On 2016-09-04, Jim Wilkins wrote:
"DoN. Nichols" wrote in message
...
On 2016-09-04, Jim Wilkins wrote:
.....
That company built Analog Devices' production-line parametric
testers,
the machines that confirm each part meets specs, and had an
arrangement to get enough of their hand-selected highest-performing
op
amps to build more sensitive and accurate analog circuits than
anyone
else. Most of them went back in testers for AD.
Eventually, that should saturate, and more make it out to the
rest of the industry. :-)
The hand-selected ones had different part numbers that weren't in the
catalog. There's always a bell curve distribution.
Of course.
We got the upper
sigma for input bias and offset current, Radio Shack was rumored to
get the lowest one.
I believe it. :-)
Other parameters didn't matter to us so maybe some
audio synthesizer company got the ones with the highest frequency
response. Our machines didn't check for that, only the
guaranteed-by-test data sheet parameters, though I've measured Bode
plots on the bench. As long as the circuit settled to the required
accuracy within one millisecond we were happy.
Reasonable -- for what you were doing.
That was for analog measurements. The digital memory chip testers sent
out address and data at 50MHz, state-of-the-art in the early 80's when
the test vectors had to be generated in the main rack and sent out
long cables to the test head.
Ouch! Perhaps a buffer near the test head which could be
pre-loaded from the computer, and then triggered to spit it out at the
RAM on command. If the tests were repetitive enough, that would
minimize the total bandwidth sent to the test head -- and local checking
for errors and only spit the errors back to the computer to minimize the
bandwidth the other way.
I remember long ago when I worked for Transitron -- one of the
products was what they called a "ref-amp" -- a potted transistor with a
Zener and forward diode (called a Stabistor, and selected for
temperature behavior) in series with the emitter. The transistor B-E
forward drop, that of the diode and of the Zener were matched by a
minicomputer measuring large batches of each device at three
temperatures -- -50C, +50C, and +150C, and punching out a deck of cards
-- one per device. These went to the mainframe to be sorted to get the
minimum temperature sensitivity. The devices were in multi-device
carrier submerged in a bath of silicone oil -- a different one for each
temperature.
I remember when we got a few of them back from a customer, and
I had to test them through the whole temperature range. At the -50, the
other silicone oils set up to a gel, and at the +150, the low-temp one
boiled, so we mixed about 50-50 of high and low temp oils. Put the
DUT in the bath, toss in a handful of dry ice chunks, and wait until it
got below the -50, and then turn on the hotplate under it and take a
reading at ever 10 degrees C on the way up. Then turn off the hotplate,
wait for it to cool down, swap in the next device, and repeat. (Yes,
the devices did go out of spec between the three points at which they
were tested before assembly. :-)
Anyway -- someone else wandered into the room, and tossed in
some dry ice while the bath was still quite hot. It boiled like mad
(into the pockets of CO2 in the liquid), with vapor spilling over the
edge of the really large beaker, spreading out over the bench, and
curling in to the still hot hotplate coils.
FWOOF!
It was interesting seeing the silicone oil vapor burn. We had a CO2
extinguisher handy, and put it out, and were able to continue with the
tests. One thing which I had not expected was that there was fine white
sand over everything, from the burning Silicone oil. :-)
I bought a batch of Chinese Schottky solar panel isolation diodes that
all are slightly below the reverse voltage leakage spec at room
temperature, and worse above it, as though they were the rejects from
a production tester.
Sound adequate for the purpose, at least. :-)
In class they treat op-amps as ideal devices. I was exposed to the
nitty-gritty of all the ways they aren't, and how to measure the
discrepancies.
Yep. In class, the input impedance is infinite, the output
impedance is zero, the open-loop gain is infinite, balance between
inputs is perfect, and no capacitance anywhere to delay signal changes.
For most applications, you can get away with treating them as being like
that -- but of course they are not.
My first exposure to op-amps was the plug-in modules with two
tubes which were used as part of the Beta testers on the transistor
production line. I had no idea how they worked at the time, so the
schematics rather puzzled me. :-) Trying to remember who made them, and
I *think* that it was Philbrick -- who also made ones with discrete
transistors in little metal bricks.
I first learned how to use them from the Burr-Brown application
booklet.
http://www.analog.com/library/analog...surements.html
Enjoy,
DoN.
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