[time-nuts] EFC tracking

[Steve Rooke]

JDB,

On 27 June 2010 01:19, J.D. Bakker <jdb at lartmaker.nl> 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.

I'd be looking at buffering the inputs to provide a low impedance
there and choose values of the R's and C's around the op-amp to try
and alleviate problems caused by the 4066's contact resistance. I am
aware of the higher spec devices but will have to see if it's easy to
source them over here in New Zealand without having to order them
overseas.

> 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.

Do you have any part numbers to hand that I could go and look up
please? Certainly if there is a decent custom solution it may be
better but I'd have to bear in mind the cost of probably shipping this
in from abroad.

Thanks for your help,
Steve

>
> 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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-- 
Steve Rooke - ZL3TUV & G8KVD
The only reason for time is so that everything doesn't happen at once.
- Einstein