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From: Jean-Rene David
Subject: Unusual latchup problem
Organization: PubNIX Inc (http://www.pubnix.net)
Date: Wed, 06 Nov 2002 02:10:15 -0500
NNTP-Posting-Date: Tue, 05 Nov 2002 23:12:13 EST
This is a circuit I didn't design but which I'm troubleshooting.
It seems I have a latchup problem on a circuit with no connection to the
exernal world. There are four supplies (+-12, +-5) but no part is connected
to both (I removed them all) and no +-12V part is connected to any +-5V
part. All the +-12V parts are opamps or instrumentation amplifiers.
Now the symptoms are strange in the following way: when I fire up the power
supply with the circuit plugged in, the -12V latches up 90% of the time.
When I turn the supply on and THEN connect it to the circuit, the -12V
latches-up about 10% of the time.
Since the circuit is not connected to the external world, I thought ringing
from a steep rise on the P/S might cause the latchup. However the rise time
when turning the P/S on with the circuit plugged in (the case that latches
up) has SLOWER rise time (about 3ms) than the other case (a few
Also, when the -12V *doesn't* latchup, it actually goes up to 0.7V for
50-400ms after the +12V rise before coming back down again to -12V. The
delay seems random.
There are more than a dozen opamps on the thing and though I will do it if
I have to, I don't really feel like adding protection diodes or current
limiting resistors on each one.
If anyone has any idea on 1) the possible cause of this problem or 2) a way
to troubleshoot the problem or otherwise find the faulty device, I would
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