JDB, On 27 June 2010 01:19, J.D. Bakker <[email protected]> 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/ > > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to > https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. > -- Steve Rooke - ZL3TUV & G8KVD The only reason for time is so that everything doesn't happen at once. - Einstein _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
