Dario Greggio wrote:
> Basically I need to measure 220V mains,

Measure the actual voltage?  Peak?  RMS?  What?

> by mean of a 220->6 tiny
> transformer and a partitor and a protection diode, staight into a
> PIC's ADC.

So the 0 value is Vdd/2 and the peaks are within the A/D range?

> I'm sampling the sine every 1mS and was wondering of a FFT or average
> or else... I need some 5% accuracy.

It sounds like you are trying to measure the mains voltage, so it being
close to a sine wave is a good assumption.  I don't see anything useful a
FFT is going to do for you, although it's still not clear exactly what you
want out.

If you have the memory, you could keep the last 20 samples, which will
always contain the most recent top and bottom peaks.  Each sample is spaced
18deg apart, so there is always a sample within 9deg of each peak.
COS(9deg) = 0.988, which is well within your 5% error tolerance.  In other
words, peak detection of your samples yields at most 1.2% error due to
missing the peaks a little.

The max minus min of the stored samples gives you the peak to peak voltage.
That divided by 2*sqrt(2) yields the RMS voltage, assuming sine wave.

There are a lot more details depending on what you want to do about short
term spikes, harmnoic content, etc.  If memory is a issue, there are ways to
get the peaks without storing a whole cycle worth of data.


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