On 27 June 2010 01:40, jimlux <jim...@earthlink.net> wrote: > 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 really hadn't thought of factoring in the resistance of the 4066 or environmental effects on it I must admit. I wonder how long a mercury wetted reed relay would last at 100Hz though. The sampling rate would be much higher than this to discard the transition readings. >> >> 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. So I need to locate this if possible, any pointer please? Thanks for your help, Steve > > > > >> >> JDB. >> [had just been looking into this for a transistor matcher/noise test rig >> I'm working on] > > > _______________________________________________ > time-nuts mailing list -- time-nuts@febo.com > 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 -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.