DAC failure mode

Hi folks,

currently I'm looking at a piece of equipment (a PC32 DSP board from
Innovative Integrations) that has an AD669 D/A converter on board. Instead
of the desired sweep from -10V to +10V, I get an odd assortment of linear
pieces that looks like this:

output
^
+10 | / /
| / /
| / /
+5 |/ /
|
|
|
0---------------------> command (0-FFFF)
/ /
/ /
/ /
-5 / /

Or, in words: When the output should be -10V (input 0000), it is -5V and
rises linearly. On the 3FFF->4000 transition, the output jumps back to -5V
and then rises properly to 0. On 7FFF->8000, I get a jump to +5V and a ramp
to +10V, at BFFF->C000 back to +5V, and another ramp to +10V which is
reached at FFFF.

Question: How can such an error arise? Is this a typical DAC failure mode?
Is it possible to cause it through ESD at one (buffered) analog output?
Stupid question of course. Anything can be caused by ESD. I wouldn't have
expected a commercial product to be that susceptible though.

The background is as follows: I've been asked by a colleague to look into a
problem that he seemed to have with some circuit connected to the DAC output
in question. To test that other thing (which turned out to be healthy) I
connected it to the PC32's output, found weird behavior, and then found the
problem on the PC32 itself. Now I'm being accused of having destroyed the
PC32 by touching its outputs. I won't go into the details of the
accompanying issues here, but up to this point I was the only one who didn't
have severe problems with this socially weird fellow, who is now convinced
that I have destroyed his card and expects me to repair it. Personally I
suspect that the card had been shot before and he'd just not noticed it in
his measurements (which isn't all too plausible either). Malevolence on his
part, weirdness aside, can be excluded.

robert
.

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