[time-nuts] EFC tracking
jimlux at earthlink.net
Sat Jun 26 13:40:41 UTC 2010
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.
> [had just been looking into this for a transistor matcher/noise test rig
> I'm working on]
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