[time-nuts] Zero-Crossing Detector Design?

[Hal Murray]

albertson.chris at gmail.com said:
>> Hysteresis does nothing to eliminate jitter or temperature

> Maybe, but it is absolutely needed if there is any noise on the signal.   A
> perfect comparator with zero hysteresis would dither on every zero crossing.

Hysteresis doesn't eliminate the dither from noise.

Assume 5 volt CMOS logic with a switching point at 2.5 V.  With hysteresis, 
the low-to high direction switches at (say) 3 V and the high-to-low direction 
switches at 2 V.  If you have noise, whatever happens at 2.5 V without 
hysteresis happens at 3V with hysteresis.

Actually, if you are working with a sine wave, shifting the switching point 
off center will make the problem worse since the slope will be reduced.


Hysterssis will eliminate spikes or double pulses that are caused by noise on 
a signal with low rise time as long as the noise isn't too big.


-- 
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