J.D. Bakker wrote:
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.
Or the traditional chopper approach of a mercury wetted reed relay?
If you're processing with a sound card, you have the advantage that you
don't need to process the samples coming from the time of transition
(unlike a traditional analog chopper with synchronous detection), so a
fairly crummy relay would probably work. The key is that it can toggle
at, say, 100Hz, forever.
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.
The eval board for the part may have a computer interface built into it.
JDB.
[had just been looking into this for a transistor matcher/noise test rig
I'm working on]
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