Raw data acquisition should always be done at a sample rate more than twice
the upper frequency passed by the amplifier. Sample averaging to a lower
sample rate is not the way to go, as simple sample averaging will reduce,
but not eliminate, aliasing. Instead, low-pass filter the raw data to an
upper frequency less than one half the reduced sample rate. Reduction to th=
e
final rate can then be accomplished by simple decimation, or by sample
averaging.

I have done experiments to verify the above statements. I would recommend
that the digital low-pass filter be of fourth to sixth order.

Bob

On Wed, Jul 6, 2011 at 11:11 AM, Randall Pratt  wrote=
:

>  I=92m following the discussion of Geoff=92s noise with interest.  I woul=
d
> like to ask some questions on alias noise.  ****
>
> ** **
>
> If one were to vary the sample rate and sample at say 60sps, 40 sps, 15
> sps, 6 sps is there a formula to take the peaks from the various ffts and
> arrive at a list of probable noise alias problems?   I have done this and
> can see some changes in noise peaks on the spectrums but I have not
> attempted to calculate a relationship.****
>
> ** **
>
> Secondly, if the AD runs at 120 sps and averages to 6 sps recording rate
> will frequencies between 3 and 60 hz alias into the record?****
>
> ** **
>
> Randy****
>
> ** **
>
Raw data acquisiti=
on should always be done at a sample rate more than twice the upper frequen=
cy passed by the amplifier. Sample averaging to a lower sample rate is not =
the way to go, as simple sample averaging will reduce, but not eliminate, a=
liasing. Instead, low-pass filter the raw data to an upper frequency less t=
han one half the reduced sample rate. Reduction to the final rate can then =
be accomplished by simple decimation, or by sample averaging.



I have done experiments to verify the above statements. I would recomme=
nd that the digital low-pass filter be of fourth to sixth order.

Bob=


On Wed, Jul 6, 2011 at 11:=
11 AM, Randall Pratt <rpratt@.............> wrote:















I=92m following the discussion of Geoff=92s =
noise
with interest.=A0 I would like to ask some questions on alias noise.=A0 =


=A0

If one were to vary the sample rate and samp=
le at say 60sps,
40 sps, 15 sps, 6 sps is there a formula to take the peaks from the various
ffts and arrive at a list of probable noise alias problems?=A0=A0 I have
done this and can see some changes in noise peaks on the spectrums but I ha=
ve
not attempted to calculate a relationship.

=A0

Secondly, if the AD runs at 120 sps and aver=
ages to 6 sps
recording rate will frequencies between 3 and 60 hz alias into the record?<=
u>

=A0

Randy

=A0