From 6c680d07902fa90e548efc186fd75136e2ab6e66 Mon Sep 17 00:00:00 2001 From: David Schleef Date: Fri, 7 Jun 2002 00:17:35 +0000 Subject: [PATCH] More questions --- doc/FAQ | 54 +++++++++++++++++++++++++++++++++++++++++++++++++++++- 1 file changed, 53 insertions(+), 1 deletion(-) diff --git a/doc/FAQ b/doc/FAQ index 2226fe8..bfbffaa 100644 --- a/doc/FAQ +++ b/doc/FAQ @@ -55,7 +55,59 @@ and send the information to David Schleef . 6. Could you please add a search engine to the mailing list archive? No. Please use generic web search engines such as Google, where you -can add the search term "host:www.comedi.org" to limit the search to +can add the search term "site:stm.lbl.gov" to limit the search to the site where the Comedi archives are located. +7. I apply a signal to channel 0, but I see the signal on other +channels. + +Typically, this is an electrical problem that can come from multiple +sources. First, any inputs that are not specifically driven or +grounded will tend to float around, measuring random electrical +signals in their environment -- the strongest of which are usually +nearby channels, 50/60 Hz line noise, or the A/D converter itself. + + +8. An FFT of measured data shows lots of sharp peaks. + +Yep. Not much that can be done about that. Well, actually, there is. +Peaks near multiples of 50 or 60 Hz are electromagnetic coupling from +the power mains. Almost all the time, this indicates a ground loop +in the measurement path. Peaks that are multiples of 100 or 120 Hz +on photodetectors are because of the room lights. Peaks in the 100-1000 +Hz range, with harmonics, are sometimes stray fields from electric +motors. Strong, sharp peaks in the 1-100 kHz range are typically due +to the horizontal refresh of CRT displays (like your computer monitor). +Less strong peaks can be due to electronics in a computer or other +device. + +However, a sharp peak in an FFT does not always indicate a problem. +Remember that you always have noise, and that some of it will be +periodic. You just have to make sure that the particular type of +noise that you have does not affect your analysis. + + +9. Ok, I'm getting good results. What should I look at to get great +results? + + - Make sure that all unused inputs are grounded. Electrically floating + wires act as antennas, picking up stray electrical signals, and often + strongly coupling them into the signals you actually want to measure. + + - Get a flat panel monitor. CRT displays emit strong magnetic fields + oscillating at the horizontal refresh rate. It is most easily + observable in a Fourier transform of an input signal. + + - Calibrate your board. + + - Read up on ground loops. Make sure that all your electrical + shielding has exactly one path to a common ground. + + - If you use a laptop for data acquisition, make sure it is grounded + properly. Laptops usually are not grounded, and you generally + don't want your data acquisition cable to also be your grounding + wire. + + - + -- 2.26.2