[sdiy] Temperature Compensated Exponential ConverterUsingSSM2164

[David G. Dixon]

> No. You still have 3300ppm/K after division.

Yes, of course, you're right.  I shouldn't do this stuff at night!

I think this leaves two options:

1.  The "obvious" option:  Take 15V straight from the supply rail and divide
it down according to the scheme as outlined before.  I'm presuming that the
supply rail won't bounce all over the place!

2.  The "clever" option:  Drop the rail voltage with a zener diode rated
between 4.7 and 5.6 volts...

The temperature coefficient of zener diodes depends on which breakdown
mechanism predominates.  Below about 5V, the Zener effect is predominant,
and this has a negative TC.  Above about 5V, avalanche breakdown is
predominant, and this has a positive TC.  At about 5V, the two effects are
roughly equal, and their TC's cancel each other out.  Looking at the
datasheets, I would opt for a 5.1V zener in this case.  The TC of 1N751, for
example, is stated as +/- 0.01%/K, which suggests that it may even be zero
with a bit of luck.  In the worst case, this gives +/- 0.51mV/K which, when
divided down to 300mV, would give only +/- 30uV/K in the tempco voltage.
This is well within calculated tolerance for stable tuning.

I think we have a winner!

(To paraphrase our glorious past-president, George "W." Bush, "Is our
synth-diyers learning?"  Yes, we is!)