Russell McMahon wrote:
> Transformer and 2 resistors in basic form.
> Connect one leg to Vdd, other via divider to ground and tap to ADC in.

You could just as well connect one leg to ground, and the divider between
the other leg and Vdd.

> Sounds super easy, no body diodes at risk, no diode drops,
> minimum components, full cycle of waveform, ...

No.  Go show us real component values and you'll find the flaw in your
topology.  The transformer produces 6V RMS out, which is 17V peak to peak or
+-8.5V if one leg is connected to ground (this is the same as you suggested
just flipped around to make it easier to think about).  Obviously no
resistor divider from 0V to 3.3V can possibly keep a 8.5V input clipped to
3.3V.  There will always be some excursion above Vdd (or below ground
flipped your original way).  That means "full cycle waveform" is impossible
with this method.

To get what you want, you need to add another resistor.  That's basically
the circuit I showed, except that while I was at it I AC coupled it by
adding a cap.  I also added a little low pass filtering cap, which you
mentioned as optional with your approach too.


********************************************************************
Embed Inc, Littleton Massachusetts, http://www.embedinc.com/products
(978) 742-9014.  Gold level PIC consultants since 2000.
-- 
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist