> This is a concern, as I have to have 3 wires in the set. Would this > create more interference than it eliminates? Should I see about > running just Ground and Signal with 5V from a nearby source not in the > cable? This was something I forgot to ask in the original question and > I'm glad you mentioned it. No, the point was that twisted pairs are exactly that - pairs of 2 wires, prefferably balanced impedance. It's done for a specific reason and the effect is not there with 3 wires. You need 3 wires so you use a 3 wire + shield cable and connect the shield to 0V at the PIC end. A 2 wire + shield cable may be easier to find but is not quite as good. In that case make the shield the 0V connection to the pot. >>>Don't forget the protection diode like Jinx says, or use a zener. >>>Put the diode after the R of the RC filter so the diode current will >>>be limited. > >>Using a Schottky diode is not a good idea if you place it after the >>series resistor - the leakage current is too high. However, I have >>found that I have NEVER needed the diode in the arrangement described >>- 3 wires off to the pot, low pass RC network feeding the a/d input. > > This seems like a nice simple solution, how critical is proximity to > the input pin for the low-pass network? Should it be as close as > possible to the potentiometer to filter the input sooner and a short > PCB run to the input pin, or should it be as close as possible to the > input pin to eliminate any induced interference on the PCB? Put the RC low pass filter as close to the PIC input pin as possible. Most of the noise will be induced in the long cable running to the pot. As indicated above, you can get away without the diode protection. It makes things simpler. The only things that help reduce pickup in the cable are shielding, balancing & twisting, reducing impedance or buffering. Just like they do in audio. Making the pot the lowest practical resistance is good, shielding is easy, everyhting else is not really worth the effort for this application. -- Brent Brown, Electronic Design Solutions 16 English Street, Hamilton, New Zealand Ph/fax: +64 7 849 0069 Mobile/txt: 025 334 069 eMail: brent.brown@clear.net.nz -- http://www.piclist.com hint: To leave the PICList mailto:piclist-unsubscribe-request@mitvma.mit.edu