baohulu
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china
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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
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