Thanks very much for the suggestions.  The load impedance change was caused 
by a change in resistor values around the 5532 summing amp that follows the 
latching-up LF353.  I've now reduced those values by a factor of 10 and I'm 
retesting.  It took about 22 hours for the fault to appear each time, so 
it's too early to tell whether this change will fix it.

In researching this problem, I came across an article that claims that the 
TL072 is particularly susceptible to latch-up, although I've used them for 
years without problem.  I'll provide the link if I can find it again.

This is a 2-layer board with an external switcher, but the bottom layer is 
virtually all ground.  Should that suffice?

What would be the simplest way to protect the op amp input from spikes? 
Zener to ground?

Thanks again,
RR

----- Original Message ----- 
From: "Gerhard Fiedler" <lists@connectionbrazil.com>
To: <piclist@mit.edu>
Sent: Wednesday, January 25, 2006 3:46 AM
Subject: Re: [EE]: op amp latch-up problem


> Rob Robson wrote:
>
>> Besides power supply noise, the only other difference betwee n the
>> functioning legacy circuit and the faulty new one is the impedance
>> they're each looking into: 2.7k for the former; 22k for the latter. Does
>> anyone have any ideas about what might be causing the op amp to latch
>> up?
>
> Not really. But possibly you're one step further if you check whether your
> circuit works with a 3k resistor from the output(s) to ground (so simulate
> the earlier impedance).
>
> What did you change to cause the impedance change?
>
> Gerhard
>
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