At 03:42 PM 6/11/02 -0400, Olin Lathrop wrote: > > Microchip specifies that your source impedance should be no higher > > than 10K - you have a worst case source impedance of 25K (when the pot is > > in the exact middle). The cure is simple - add a 0.1 - 1 uF cap from the > > analog input to gnd. The cap supplies the charge required when the input > > is sampled and restores full accuracy to the a/d input. > >Not so fast. Yes, a large cap will hold the pin voltage constant during the >brief acquisition interval, but accuracy may still be off and the "cross >talk" between channels may become worse. Nope - I disagree here. Been there, done that, spent rather a long time verifying performance. Cross talk is definitely NOT made worse by adding the cap. But I should qualify my statement: the addition of the cap reduces the effect of series impedance so long as the sample rate is rate is slow enough that the charge removed (or added) during the sample process is completely restored before the next sample occurs. A quick look at the ratio between how long the sample switch is closed on a particular channel vs how long it is open suggests that this is not a problem if sampling only 2 channels at the fastest possible sample rate and definitely not a problem is sampling 4 channels or more. Its absolutely not a problem with 100K series resistors and 1 uF caps when sampling channels at a 1 ms rate - again, I spent a lot of time verifying accuracy in one project before deciding it was OK. But take away the capacitors - not acceptable. dwayne -- Dwayne Reid Trinity Electronics Systems Ltd Edmonton, AB, CANADA (780) 489-3199 voice (780) 487-6397 fax Celebrating 18 years of Engineering Innovation (1984 - 2002) .-. .-. .-. .-. .-. .-. .-. .-. .-. .- `-' `-' `-' `-' `-' `-' `-' `-' `-' Do NOT send unsolicited commercial email to this email address. This message neither grants consent to receive unsolicited commercial email nor is intended to solicit commercial email. -- http://www.piclist.com hint: PICList Posts must start with ONE topic: [PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads