Re: Slow varying inputs

Hi,

Why not analog filtering? If your signal varies at under 1 Hz rate, as your 
sampling frequency suggests, and you suspect tens of Hz noise, you can easily 
cut that off by say, a 2nd order low-pass filter, which should be fairly easy to 
make with operational amplifiers.

Sandy Pond wrote:

> On Wed, 2005-03-02 at 22:25 -0500, Daniel Nilsson wrote:
> 
>> Hi All,
>> 
>> I'm working on a project where I'm trying to sample more really slow
>> varying inputs, I'm only interested in getting a new value every 500ms
>> or so and the data will not vary much in this time period.
>> 
>> From looking at the Comedi API I find function designed to help out
>> sampling such data, though they all have the status deprecated. Why
>> are the functions deprecated ? Is there something else that provides
>> the same functionality ?
>> 
>> If not, does anyone have any filter code that you can share the is
>> designed to filter out any noise that might be present on the inputs ?
>> The issues that I'm having right now is that the inputs don't vary
>> much but I'm getting a measurement values that move back and forth
>> around the actual value. I assume some of this is digital noise and
>> some is analog noise (60Hz noise from the mains for example). I would
>> think someone had already written some code to filter out such noise ?
>> 
>> Any other suggestions would be welcome. I'm trying to measure the
>> voltage and current of a Ni-Cd battery that is being discharged
>> through a resistor if anyone was curious...
>> 
>> Thanks
> 
> 
> First a little DSP info: If there's noise on your input you'll need to
> sample at twice the maximum frequency of the noise in order to digitally
> filter the noise.  Otherwise you'll end up with alias errors in your
> recorded input.  If this isn't possible then you need to alter your
> setup to reduce stray noise or install a hardware filter before the
> input to eliminate the higher frequency noise.  
> 
> Once you collect the samples it is easy to digitally filter the signal
> by either using a commercial program like matlab, open source tools like
> python, of writing a small C program.  If you google around on "digital
> filtering" you will find programs.  For instance:
> 
> http://ccrma.stanford.edu/software/stk/
> 
> 
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-- Tuomas
--------------------------------------------------------------
Tuomas Räsänen               tuomas.rasanen_at_ssf.fi
Space Systems Finland        +358 9 613 28654
Kappelitie 6, 02200 Espoo    mobile: 050 330 1758

Received on 2005-03-03Z08:26:27