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Design >> Analog Design >> how 2 reduce the systematic offset of a folded cascod comparator
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Message started by Tofu on Apr 7th, 2009, 4:09pm

Title: how 2 reduce the systematic offset of a folded cascod comparator
Post by Tofu on Apr 7th, 2009, 4:09pm

Hi:

I made a comparator based on folded cascode structure due to headroom issue. It's systematic offset is in the range of 3mv which is a bit large than the spec (<900uv). I wonder if there is any knobs (e.g., current, diff pair, load, etc) for this type of architecture that I can tune to decrease the number?  and how?

thanks a lot!

Title: Re: how 2 reduce the systematic offset of a folded cascod comparator
Post by wave on Apr 7th, 2009, 5:52pm

Sounds like your folded legs may not match.  Check their currents, Vds, and Early effects.

;D
Wave

Title: Re: how 2 reduce the systematic offset of a folded cascod comparator
Post by raja.cedt on Apr 8th, 2009, 12:49am

have you got expected output common mode voltage?and check common mode at two outputs,if they are not symmetric..as he said its like current mismatch problem,.And  if possible decrease the overdrive voltage of differential pair..it may help

Thanks,
rajasekhar.

Title: Re: how 2 reduce the systematic offset of a folded cascod comparator
Post by loose-electron on Apr 17th, 2009, 10:50pm

Offset is almost always dominated by the mismatch of the differential pair at the input.

You should consider an architecture that has minimal systematic offsets and then uses a method of offset cancellation to deal with mismatch issues.

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