PSN-L Email List Message

Subject: PSD noise trouble
From: Chuck / Judy Burch cjburch@...........
Date: Wed, 26 May 2010 14:39:43 -0600



Thanks to everyone who made suggestions on eliminating high frequency 
noise pulses produced by an analog switch, especially Karl Cunningham 
and Walt Jung.

The problem was that high frequency switching pulses got through the DC 
power bypass capacitors at the chip and propagated on the power supply 
lines and ground over the whole circuit.  I was using 0.1 uF bypass caps 
on each chip power connection.  When I used a 100 uF electrolytic in 
parallel with a 1.0 uF film cap to bypass the switch's power connections 
the problem effectively disappeared.  Other schemes might work just as 
well or better - these were components I had on hand.

Some conclusions:

1.  Bypassing PS connections at each chip is not overkill.

2.  Ordinary bypass practice (0.1 uF to ground at each power connection) 
is fine in most circumstances - but not all.

3.  Try not to mix low level analog circuits with switching circuits - 
and be very careful if you do.

4.  Don't use that switching power supply you salvaged from a dead PC, 
except as a source of parts.


Thanks again,

Chuck Burch
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