Em 28/4/2012 04:56, Jan-Erik Soderholm escreveu: > 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. Once I designed a digital echo chamber. It used two audio channels, one for input and another for output. In the first design there was some crosstalk between channels and the sound never faded completely, even with muted input and high decay rate. It was not two A/D channels but the effect can be the same for some applications. Isaac --=20 http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .