 vs Freq can be ignored and no load scenario can be assumed. I have found a suitable op amp LM7171 but with the large gain intended 4 stages of AV 2.25 the input offset voltage of ~100uV will be too much. I have simulated and tested a unity gain offset voltage compensated op amp with both low impedance offset source and voltage divider offset source. While I am able to get very good offset for a specific voltage the drift over the entire range is quite large and essentially equivalent to the original offset voltage. Does any one have any suggestions on how to get less drift over the entire range.

Here at 2v we have very good offset adjustment
975120-0

Here at -2v we are pretty far out
975124-1
 

Offline Vovk_Z

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Re: offset voltage compensation
« Reply #1 on: April 22, 2020, 12:34:21 am »
All high-speed opamps have about 1 mV offset or more. You can adjust it for some temperature but can't adjust it for all temperature range, except you use additional integrator.
If you don't need 0 Hz (DC) then may be you may use a capacitor at the output?
 
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Offline nabardwellTopic starter

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Re: offset voltage compensation
« Reply #2 on: April 22, 2020, 12:47:05 am »
I was planning on being able to amplify both ac and dc signals. The overall gain after 4 stages will be x25 and if I cant compensate the offset voltage on the first two stages to be better than ~50uV I will have over 1.25 mv on the output from the offset voltage. I am most concerned about the offset drift over Vin when I add offset compensation I get a large offset variation over the Vin range +-2v.
 

Offline Vovk_Z

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Re: offset voltage compensation
« Reply #3 on: April 22, 2020, 12:41:29 pm »
I think you should look an Art of Electronix  great book. There is an example of how to calculate opamp circuits error budget (and ways to compensate offset). Because I'm not sure you can simulate circuit you want before you'll understand how to do.
« Last Edit: April 22, 2020, 12:47:31 pm by Vovk_Z »
 
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Offline mycroft

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Re: offset voltage compensation
« Reply #4 on: April 22, 2020, 03:02:40 pm »
You can use a composite amplifier where a slow op amp handles the offset compensation of a fast op amp. Take a look at Composite Amplifiers by Jim William in LT AN-21 (https://www.analog.com/media/en/technical-documentation/application-notes/an21f.pdf).
 
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Offline nabardwellTopic starter

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Re: offset voltage compensation
« Reply #5 on: April 22, 2020, 08:25:25 pm »
Using a composite amplifier seems like the ticket I will build this with a max4239 for the offset stage and LM7171 for the gain portion. Thanks for the suggestion.
 


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