Why not take advantage of the offset?  If you're a/d range is 0-5, 2.5 would
be perfect if you want to look at the whole wave.  Just subtract half of the
range if you want to compare with ground.  Keep in mind if you're sample
rate is high, and your system has a little noise, zero crossings can be
fuzzy at transitions.

----- Original Message -----
From: Vasile Surducan <vasile@L30.ITIM-CJ.RO>

> if the DC offset is constant I don't see any problem except you need
> probably a zero crossing detection, the easyest way is maybe to
> extract the AC voltage with a capacitor and to compare with gnd, but
> may be any other solution ( compare directly with the offset ).

> On Wed, 12 Dec 2001, Micro Eng wrote:
>
> > If some recall, I was looking for a current measureing device, and I
found a
> > relativly inexpensive unit, that is a hall effect device. Basically you
> > connect up 5VDC, ground and the device under test, and you get a AC
waveform
> > riding on a 2.5VDC signal.  The amplitude of the DC referenced AC signal
is
> > supposed to vary according to the amount of current but I still need to
run
> > some tests since I was using a dimmer switch to vary the current, but it
> > simply chops it to lower the voltage and thus the current.  Need to buy
some
> > various voltage light bulbs.
> >
> > Anyway, has anyone ever measured the AC voltage relative to the DC
offset?

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