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Path: cantaloupe.srv.cs.cmu.edu!das-news2.harvard.edu!news2.near.net!howland.reston.ans.net!cs.utexas.edu!csc.ti.com!news.ti.com!mksol.dseg.ti.com!strohm
From: strohm@mksol.dseg.ti.com (john r strohm)
Subject: Re: Help: noise on op-amp output from digital signals
Message-ID: <1994Nov23.210813.21410@mksol.dseg.ti.com>
Organization: Texas Instruments, Inc
References: <mark.stephens-2211940850140001@mstephens.gsfc.nasa.gov>
Date: Wed, 23 Nov 1994 21:08:13 GMT
Lines: 46
Xref: glinda.oz.cs.cmu.edu sci.electronics:109152 comp.robotics:15637

In article <mark.stephens-2211940850140001@mstephens.gsfc.nasa.gov> mark.stephens@gsfc.nasa.gov (mark stephens) writes:
>While I've made progress in getting an op-amp to produce a fairly stable
>signal I have discovered yet another noise phenomenom.  When I'm looking
>on the oscilloscope at a DC op-amp ouput (LP324 quad), there is a periodic
>1v PP "buzz".  After stairing at it awhile, I realized it was the exact
>same pattern produced from a port on the HC11 which toggles an LED.  Each
>time the MPU wakes up after a real time interrupt (a WAI instruction), it
>toggles the LED off (signal low) and somehow creates the "buzz" on the
>op-amp output.  The LED is switched off (signal hi) just before the MPU
>goes into the wait state.
>
>The op-amp is powered from the same switching supply as the digital at +-12v.
>
>Is this the ground loop monster?  Or the horrific ground bounce?  All of
>this stuff is on a breadboard, sharing the same ground.  Should I lead all
>of the analog grounds to a single point?

This sounds like the "no deglitching capacitors" monster.  Try putting 0.1 uF
directly across the op amp plus and minus rails, and also put about 10 uF
across the breadboard.

And deglitch ALL the digital ICs: check Don Lancaster's cookbooks for rules
of thumb.

As a general rule, you should NEVER run your analog and your digital stuff off
the same power supply, but ALWAYS put isolation between them.

What is most likely happening is that the LED pulse is pulling a big surge
from the positive rail, which is pulling down the positive rail slightly with
reference to the ground and negative rails.  The op amp sees this as a change
in the input level (measured between the positive and negative rails) and
amplifies it, as advertised.

Deglitching capacitors are wonderful things; many engineering school courses
mention them in passing, but never actually manage to put the student in a
position where he HAS to put one in to make something work.  Lots of lab
breadboards have stiff enough power supplies that deglitching isn't needed.
But not all.  I learned about deglitching on a battery-powered CMOS project
that worked perfectly on my bench supply but refused to work on the battery.

>
>Any help, comments would be appreciated.
>
>mark


