Comparators—what’s all the chatter?

RE: Comparators—what’s all the chatter?

Az-eddine Haida — Wed, 05 May 2021 15:48:47 GMT

Hi,

RE: Comparators—what’s all the chatter?

Fabio Barone — Mon, 11 Dec 2017 00:28:58 GMT

Hello Bruce,

good read. If I may, I have one suggestion for reducing chatter for comparators with slowly moving signals: place a capacitor in **parallel** with R2 . You mentioned having one in series with R2, but I have found that placing a cap in series with the feedback (hysteresis) resistor is very effective in eliminating chatter. Sometimes an additional resistor in series with the new capacitor is also needed.

By having the capacitor as a parallel path to R2, one can think of the feedback as having two parts:

1. a DC part (set by R2 and R1 in the circuit above), and

2. an AC part, which would be set by the capacitor, its series R (if any), and R1.

The AC part provides a high positive feedback, which forces the output to change rapidly, and avoids chatter during the transition. Once the output transition is done, the DC part of the feedback sets the new threshold for the opposite transition. Suggest set the time constant of the AC part as being 2 to 5 times the time delay of the comparator (time from input crossing threshold to output reaching 90% of of final value), but of course, there is a trade off since this time constant would also set a lower limit on how quickly the compartor can then react to an input swapping over to the opposite state.

Loking forward to more posts on analog stuff!

Thanks.

RE: Comparators—what’s all the chatter?

Bruce Trump — Sun, 03 Feb 2013 19:20:39 GMT

Ken-- The upper and lower threshold are shifted by the offset voltage so the hysteresis band remains the same. -- Bruce

RE: Comparators—what’s all the chatter?

Ken Dillinger — Sun, 03 Feb 2013 18:31:17 GMT

What about the offset voltage? Vt + Vos + Vhys would be the trip point.

RE: Comparators—what’s all the chatter?

Bruce Trump — Wed, 30 Jan 2013 02:19:59 GMT

David--In my experience, a slowly rising input eventually causes a sudden onset of chatter. Chatter begets more chatter due to self-created noise. As the ramp continues, it dies out more gradually. With appropriate hysteresis, it's clean. I was deliberately vague on the specifics of ramp rate. It depends so much on the speed of the comparator and magnitude of the noise in your inputs. kHz. == Bruce

RE: Comparators—what’s all the chatter?

David Escandon — Tue, 29 Jan 2013 21:25:40 GMT

Thanks Bruce. In your experience, is "chatter" a normally distributed effect? Also, by fast moving ramp rates are you thinking kilohertz or more? Just wondering. Thanks.