A guy recently asked me how I would look for a voltage reference that's
stable versus temp cycling. I told him I would take several of the best voltage
references I had and use a dual-slope DVM of at least six digits to
compare them to the units in question. He then asked if comparing some references to some other ones was kind of incestuous. This is
not rocket science.
You take several good voltage references and leave them the
heck alone! Apply some bias and just let them run undisturbed. I
have done this many times. But if possible, measure each of
them, at least once or twice per day. Gather up the trends. Look at
the data. Study them, study the standards, and study the DUTs.
I once had a set of 16 good references, LM399-types, that had
a subsurface (buried) zener and a heater to hold them at 88°C.
Each output was 6.9 V ±2%, and I averaged the output through
some 499-O resistors. The output impedance was 65 O. I measured
the average of eight versus eight. The relative stability
seemed to be very good. The noise seemed to be excellent -
better than 0.1 ppm p-p, out of 7 V, in a bandwidth of about 1 Hz.
But the point is that if you leave something alone, and cycle
something else through experiences, you learn something.
What are you trying to learn? A drift versus temp is easy to spot
if one part goes through a temp cycle and another doesn't.
Cycle the DUTs twice or four or eight times. Look for trends.
Check the VREF of each of these against the average of the two
or four or eight uncycled references. You may learn something.
What do you see for trends? Do you see a drift that decreases
gradually versus experience? Or is there some hysteretic drift
that keeps coming back?
Back in the day
Back 24 years ago, we set up some
excellent long-term stability tests for the LM199AH. We read their
data every week for six weeks, compared to a battery of other reference
voltages, including ovenized saturated standard cells,
4 1.018 V dc. We convinced ourselves that these references
were mostly drifting less than 10 or 15 ppm per 1000 hours.
We sold them with a guarantee of 20 ppm per 1000 hours.
When we ran out of such customers, we shut down that aging/
testing program. But it was challenging. What if one or more of
the tested references had some drift? We were prepared to use
our judgement to decide that the apparent drift of one reference
could be ignored - if all the other references seemed stable.
What if the DVM's reference started drifting? This was a ratiometric
test, so we could correct for that. So long as most of the references
seemed stable, we could compensate. Of course, we had
to rely on the ratiometric linearity of the DVM, which was guaranteed
and inherent, better than 1 ppm. It was a very good HP3456.
To be fair, I shouldn't just call it a dual-slope DVM. It was more
like quad slope or multi-slope. Its measurement scheme used a
lot of rubbing and polishing. I have seen several other kinds of
six-digit DVMs that had linearity flaws, but the Hewlett-Packard
ones never showed any such error. Now they are called Agilent.
Since that era, we have used groups of LM399s as references
for testing other kinds of band-gap references. Of
course, using a group of four references can increase the
chances that one of the group will start drifting.
But if you have four groups of four, the chances that one will
start drifting and won't be apprehended are quite small. Longterm
drift can be fairly dependable. We sent some LM399s to
the NBS/NIST, which found a long-term drift rate of about 1 ppm
per 1000 hours when the die was self-heated to 88°C.
Other people have observed that if the LM399 isn't heated
to 88°C, but just kept at room temp, the long-term drift rate
can be less than 1 ppm per 1000 hours. So should you heat
them up only an hour per month? Maybe so.
The VOS of an op amp is almost trivial - and it is not trivial, nor
is it trivial to guess, what will happen on the next full temp cycle.
Sometimes an op amp will drift 1 or 2 µV. But other times it may
drift 3 or -4 µV after cycling around a full set of temperature tests.
The next time it goes around the cycle, it might drift 4 or -3 µV.
This is due to stress on the die. Can you predict this? I don't think
so. So even an op amp requires some respect in its testing.
A computer can predict how much stress will be on the first
die, at various places, when it is packaged. It can predict how
much stress will be on the second die - and the third. It's all the
same. So offsets and drift and hysteresis will be the same,
right? Not so. So much for computers. So much for CAD. I prefer
to admit the reality of computer-hindered design.
These stresses apply also to band-gap references. They, too,
have drifts as they are temp-cycled and brought back to the
original temperature. I don't know any circuits that aren't more
stable if you just leave them alone.
Comments invited! rap@galaxy.nsc.com - or:
Mail Stop D2597A, National Semiconductor
P.O. Box 58090, Santa Clara, CA 95052-8090