[time-nuts] EFC tracking

J.D. Bakker jdb at lartmaker.nl
Sat Jun 26 13:19:10 UTC 2010

>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.

[had just been looking into this for a transistor matcher/noise test 
rig I'm working on]
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