[time-nuts] EFC tracking

jimlux 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.

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

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