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Comparing Comparators (Read 3083 times)
AnalogDE
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Posts: 137
Comparing Comparators
Oct 26
th
, 2009, 10:25am
I'm trying to decide between the following two textbook comparators. One is a clocked comparator design from Razavi, the other is a non-clocked design from Baker.
What are the pros/cons of each architecture? I've listed what I can think of below.
Also, why in the Baker architecture is there a self-biased diff-amp in the output stage? Why not a regular diff-amp with NMOS current src tail like in the pre-amp stage?
Thanks!
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BakerComparator.JPG
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AnalogDE
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Re: Comparing Comparators
Reply #1 -
Oct 26
th
, 2009, 10:26am
Here is the Razavi clocked comparator:
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RazaviComparator.JPG
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HdrChopper
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Re: Comparing Comparators
Reply #2 -
Oct 26
th
, 2009, 5:24pm
Hi jiesteve,
Bakker approach:
Pros: less offset and noise due to large input gain (the active loads can be made large and with low gm vs the latch devices which are typically large gm and small area). More gain also means less prop. delay (important for some comparators).
This design can easily be converted into a clock comparator if M12 is clocked (and a few additional switches are added to guaranteed output state).
Biasing could be a normal one (I guess self biasing for saving area)
Razavi approach
Cons: more offset (due to lack of large input voltage gain and large gm of latch devices). More prop delay due to smaller gain.
Bottom line:
More power --> better noise and offset. This trade off is unavoidable.
Hope this helps
Tosei
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Keep it simple
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raja.cedt
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Germany
Re: Comparing Comparators
Reply #3 -
Oct 26
th
, 2009, 9:34pm
hi,
in addition to tosie comments, razaavi circuit have provision for reseting the output to some intermediate value and at the same time razaavi circuit has more gain, hence less regeneration time.
thanks,
rajasekhar.
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spunky
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Analog Designer
Posts: 4
USA
Re: Comparing Comparators
Reply #4 -
Oct 28
th
, 2009, 12:53pm
For a high speed comparator, I think the pre-amp should have high gain but it is not necessary true for the following stages such as the latch or the intermediate stage. Because when the small difference is amplified by preamp it becomes a signal with relatively large amplitude, while the slew rate limitation prevails.
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