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Comparator for SAR ADC

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I designed a 8bit S A R A D C and have trouble making the comparartor to work when the comparator inputs are very small, as a result I'm getting incorrect digital code.

e.g.

in- = 0.039139V
in+ = 0.04000V
Both out+ and out- will be zero
when out+ should have been one.

The inputs go to the gate of 2 nmos, when they are below Vt (like the e.g. above) there won't be any Id, how can I make it work??

Any help is greatly appreciated.
 

You need to know what is the possible input range. If input is rail to rail, the preamp of the latch comparator has to be rail-to-rail structure. If input range is starting from 0 up to some voltage where less than Vcc, you may need to use PMOS as input stage.

NMOS is impossible for your case, though it gives higher gain compare to PMOS.
 

    T

    tofu

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I think that you should also use a RS flip flop to make sure the two output not zero at the same time.
Also you can use a dc shift circuit to make sure that your input circuits are not cut off at the same time
 

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    tofu

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