[sdiy] ADC filtering (was Re: A/D converter with good accuracy for a reasonable price?)

[Colin f]

> For a moving sample filter of size 4, you will start with an 
> array like a[4] filled initially with zeroes.  You use that 
> as a ring buffer and each sample time compute the new average as:
> 
> Out=(a[0]+a[1]+a[2]+a[3])/4;
> 
> Note that divide by 4 can be done with 2 right shifts.  You 
> maintain a modulo 4 update pointer so that it simply wraps 
> and always destroys the oldest value to insert the newest 
> value.  The update pointer is incremented each sample time.
> 
> The value Out is what you use as a "cleaned up" or filtered sample.

As a simple speed optimisation, you can also keep a running total for the
average.
Before you over-write the update pointer, subtract the doomed value from the
running total, and add the new one.
Then your out calculation needs far fewer adds for a long buffer.

Cheers,
Colin f