AVR-Chat

I second Larry's suggestion for using IIR filtering.  I use a sequence 
of them for
my noisy accelerometer applications, (as well as other sense inputs), 
and gain
flexible low-pass filtering and increased resolution.  They are very easy to
implement in AVR.

Ken


Larry Barello wrote:

>A little more detail on sensor used, how it is read and the nature of the
>noise that is troublesome and what the application is would help in
>determining what, if any, filtering is needed.  
>
>You can get only so much resolution using RC filter.  In the case of the AVR
>the ADC is only 10 bits.  However, if you use an IIR filter (i.e. weighted
>average, or exponential filter) you can not only reduce bandwidth and filter
>out noise, but you can also increase your resolution one or two bits. The
>weighted average filter is the digital version of a single stage RC filter.
>A weighted average filter looks something like this:
>
>Output = (prev_output - input)* K1 + input
>
>Where K1 is equal to e^-(dt/tau).  Dt = the sampling period and Tau is the
>rise time desired (in seconds).  The frequency response is 1/2*PI*tau.  Or,
>tau = 1/2*pi*frequency.  I recently implemented this filter in the following
>way:
>
>Int32_t results;
>
>Void InitFilter(int16_t val)
>{
>	Results = val * 256;
>}
>
>void DoFilter(int16_t val)
>{
>	int32_t t = (int32_t)val * 256;	// Turn into 24.8 fraction
>
>	results = ((((int32_t)K1 * (results - t) + 128L) / 256L) + t);
>}
>
>uint16_t GetFilter(void)
>{
>	return results / 256;	// Divide by 128 or 64 for more 
>}					// bits resolution...
>
>K1 is a 8.8 binary fraction e.g.  K1 = 256 * e^-(dt/tau) 
>
>The intermediate result is a 24.8 bit number.  I picked an 8 bit fraction
>since it is so easy to convert back & forth if the compiler is clever: just
>shuffle bytes.
>
>Weighted average filters (IIR) are compact, computationally efficient and
>don't require storing lots of previous values, but they have limited
>frequency response & phase shift characteristics.  I recommend Smith's "The
>Scientist and Engineers Guide to Digital Signal Processing" for an easy to
>read tutorial on practical filter design.
>
>Cheers!
>
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> 
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