PSN-L Email List Message

Subject: Re: ADXL202 Sensor
From: Karl Cunningham karlc@..........
Date: Thu, 24 May 2001 23:12:19 -0700

Hi Casey --

I was thinking if you were looking at the output as a file of numbers on 
the computer.  If you were to take 100 readings, then throw out the highest 
and lowest (assume they might be glitches from power lines, etc), then what 
are the highest and lowest readings that are left.

Here's another way I've heard to measure random noise with an oscilloscope: 
Feed the signal source into a vertical channel of the scope, and adjust the 
vertical sensitivity so the the trace is around two major divisions on the 
screen.  Then connect another vertical channel to the same signal and set 
its sensitivity to the same value as the first channel.  If the noise is 
random, each channel's trace will be brightest near its center and get 
dimmer toward the top and bottom edges of that trace.

Now adjust the vertical position of one of the channels until the traces 
overlap in such a way that the brightness of the display between the two 
channels is almost constant.  That is, at the point where the lower part of 
the upper trace is starting to dim, the upper part of the lower trace is 
brightening so as to compensate.  Then ground the input to both scope 
channels and observe the difference in vertical position between the two 
traces.  The number of divisions between the traces times the vertical 
sensitivity is a measure of the noise of the signal.

Unfortunately, I don't remember if this number is supposed to be peak, RMS, 
or what.  Maybe someone out there knows.


--On Thursday, May 24, 2001 17:35 -0700 Casey Crane  wrote:
> Karl,
> In reguards to to reply, first thanks, then 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 ?
>         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.


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Larry Cochrane <cochrane@..............>