I have read a few of these posts and if I am correct (which may not be the case) the question is about the reference voltage on an A/D. I have always used a band gap reference which has a low enough impedance to help minimize noise. I sometimes have used zener circuits. In simple terms the low impedance demands more current from the noise than the noise can source. A 500 ohm pot may actually be reasonable, but 10K may be pushing it a bit. Noise that can source 1 uA is 1 uA*10K=10 mV of noise. But for Z=500R, noise that can source 1 uA is 1 uA*500 = 500 uV. I did not understand the part about using a 100 foot cable. Was this for the reference voltage? I would think that such a cable would have to be terminated with an appropriate Z. ----- Original Message ----- From: "peter green" To: "Microcontroller discussion list - Public." Sent: Wednesday, August 13, 2008 8:05 PM Subject: Re: [PIC] A/D impedances > John Ferrell wrote: >> 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. > This sounds like it could be a transmission line effect to me, my guess > is that the spikes from the ADC were getting reflected off the far end > of the cable becoming effectively noise on future readins. > > > -- > 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