On Tue, 2008-08-12 at 23:59 -0500, Mark Rages wrote: > On Tue, Aug 12, 2008 at 10:45 PM, Jinx wrote: > > Just out of curiosity (and a little $$$ saving), is a capacitor > > acceptable as a low-impedance "buffer" for inputs ? > > > > I've tried that in the past, without success. I put a largish > capacitor on the ADC input, but I still noticed a lot of noise in the > reading. An op-amp buffer made the noise go away. > > My (speculative) theory is that the average input currents change > depending on how the successive approximating goes and how much charge > gets taken from C_hold. Did you try just the cap? If so that's your problem. The cap alone won't solve much, what you are after is a low pass filter, and for that you'll need a resistor as well. The values are really up to your specific situation. Offhand, make your external cap >> then the S&H cap in the pic. Then select a resistor that forms a low pass filter with the cutoff you need. TTYL -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist