I wonder if anyone has done something like this before and could share
their experiences.
The general principle should work. However, as you're interested in
slow changes, there are some error sources that might be
unacceptable, including the drift of (differential) channel
resistances for the 4066 over temperature, voltage and time. As shown
the scheme is also sensitive to impedance mismatch/drift on the two
inputs. Charge injection is a bit on the high side on a 4066; a more
expensive (A)DG4xx-series chip may improve on that.
I don't know if it qualifies as simple/cheap, but Analog Devices and
others have single chip low-rate sigma/delta converters with good to
excellent properties; these were meant for strain gauges but should
be able to track slow-moving control voltages just fine. Interfacing
them to a parallel port (or USB PP adapter) should be close to
trivial. Do have a close look at the data sheet: some parts have
unbuffered inputs, and present a fluctuating input impedance which
might couple onto EFC lines. A simple isolation amp with one or two
precision op-amps should fix that.
JDB.
[had just been looking into this for a transistor matcher/noise test
rig I'm working on]
--
LART. 250 MIPS under one Watt. Free hardware design files.
http://www.lartmaker.nl/
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