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Design >> Analog Design >> opamp comparator vs. latch comparator
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Message started by analogic on Mar 24^th, 2005, 12:10pm

Title: opamp comparator vs. latch comparator
Post by analogic on Mar 24^th, 2005, 12:10pm

in my understanding, latch based comparator has the advantage of high speed, low power consumption compared to opamp based comparator.

can any one tell me what is advantage of opamp based comparator? and in what applications, we should use opamp based comparator.

Thanks.

Title: Re: opamp comparator vs. latch comparator
Post by 101questions on Mar 24^th, 2005, 1:23pm

Excellent question.
Op-amp comparator has an advantage of absense of clock.
Also, its offset is potentially smaller, though it depends on implementation.
They are used in PWM, RC timer circuits (555 likes) and other asyncronous analog circuits.

Title: Re: opamp comparator vs. latch comparator
Post by analogic on Mar 24^th, 2005, 8:07pm

Thanks for your reply. but i still have some confusion:

"Op-amp comparator has an advantage of absense of clock. "
- do you mean latch comparator needs a reset phase while opamp comparator doesnt?
Reset phase improves the conversion speed of latch comparator, will it improve the conversion speed if introduced to opamp comparator?

"Also, its offset is potentially smaller, though it depends on implementation. "
- without autozero, the offset of both latch comparator and opamp comparator is only dependent on transistor size, right? why it is smaller in opamp comparator?

"They are used in PWM, RC timer circuits (555 likes) and other asyncronous analog circuits."
- could you explain a little bit why latched comparator is not suitable for PWM application, PWM circuit is also clocked circuit, why opamp comparator has advantage in it? Thanks.

Title: Re: opamp comparator vs. latch comparator
Post by sheldon on Mar 25^th, 2005, 12:04am

For what its worth,

First, you should not be using op-amps as comparators.
They are linear circuits and does always behave well when
overdriven. If you want to monitor voltage in continuous-
time use a non-latched comparator, i.e., a series of
differential stages with a level shifter at the end.

When events occur asynchronously, as in a Switching
Regulator, the comparator needs to respond instanteously
so being unclocked is good. If the output transistor is
frying you don't want to wait for the next clock to turn
it off. In applications, like data converters being clocked
is acceptable. Clocking[latching] allow you to increase gain,
i.e., use regenerative gain, and reduce the likelyhood
of metastability.

In general, the input stage of continouos-time
comparator is a diff pair, one offset. Depending on the
design of the latch comparator, the input could be
several matched pairs, more pairs more offset.

Regards,

Sheldon

Title: Re: opamp comparator vs. latch comparator
Post by 101questions on Mar 25^th, 2005, 2:45am

analogic wrote on Mar 24^th, 2005, 8:07pm:

Reset phase by itself does not improve speed. Regenerative gain phase is what improves gain.

analogic wrote on Mar 24^th, 2005, 8:07pm:

"Also, its offset is potentially smaller, though it depends on implementation. "
- without autozero, the offset of both latch comparator and opamp comparator is only dependent on transistor size, right? why it is smaller in opamp comparator?

Latched comparator offset depends also on kick-back effects (hysteresis-like thing), parasitic capacitance mismatch and some other additional sources of mismatch. Linear-type comparator offset is diminated by transistor mismatches in most cases.

analogic wrote on Mar 24^th, 2005, 8:07pm:

"They are used in PWM, RC timer circuits (555 likes) and other asyncronous analog circuits."
- could you explain a little bit why latched comparator is not suitable for PWM application, PWM circuit is also clocked circuit, why opamp comparator has advantage in it? Thanks.

sheldon gave an excellent answer on this.

Title: Re: opamp comparator vs. latch comparator
Post by analogic on Mar 25^th, 2005, 8:56am

thank you two for the excellent explanation!