Page 20 - Troubleshooting Analog Circuits

Consider Appointing a Czar for a Problem Area 7

+5v
0
R128
1K

POINT A

r

R1
1K
- NOTE: ALL RESISTORS 1 K
-
Figure I .2. If you discovered that the midpoint was not at 2.5 V, but at 0 V. how would you troubleshoot
this circuit? How would you search to detect a short, or an open?

woking with 2-layer metal, I bring nodes up from the first metal through vias to the
second metal. Sometimes I leave holes in my Vapox passivation to facilitate probing
dice. The subject of testability has often been addressed for large digital circuits, but
the underlying ideas of Design For Testability are important regardless of the type of
circuit you are designing. You can avoid a lot of trouble by thinking about what can
go wrong and how to keep it from going wrong before the ensuing problems lunge at
you. By planning for every possibility, you can profit from your awareness of
Murphy’s Law. Now, clearly, you won’t think of every possibility. (Remember, it
was something that couldn’ f go wrong that caused the problems with Stanier’s loco-
motives.) But, a little forethought can certainly minimize the number of problems
you have to deal with.

Consider Appoihting a Czar for a Problem Area
A few years ago we had so many nagging little troubles with band-gap reference
circuits at National, that I decided (unilaterally) to declare myself “Czar of Band
Gaps.” The main rules were that all successful band-gap circuits should be registered
with the Czar so that we could keep a log book of successful circuits; all unsuccessful
circuits, their reasons for failure, and the fixes for the failures should likewise be
logged in with the Czar so that we could avoid repeating old mistakes; and all new
circuits should be submitted to the Czar to allow him to spot any old errors. So far,
we think we’ve found and fixed over 50% of the possible errors, before the wafers
were fabricated, and we’re gaining. In addition, we have added Czars for start-up
circuits and for trim circuits, and a Czarina for data-sheet changes, and we are con-
sidering other czardoms. It’s a bit of a game, but it’s also a serious business to use a
game to try to prevent expensive errors.
I haven’t always been a good troubleshooter, but my “baptism of fire” occurred
quite a few years ago. I had designed a group of modular data converters. We had to