> From: Casey Crane> Date: Thu, 24 May 2001 17:35:29 -0700 > >How do you define "Counts of > noise" ? I have an O'scope But how do I measure this random or > psuedo-random stuff without a spectrum anylizer ? You don't need a scope, just the old eyeballs. And you need to use the gravitational field of the earth to calibrate your ADXL circuit board. What is your resolution? You can check that rotating your sensor from + to - 1g, where "g" is the gravitational field of the earth. For example, Larry Cochrane set his ADXL05 up so that his 16 bit a/c gave about 16000 counts/g. Then each count represented about 0.06mg (thousands of a g). See http://psn.quake.net/strongmotion.html What change in counts do you get when you rotate your board? What does each count represent in your system in mg? Now how many counts +/- do you get when the unit is sitting still? Larry got about +/-30 counts, and since his resolution was 0.06mg, the noise level was about about 2 mg. Just eyeball the output for a minute or two to get a rough reading of the range of the noise readings. The ADXL202 spec sheet has a formula that relates bandwidth (b) to noise level (n). It is approximately n=0.5*sqr(b) in mg. So if your bandwidth is 15 hz, your noise level is n=0.5*sqr(15)=~2mg. If you see a lot more noise than that you should recheck your circuit. > My AtoD is quiet as a mouse. I'm using an ADC0809 eight channel > AtoD and am using but four of these channels. I'm using the printer LPT1 > interface. Perhaps your resolution is much larger than 2 mg and that is why you are not seeing any noise? If you rotate your board from +/- 1g and the a/d does not change by at least 1000, then you need more gain or more a/d bits to make use of all the performance the ADXL202 is capable of. The ADXL202 has the same noise specs as Larry's ADXL05. The newer single channel ADLX105 has half the noise level of the ADXL05 and ADXL202. I recently built some boards with the 202 and 105 chips but my results for both were similar to Larrys. I did not see much improvement in the 105. If the ADXL noise level is about 2mg then the "shake maps" on the web suggest you have to be very close to a 4m quake to see anything with an ADXL based sensor. http://www.trinet.org/shake/ Ciao, Dave The Inexpensive Seismometer Project http://www.infiltec.com/seismo/ __________________________________________________________ Public Seismic Network Mailing List (PSN-L)
Larry Cochrane <cochrane@..............>