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https://designers-guide.org/forum/YaBB.pl Design >> Analog Design >> opamp comparator vs. latch comparator https://designers-guide.org/forum/YaBB.pl?num=1111695002 Message started by analogic on Mar 24th, 2005, 12:10pm |
Title: opamp comparator vs. latch comparator Post by analogic on Mar 24th, 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 24th, 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 24th, 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 25th, 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 25th, 2005, 2:45am analogic wrote on Mar 24th, 2005, 8:07pm:
Reset phase by itself does not improve speed. Regenerative gain phase is what improves gain. analogic wrote on Mar 24th, 2005, 8:07pm:
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 24th, 2005, 8:07pm:
sheldon gave an excellent answer on this. |
Title: Re: opamp comparator vs. latch comparator Post by analogic on Mar 25th, 2005, 8:56am thank you two for the excellent explanation! |
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