Re: DAC failure mode

Robert Latest wrote:
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? [...]

From the looks of it, I'd say the bit with weight 4000 is stuck on. If
that isn't visible at the DAC input bus, then we must conclude that
something inside the chip is shot.

Regards,
Jeroen Belleman
.