why don't chopper comparators exist? - Page 1
Forum section: electronics / projects

author: incf
author_rank: Frequent Contributor
timestamp: March 30, 2025, 12:14:54 pm
content: Sometimes it is nice to have very close to zero offset voltage.Why don't chopper comparators seem to exist? Is this some inherent issue with choppers?A chopper op-amp would likely saturate and be very slow if operated as a comparator. It also would be very leaky between the inverting and non-inverting inputs. (But I would like to think that a chopper comparator could be made to not exhibit those issues)

author: moffy
author_rank: Super Contributor
timestamp: March 30, 2025, 12:49:32 pm
content: Chopper amplifiers are a sampled data system:https://www.analog.com/en/resources/analog-dialogue/articles/zero-drift-operational-amps.htmlThey have inherent delay due to the chopping frequency and can introduce aliasing products, they are relatively low frequency and unsuitable for comparators.

author: mtwieg
author_rank: Frequent Contributor
timestamp: March 30, 2025, 01:19:19 pm
content: Interesting question. I'd assume that chopper/autozero techniques only work well when the differential input voltage is very small (comparable to the offset voltage itself), as should be the case with a properly designed op amp circuit. But with a comparator, the differential input voltage could be several orders of magnitude larger. So the chopping circuitry would have to handle a far larger dynamic range.Quote from: moffy on March 30, 2025, 12:49:32 pmI'm sure a lot of engineers would appreciate a slow (>1us delay) comparator with a very small offset (<1uV).

author: moffy
author_rank: Super Contributor
timestamp: March 30, 2025, 01:29:23 pm
content: Quote from: mtwieg on March 30, 2025, 01:19:19 pmDelay is only one of the issues.

author: Marco
author_rank: Super Contributor
timestamp: March 30, 2025, 02:14:24 pm
content: Quote from: mtwieg on March 30, 2025, 01:19:19 pmMicrochip has a composite auto-zero amplifier and comparator circuit in their datasheet, Google tells me SAR ADCs use auto-zero comparators internally too. It's weird that no one delivers it as an IC.

author: Someone
author_rank: Super Contributor
timestamp: March 30, 2025, 11:08:18 pm
content: Quote from: mtwieg on March 30, 2025, 01:19:19 pmAs above, the solution is a composite. Add some gain with a chopper stabilised opamp and then compare against the amplified signal. Well proven in design.

author: David Hess
author_rank: Super Contributor
timestamp: March 31, 2025, 04:52:25 am
content: They do exist, however offset correction requires the input differential pair to be balanced which is the opposite of how a comparator works, and comparators are almost never unity gain stable.

author: Berni
author_rank: Super Contributor
timestamp: March 31, 2025, 08:02:29 am
content: To answer the question, ask yourself how a chopper amplifier works.The chopper samples the voltage between the differential inputs, then it subtracts that away from them. Doing this back and forth to march towards zero. It can do this because when an operational amplifier is used as an amplifier (and is not saturated) the voltage between the diff inputs should ideally be 0V, so if you see any voltage there it means that's the opamps fault, so that's the offset voltage that must be trimmed away.However a comparator is usually fed completely different voltages in each diff input, hence you would be constantly trying to compensate away your actual input signal, hence such a chopper comparators input offset would start wildly flailing all over the place.So best is to indeed use the "amplified comparator" trick mentioned above in the thread to amplify the signal before comparison. HOWEVER careful what kind of chopper opamp you use. Some choppers have the ability to compensate away very large offset voltages, so it might still try to compensate away your signal when it hits saturation. So make sure to test the particular chopper part number you want to use and write down a warning to not use alternatives for this part.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: March 31, 2025, 08:58:34 am
content: Quote from: incf on March 30, 2025, 12:14:54 pmThis will also be a volume thing.There is some middle ground, between auto-Zero opamps (low uV) and comparators (where it is rare to find anything sub 1mV max).There are low offset OpAmps ( << 1mV) that are not auto-zero, that can be used as comparators, so that covers a lot of use cases.True auto-zero cannot be used, directly on a comparator input, as mentioned above.If you do not need very high impedance on both pins, there are many current sense opamps that have low offset voltages too.

author: Kleinstein
author_rank: Super Contributor
timestamp: March 31, 2025, 09:19:09 am
content: For an AZ amplifier to work, it has to be working in it's linear range. As a comparate the input is most of the time not in it's linear range. So directly an AZ comparator does not work.One can however have a kind of nonliner auto zero preamplifier for the input signal, that saturates gracefully, e.g. with diodes in the feedback. A comparator can than follow. Effectively this can reduce the offset of the comparator by the gain of the amplifier part. It add a little delay, but can still speed things up for small signals with little overdrive.To some degree the old ICL7650 can be used as comparator. It has a special clamping feature inclucded so that it can avoid saturation of the chopper / AZ part even with a large input signal. It is still a relatively slow amplifier and a fast low offset OP-amp (e.g. OPA141) could be a faster alternative.

author: jwet
author_rank: Frequent Contributor
timestamp: April 01, 2025, 05:54:00 pm
content: As others have said, but here are other things to consider.Op-Amps need low offset because they can be called on to have high DC gain. A .5 mV offset for even a cheap op-amp isn't too bad if it doesn't drift and you're making a follower- the problem comes in when you have a gain - a gain of 50 gives you 25 mV. Comparator offset doesn't get gained up.Comparators also have "overdrive" requirements- their response time is a function of how much overdrive there is between plus and minus. Its an artifact of having finite gain before their logic output takes over. Where would you specify offset? You'd have to have some overdrive.Nuclear instrumentation is one of the more demanding comparator applications but offsets less that a mV or so aren't required.Maxim made the MAX9015 and others that had a 150 uV Vos (typ). These were for use in flea powered signal chains where the customer didn't want to preamplify a sensor signal used for wake up signals, etc. All our standard comparators were about 1 mV.Keep thinking.

author: David Hess
author_rank: Super Contributor
timestamp: April 01, 2025, 06:36:28 pm
content: A preamplifier using a fast precision operational amplifier can always be added to reduce the effect of the comparator's input offset voltage. The preamplifier's feedback can be clamped with antiparallel diodes to keep it within its linear range, allowing the use of chopper-stabilization if required. This configuration was common in discrete fast 12-bit successive approximation ADCs.