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Design >> Mixed-Signal Design >> sar adc comparator offset rejection
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Message started by baohulu on Sep 18^th, 2012, 1:33am

Title: sar adc comparator offset rejection
Post by baohulu on Sep 18^th, 2012, 1:33am

hi,all
in sar adc, during the conversion time, comparator offset is usually rejected by the open-loop opam before it, the opam may be 2,or 3 simple opam, each with less than 20dB gain, then the effective input offset of the comparator is voff_comp/opam-gain.
however, I have a question, when the input of the open-loop opam is not that small, because they are open-loop, then, their operating point is not in the small-signal model, or say, their gain is not the ac-simulation gain, may be much smaller than it because maybe some mosfet is not in the saturation region.
then the comparator input effective offset is larger than the calculation. for a ADC, the offset may be larger than the LSB.

of course, if it is used in a receiver, they only care the SNR of ADC, then , a large signal with a larger offset can be accepted. however, if I use it as a sensor, it will mean that I can't measure with a good accuracy, right?
for example:
if the input is 1v, the offset may be 20mv
if the input is 20mv, the offset maybe 20uv
when it is used to measure something, like temperature, it mean, the relative accuracy is limited by the 20mv though when the signal is small it has a better accuracy

Title: Re: sar adc comparator offset rejection
Post by dcic on Sep 19^th, 2012, 1:26am

Assuming 1b/conversion SAR ADC, the offset of a single comparator (or its preamp) can be calibrated out digitally, given that the offset+input do not exceed the dynamic range of converter. If so, ENOB will be compromised.

Title: Re: sar adc comparator offset rejection
Post by ywguo on Oct 6^th, 2012, 5:40am

Hi Baohulu,

The comparator must have correct output if the input is 1 V and the offset is 20 mV. The same thing if the input is 20 mV and the offset is 20 μV. So why do you worry about the input referred offset of the comparator?

Best Regards
Yawei Guo