Denny Esterline wrote 2012-04-28 02:25: > On Fri, Apr 27, 2012 at 3:20 PM, Harold Hallikainen > wrote: > >> In my opinion, there are two things that set the maximum source >> resistance driving a PIC ADC. The first is the hold capacitor charge >> time. If you are measuring a near DC voltage and not using the PIC >> input multiplexer to read multiple ADC inputs, this should have no >> effect. The source resistance and the input hold capacitor form a low >> pass filter, but does not introduce a steady error. >> >> If the input is multiplexed, this becomes an issue. You'll see >> crosstalk between inputs as it takes time for the input holding >> capacitor to change voltage from one channel to another. >> >> >> > It's been forever and a day ago since I read that section, but I > thought the multiplexer specifically discharged the sample& hold > capacitor between conversions specifically to prevent this type of cross > talk. > That will give you a value with an error towards whatever level is used for the "discharge" (instead). I do not see how that is better then a value that is somehowe baised on interference from another channel. If you have two channels with, say, 3.0V and 3.1V on each, and you *do* have problems with aquisision time, you will get a larger error if you always discharge to 0V between each channel switch. Besides, I do not remember reading about that discharge for each conversion either. :-) Jan-Erik. > Or is my memory going like my hair? > > -Denny --=20 http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .