PSN-L Email List Message

Subject: Re: Advantages of more ADC bits
From: "GMV" gmvoeth@...........
Date: Mon, 9 Feb 2009 17:47:06 -0700

note on alising alizing ??

It is my understanding that all inputs which are sampled
must be low pass filtered at like 0.4 of the sample rate
in hertz so if you sample at 100 samples per seconds
you need to prefilter prior to input to the a/d with a
n4 to preferred(n8 eight stages of a simple rc filter) at like 40 hertz.
alizing is kinda a law of physics (math) you must deal with
or get phony signals like 50 or 60 hz artifacts in your
alizing is like with 100 sps:
50 hz wraps back to zero(DC) and 51 becomes one Hz(ac) ??
you must eliminate all freqs above 1/2 the sample rate and since
filters are not perfect that means like 0.75/2 instead of 1/2.
You want zero Dbv ( x1) or less above 1/2 the sps in hertz.

Im no scientist but I do understand this limitation since
working with sampled data in the sonar field.

I kept getting phony 60hz artifacts just above 5Hz till i filtered my signal
before sampling it.


----- Original Message ----- 
From: "rsparks" 
Sent: Sunday, February 08, 2009 1:13 PM
Subject: Re: Advantages of more ADC bits

> Thanks to both Chris and Brett for their thoughtful replies.  I better 
> understand that an upgrade to 24 bits is not a "no brainer"  upgrade, 
> but part of design strategy.   All the parts must fit together to get 
> superior results and fulfill site goals.
> There are new devices that may hold promise for future amateur use.  One 
> such device is the Cirrus Logic CS5530 24 bit ADC with integrated 
> amplifier.  I don't know if this could stand alone with a dedicated 
> computer (or maybe not dedicated) using a serial port, or if it must be 
> combined with a microcontroller.  I also don't know if the input must be 
> pre-filtered to prevent aliasing, or if the integrated amplifier is 
> sufficient  for amateur seismic work.
> Thanks for your comments
> Roger
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