I fought this problem pretty hard dealing with a 500 ohm pot (voltage divider) at the end of a 100 foot cable. The A/D looked good on the scope without the cable and the cable looked good without the A/D. Op amp seemed to help but added complexity, Capacitors just changed the problem. The solution was a big RF choke (2500 milli-henry?) from the junk box. An iron core transformer also worked fine but was too big & bulky to consider. Since this is a one-off kind of project I have not been too worried about the details past the point that it is working. Side note: I tried to go back to the archives to put the resolution on the original thread but I seem to lack the talent to do so. The project has been slow because it has spanned a couple of hospital stays and surgeries. Now it is hard to get back to the workbench because of the new found freedom! John Ferrell W8CCW "All that is necessary for the triumph of evil is for good men to do nothing." -- Edmund Burke http://DixieNC.US ----- Original Message ----- From: "Jinx" To: "Microcontroller discussion list - Public." Sent: Tuesday, August 12, 2008 11:45 PM Subject: [PIC] A/D impedances > Just out of curiosity (and a little $$$ saving), is a capacitor > acceptable as a low-impedance "buffer" for inputs ? > > d/s says, for Vref for example > > "In order to maintain the A/D accuracy, the voltage reference source > impedances should be kept low to reduce voltage changes. These > voltage changes occur as reference currents flow through the reference > source impedance. The maximum recommended impedance of the > VREF+ and VREF- external reference voltage sources is 250R" > > I'm revising a circuit that currently uses op-amps and voltage reference > ICs, and wonder if much of that can be dispensed with. The signals to > be measured are fairly steady DC voltages, on a minutely basis > > The ADC sampling is over in a few microseconds, so if the answer to > my first question is "yes", then what calculation is needed for the value > of C ? Or would one simply increase the acquisition time ? > > I've already trialled a 48V input using a 910:10k divider (FSD is > 61.44V) followed by a 47nF capacitor, and can't see any difference > between that and with the existing op-amp buffer in place, even with > the very short acquisition time of a 40MHz PIC > > -- > http://www.piclist.com PIC/SX FAQ & list archive > View/change your membership options at > http://mailman.mit.edu/mailman/listinfo/piclist > -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist