Re: DAC failure mode

Fred Bartoli wrote:

Joerg a écrit :

Fred Bartoli wrote:

Robert Latest a écrit :

John Larkin wrote:

Inspect the soldering. Most problems like this turn out to be visible.



In fact I did, and it looked promising: The DAC712 (not the AD669, contrary to the board's data***), had obviously been reworked in the past. It was hand-soldered and came from a different batch than the other two. Its D14 pin (being in a corner of the package) was easily inspectable under a microscope and looked suspicious, and indeed, I could get a scalpel into where there should have been solid solder. Rather than taking the chance of a pad that might have been damaged during the rework, I directly soldered a wire to the D14 pin of the neighboring DAC712 (both on the same data bus).

Seldom had I been more confident of having found the right fix, so I was quite disappointed when the problem remained (intermittent, by the way, as I've found out in the meantime. Sometimes the darn thing works).

Lacking proper logic analysis equipment, I clipped a scope to my wire and played with the DAC's inputs (unfortunately I can't set them directly but have to mouse-move a slider on the application that talks to the board). The D14 line sits frozen at 0V, although it should of course cycle high and low at times when it should be set high for this particular DAC.

Then I started the part of the application that ramps the other DACs (all of which work flawlessly). Once this is running, the D14 line starts hopping merrily between 2.5 and 5V (!!!). When I stop the scanning process, the D14 line sits back at ground. Capacitive coupling through a faulty trace? Why the three-level thingy, then? Could it be a measurement thing -- because I don't have an ISA riser board, I couldn't fit a proper scope probe onto the board but had to use clips and banana-plug cables.


Not capacitive. Just a dead short between 2 outputs, and the drivers' RDSon divide according to their status.


Or (but more unlikely) something going into tri-state.


Go hunting for another data/signal line showing the same pattern and you're almost done.


Or trace it back to the respective bus driver chip and see if that looks healthy. Quite possibly a solder joint has come off at that chip.


Tssttt... With a sig line hoping between 2.5V and 5V?
Not impossible. But very unlikely.


It depends on what else is hanging on that line. For example, you can have that very scenario with a thevenin termination. High -> 5V or whatever VCC is, tri-state -> 2.5V or whatever the thevenin ratio has been set to.

--
Regards, Joerg

http://www.analogconsultants.com
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