> I am to use a 24FJxxGA106 to (along other things) measure 220V mains
> voltage with some coarse accuracy, say 1-2%

What is the intended application.
What you are proposing is vast overkill for some applications and
maybe not good enough for others.

A transformer may somewhat modify the waveform. If you have heavy loads
present which modify the voltage from open circuit to on-load then
waveform shape may change depending on whether the load is resistive or
reactive or some mix (or changing).  Other factors such as energy return from
rotating machines or electronic supplies MAY make a difference. But
may not, or may not matter.

If you have a reasonable sine wave you could consider peak measuring a
lightly loaded secondary and scaling (by 0.7071 in ideal case).

If you use an FFT you'll want to measure integral half cycles if you
want to avoid windowing complexities.

You could consider a precision ("ideal") rectifier and DC measurement
or just peak measurement with ADC or even measurements at fixed points
along cycle from zero to zero to confirm the sine shape with minimal
calculation.

Or ...

So, knowing intended use may help provide a cheap or low complexity answer.



       Russell
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