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Design >> Analog Design >> How to sim opamp's input referred system offset?
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Message started by pipeline on Sep 13^th, 2007, 9:32am

Title: How to sim opamp's input referred system offset?
Post by pipeline on Sep 13^th, 2007, 9:32am

Could system offset be determined by simulation? Or just by analysis and hand calculation? Is Monte-Carlo simulation only suitable for random offset? :-/I'm confused about these questions. Would any one kindly elaborate them? Thanks very much.

Title: Re: How to sim opamp's input referred system offse
Post by Berti on Sep 13^th, 2007, 10:39pm

Systematic offset is caused either by asymmetrical circuits or layout. It is therefore
straight-forward to simulate systematic offset with a circuit simulator(using post-layout
back-annotation, respectively).
For random offset you can use monte-carlo simulation assuming your design-kit provides
you with statistical data.

To simulate offset you should therefore add asymmetrics from the layout into your
schematic (at least parasitics, PWE and STI is only required for 130nm and below)
and run a monte-carlo simulation using the statistical data provided by the fab.

Regards

Title: Re: How to sim opamp's input referred system offset?
Post by pipeline on Jun 20^th, 2009, 8:24pm

Clear! Thank you very much!

Title: Re: How to sim opamp's input referred system offset?
Post by raja.cedt on Jun 21^st, 2009, 12:53am

hi berti,
i have one doubt in systematic offset, lets say i have an opamp with output biased at 1v and assume now i have connected opamp in unity gain configuration and gave .7 as input, so now offset is .3/(open loop gain) ..is this systematic offset?

Thanks,
Rajasekhar.

Title: Re: How to sim opamp's input referred system offset?
Post by thechopper on Jun 22^nd, 2009, 9:36am

raja.cedt wrote on Jun 21^st, 2009, 12:53am:

Hi Rajasekhar,

That is not a realistic scenario: since the opamp is in closed loop, the input referred offset is equal to the ouput offset / closed loop gain; assuming sufficiently large open loop gain. Therefore 0.7v input referred offset means 0.7v output referred offset, although obviously .
these are not realistic offset magnitudes.

Regards
Tosei

Title: Re: How to sim opamp's input referred system offset?
Post by aaron_do on Jun 24^th, 2009, 6:03pm

Hi Raja,

if you have an op-amp in unity gain configuration, the output should follow the input. So how do you "bias" the output at 1V?

cheers,
Aaron