On Tue, 7 Nov 2000 17:09:56 CET, you wrote: >>From: Lawrence Lile >>Reply-To: Lawrence Lile >>To: PICLIST@MITVMA.MIT.EDU >>Subject: [PIC]: The tintinnabulation of the noise >>Date: Tue, 7 Nov 2000 09:21:00 -0600 >> >>(above quote is from Poe) >>(yes, tintinnabulation is a word) >> >> >>I'm sampling a DC voltage with lots O noise. I've set up a routine = that >>takes a sample, waits a few milliseconds, then takes another sample, = etc. >>When 15 samples are taken, it takes the median of the samples as the >>result. Still get false alarms due to noise pulses every once in a = while. >> >>Now, I am not real sure if I will have 60 hz noise (picked up from the = air) >>or 120 hz noise (from a full wave rectified power supply) as the major >>noise component. It may actually be both, at various times. >> >>Does Old Nyquist rule in this situation, should I sample at twice 120 = hz or >>more to cancel out the effects of noise? I'm not interested in the >>frequency of the signal at all, just in an accurate DC level. > > > >The sampling frequency should be based on 60Hz period, when the noise = comes >from. Sampling input signal with higher frequency, for example 5 * = period, >and averaging during a whole period of 60Hz works in this case like = digital >filter. Niquist said about the theoretical minimum sampling frequency, = but >in reality, it is much higher. If you are trying to measure DC with 60Hz noise, 60Hz is exactly what you DON'T want to sample at as this is almost guaranteed to add a DC error. Avaraging samples at 120Hz ( or any multiple of 2 x the noise frequency) would be a lot better as the noise half-cycles would cancel each other out. -- http://www.piclist.com hint: To leave the PICList mailto:piclist-unsubscribe-request@mitvma.mit.edu