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DC servo loop offset cancellation technique

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Advanced Member level 5
Jan 9, 2012
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I would like to ask you why the DC servo loop is less common technique in offset cancellation. You will find much people working with chopping or digital auto zeroing but rare about this one. I have posted below an example for the DC servo method for your discussion.


Thank you in advance
Best Regards

Because the Main OpAmp technology got so good that main error
amp's own offset became the major loop error component ?

So only use was for a gross primary amp with some specific need,
like high current or power.....

Regards, Dana.
Often, chopper amplifiers are used to sense signals from DC up to a few kHz (low pass characteristic). An example is chopped instrumentation amplifiers. DC servo loops are used to cancel differential signals from DC up to a certain frequency (which of course includes offset).
Therefore, chopped/unchopped amplifiers with DC servo loops are mainly used when only a range of AC signals is of interest (high/band-pass characteristic). They are commonly used in biomedical amplifiers (for e.g. EEG) due to the AC nature of these signals and to cancel the large electrode DC offset.

In the circuit you show above, it might also be that only AC signals are of interest. Then, the input offset of Gm1 will saturate Vout. the DC servo loop will then cancel the Gm1 offset.
Dear dana and artho,

Thank you very much for your help.

i agree with arthorio, the servo loop can opnly be used for AC coupled/chopped amplifier, otherwise if we have a DC amplifier then the servo will kill the output signal as well.

This is issue is clear for me now and I have a relevant question, why do we need active integrator, why not simple passive RC integrator?

I think the "DC servo loop" circuits in the linked blog are useless and the description is misleading. It claims correcting the offset error of a DC amplifier but it doesn't. There are however slightly different topologies of composite amplifiers where a low bandwidth low offset amplifier zeros the offset of the high bandwidth main amplifier. Instead of zeroing the output voltage of the main amplifier (as done in the link) it would e.g. zero the inverting input node voltage.

I expect you'll find working examples of this kind of circuits in literature. But you should be able to design it from the scratch, just think thoroughly.

Regarding your initial question of servo loop versus chopper amplifier, please realize that the composite amplifier offset voltage can't be lower than of the built-in precision amplifier. This is in contrast to a chopper amplifier where the amplifier offset is cancelled by means of precision analog switches.
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