jimlux at earthlink.net
Sat Aug 14 15:49:05 UTC 2010
J. Forster wrote:
> FWIW, IMO any engineer who uses undocumented or uncontrolled parameters or
> instructions in a production design is a fool.
> If you are that silly, you must fully specify the selection criteria.
Or, has their back against the wall and can't do it any other way.
How is this any different than using trimpots or hand select?
For years, folks have hand selected matched pairs of devices, since the
circuit requires tighter tolerances than the mfr guarantees.
Many, many RF designs have "select at test" pads to set levels or tuning
stubs depending on what the actual gain or impedance properties of the
active devices are, or for trimming temperature dependencies.
Would you say that the engineer is a fool for not just specifying
tighter tolerances.. the tighter tolerances may not be available from
the mfr (who has to respond to many customers, most of which will be
happy with the standard performance). It's sort of a tradeoff.. do you
go to the mfr and say, I need a better grade of part, or do you buy the
run-of-the-mill part, and sort them.
You might decide to do the latter for competitive reasons, e.g. rather
than the mfr producing a better grade of part, and potentially selling
it to your competitors too, you keep the "secret sauce" in house.
(Granted you could have the mfr make/select a proprietary part for you..
that's basically changing who does the work, but doesn't change the
Even manufacturers do this, for instance with speed grades on things
like microprocessors. They don't have enough process control to
guarantee a particular speed, so they make em all, and then sort them.
The other thing is that the selection criteria might not be knowable in
a standalone sense. That is, you have to put the part into the circuit
and see if it works, rather than measuring some device parameter. I
would agree that to a certain extent, this implies that you don't really
know how the circuit works, but it might also be that the most cost
effective approach is to use empiricism, rather than analysis.
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