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From: email@example.com (c3l)
Subject: Re: Digital design help needed.
Date: 24 Sep 2002 06:09:01 -0700
NNTP-Posting-Date: 24 Sep 2002 13:09:01 GMT
"George Gonzalez" wrote in message news:...
> > The problem I face is the noise generated by the Schmitt trigger. It
> > seems (from the oscilloscope) that when the schmitt triggers the data
> > acquisition board, the is a slight spike in the output of the S/H
> > amplifier.
> Anytime you have analog signals, they're going to be picking up
> digital switching noise.
> One approach is to slow down the digital transitions a bit,
> so there's less dv/dt current induced into the analog lines.
> A simple series R/C filter will slow down the transitions.
> Place it as close to the schmitt trigger as possible, and choose
> the R/C time constant to be 2 to 10 times the observed rise-fall times.
> Another approach might be to delay the trigger signal to the
> A/D board with a small delay line, so it doesnt start converting til after
> the spike.
There is a RC filter at the input of the Schmitt trigger. Basically I
use that RC filter to delay the Schmitt trigger to the A/D board. Its
the output of the Schmitt trigger that induces that noise spike.
Thanks for the tip! I'll give it a try and see what happens. It should
help but the whole point of my circuit to measure a specific point on
the pulse. The delay will introduce some uncertainty but if it works,
then at least the point on the pulse I'm measuring is more accurate.
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