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Design >> Analog Design >> offset cancellation/correction circuit for opamps
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Message started by jovial on May 28^th, 2015, 2:39am

Title: offset cancellation/correction circuit for opamps
Post by jovial on May 28^th, 2015, 2:39am

Hello everyone,

I designed an opmap in schematic view in Cadence. It is giving no offset.
Then I did the layout for the same, and I get an offset of ~20mV between the differential output nodes.
Can anyone suggest me the way to do away with it? Or is there any circuit which when augmented with the present circuit cancels the offset.??
Any paper/book/reference is welcome...

Title: Re: offset cancellation/correction circuit for opamps
Post by loose-electron on Jun 5^th, 2015, 11:18am

research time IEEE JSSC and many of the analog CMOS textbooks will have material.

Also research transistor matching, common centroid differential pairs,

Title: Re: offset cancellation/correction circuit for opamps
Post by carlgrace on Jun 8^th, 2015, 2:27pm

20 mV output-referred offset is pretty darn good for an opamp (unless you have really small gain).

Did you run monte carlo on the schematic before layout? That will help you see how much of the offset is due to input pair mismatch. Also, I'm guessing you designed a Miller opamp? If you did than obviously you balanced the current density between the input and output current legs. Since they will have slightly different Vds after layout that could also be the source of your offset.

If you really need to, you could center the design and maybe 1/2 the offset but most likely it is good enough. Are you using the opamp in a negative feedback circuit?

Title: Re: offset cancellation/correction circuit for opamps
Post by loose-electron on Jun 11^th, 2015, 8:11am

20 mv is good for a CMOS op-amp

Bipolar can get to 1-3mV

chopper methods, alignment circuits etc are possibilities if needed

Title: Re: offset cancellation/correction circuit for opamps
Post by carlgrace on Jun 11^th, 2015, 9:34am

loose-electron wrote on Jun 11^th, 2015, 8:11am:

20 mv is good for a CMOS op-amp

Bipolar can get to 1-3mV

Very good point. BJTs are off my radar... only ever used them in bandgap circuits.

Title: Re: offset cancellation/correction circuit for opamps
Post by loose-electron on Jun 15^th, 2015, 12:58pm

if you want low noise bipolar methods are still viable, but the foundry options are somewhat limited.

what you can get for noise performance from a large geometry NPN is pretty amazing, likewise matching, likewise transconductance