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Comparator in Flash ADC

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ChikumaMatsumara

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For a flash adc, is it possible to use latch comparator with built-in threshold? I don't know how it can be implement in the circuit since it is fully differential comparator. Before this I have done flash adc using two stage op amp as the comparator, and it works....but still need some modification in terms of signal delay.
 

i have seen that someone use inverter with built-in threshold as comparator ,but havent seen latch comparator with built-in threshold,
does your flash adc with two stage opamp design work well ?
how about its inl dal snr and so on?
 
hi
it is possible to use inverter with built in threshold like tuza2000 said but it is not a good technique because each inverter has to be sized differently and accurately which might cause matching problems.
if you are using a differential comparator then how will a built threshold operation work? if it is to be a built-in type then i think it should be single-ended comparator.

regards
 

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    ChikumaMatsumara

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It will give you terrible INL/DNL problems because is no way to guaratee even the monotonicity. At least a resitive ladder is monotonic for sure.
And 2 stage op amp as a comparators sound terrible slow and inefficient.
 
Actually I use the two stage op amp in a design class. Tuza2000 was right about that, my flash adc did not perform well.
About the latched comparator, may be I was wrong. I'm not sure whether it is single ended or differential ended. Here I upload the schematic of the comparator
 

I think it qualifies as pseudo differential becuase you don't have a high impedance tail current source to reject CM signals. But is differential.
Anyway, this will not give you even a decent performance in a flash becuase:
- kickback noise at latch will upset refernces and input voltage;
- no preamplifiers means you will have a lot of metastability problems => wrong codes at the output.
- comparator have large offsets (small transistors, unmatched charge injection from switches) => wrong codes again.

A good flash has a few low gain high speed preaplifier stages (simple differential pairs, gain = 2-5) in front of a comparator.
 
to:ChikumaMatsumara
this latch comparator is very popular,
but as ocarnu said you should use preamplifier in front of the comparator.

regards
 
you can use preamp to setting small signal and prevent voltage offset and use latch stage to achieve high SR and big signal setting. both of them is important.
 

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