David,

Sorry David, I'm not completely agree on this interpretation.
Your clock will never be syncronized with the 60Hz noise,
so you will have a resonance signal detected by the a/d
and you'll se a very slow signal moving from the minimum to
the maximum of the 60Hz noise.
This will be more evident if you disconnect your sensor
from the amp.
Eric says he would syncronize the a/d clock with
the 60Hz detecting the zerocrossing phase of the a/d signal
with a detector directly connected to the power grid.
(I suggest to do so using a  optocoupler used picking the
signal from the secondary coild of the transformer you use
to supply the circuit.)
This is correct, but you will MUST sample always at 60Hz
or 120 or 30 or 15 or 240 and soo on twice and halfs.
This will not save you from other interference source, and
in my opinion a good antialias filter and other proper filters
will cutoff even 60Hz and others RF signals (spikes, motor noise,
interruptors and so on...)

Cheers
Mauro



----- Original Message -----
From: "David Saum" 
To: 
Sent: Sunday, February 10, 2002 5:44 PM
Subject: RE: 60Hz Synchronous sampling?


> You can reject 60Hz if you have control of your a/d sampling rate,
> or if you can integrate after the a/d.  For instance, suppose each
> of your a/d takes 1ms.  The period of 60Hz is 1000/60ms= 16.67ms,
> so if you can do each of your 1ms a/d separated by 16.67ms you
> will catch 60Hz interference at the same phase each time and it will
> appear as a DC component.  Another way to do this is to take
> a 1ms sample every 1ms, and then average each block of 16 or 17 together,
> so that the 60Hz components approximately average out.  Of course
> neither of these techniques gets rid of noise that is not at 60Hz
> or a multiple of it.  I use a microprocessor with an onboard
> 16 bit a/d that takes about 1/60sec, so it rejects 60Hz without
> further fuss.  Check your a/d chip data sheet to see if it gives
> guidance on minimizing 60Hz.
>
> Hope this helps,
>
> Dave
> The Inexpensive Seismometer Project
> http://www.infiltec.com/seismo/
>
> > From:    "ERIC  GUINN" 
> > Date:    Sat, 9 Feb 2002 12:50:26 -0500
> >
> > Hey all.
> >
> > I'm trying to get my ducks in a row for my first LP station with
computer
> > logging, and I'm pulling my hair out trying to find an old piece of
data.
> > Could someone please help?  Where have I heard of a sampling rate of
60Hz
> > (or 30Hz?) which was synchronous and zero-crossing-locked with line
> > frequency to allow elimination of 60Hz and aliases without multiple-pole
> > filters?  I think I'd like to replicate this bit of Kung-Fu in my
design.
> > Did I just make this up or did Larry or someone publish an article like
> > that?
> >
> > Thanks in advance,
> > Eric Guinn, AC4LS
> > Sevierville, TN
>
>
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