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Comparator (opamp type vs latched type)

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Full Member level 4
Jul 20, 2005
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latched type

I've a question about comparator type,
In general, there are 2 types of comparator.opamp type vs latched type
Latched comparator is more power efficient, small size, fast slew time than opamp type.

Then what kind of application does need opamp-type comparator?

continuous time comparator

I'm not sure, maybe less offset.

comparator op-amp

opamped comp has less BW than latched comp, maybe.
so its propogation delay will be large. if your comp doesn't care delay, so opamped comp can be used.

I think.

latching opamp

Opamp based comparator is slower than latched type comparator because the speed of opamp is limited by the BW.

Main use model difference is continuous time versus sampled time operation. OTA type comparator is continuous time and its output a continuous time analog signal whereas a latched comparator is sampled time and only provides info about its input level at the latched instant. If an ADC or logic gate need to be at the output of the comparator than it needs to be sampled time (for the logic gate case if continuous latch than at least it should have much gain so that its output at the rail so not to cause any excessive crowbar current at the logic gate it drives). More basically if output of comparator interfaces to digital circuit than latching or high gain for the output to rail is necessary. If sampling periods or sampling distortion (kickback, clock noise etc) is not proper than continuous time comparator may be is appropriate.
Speeds etc. are electrical properties and can be designed around if needs arise. Important thing is to design the comparator to the task at hand. There are uncountable applications with unique requirements so one need to consider a particular or narrow application and design accordingly.

I hope this helps.

generally,when the op-amp is used as comparator, it works as pre-amplifier in order to speed up the speed. but, if we do not need so high speed. we can use the amplifer as comparator without the compensation.
But, the latch is used as comparator when it is only in the discontinue system. in other words, latch is only used in the system with clock controlling which has two phase: reset and compare.
as a summary, op-amp as comparator in the high gain or continue system,
latch as comparator in the high speed or use offset cancellation
and discontinue system.


i would like to know something about the clocking phases in a typical offset-canceled comparator, which consists of:


in razavis book it is mentioned, that such comparators consist of three modes:

offset canceling

but what about the clocking schemes for these modes? i tried out to drive this comparator with two nonoverlapping clocks, one for offset-canceling, one for tracking and latching, and the comparator worked. is there any mistake when this comparator is driven with two nonoverlapping clocks?

thanks in advance

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