I have an application with floating (low-voltage line transformer) AC voltage that I wish to sample.
I can reduce the voltage to be +/- 2.5Vp-p.
I plann to use a +2.5V reference voltage on the inputs, thereby making it a +/- 2.5V input device on all analog channels.
Does anyone have experienced such an arrangement?
Can it work?
Any recommendation?
You can't feed a negative voltage into an analog pin, regardless of the reference voltage, without damaging the Arduino.
You could offset the AC voltage, while reducing it, so that it's range was 0 to 5V, then sample that.
Why not just sample half the AC wave by using a rectifier (diode) in series with the A/D input pin? That will prevent negative voltage at the Arduino input.
Besides, what's in the bottom half of the wave that you can't see in the top? 
Hello and thanks RuggedCircuit
Answering your question:
Rectifying a 2.5V with a diode bridge means loosing all on the diodes. (a normal rectifier bridge would eat up 3 V!).Aush!
It can be done with an active bridge rectifier, (3 op amplifiers) but will necessitate more external components.
The reason for the bipolar need is because we wish to compare several channels that may show up in opposing phase.
Electrically, the use of reference offset depends on where in the input the reference offset is injected: before or after the protection circuit.
If it is before, (and because my signal is floating) it should work (the input would see all as positive).
A simple option would be to offset before the Arduino by adding one offset differential amplifier.
I still would like to find someone who has done this before.
I've used bipolar ADC in the past (PCI boards). Quite useful in real situations.
Here's a link to mouser.com with differential ADCs, unfortunately there's not an option for just bipolar. I suppose all differential are bipolar.
http://ca.mouser.com/Semiconductors/Integrated-Circuits-ICs/Data-Conversion-ICs/ADC-A-D-Converters/_/N-6j74v?P=1z0smkk&Ns=Pricing|0
A few bucks get you several channels with 8-bit resolution. Here is the data sheet for one of them that seems to have +-2.5V range and internal reference for convenience. You just have to sacrifice a few digital I/O lines to communicate with it.
http://datasheets.maxim-ic.com/en/ds/MAX1112-MAX1113.pdf
Let us know if you have good luck with it.
If the voltage is floating then make a 2.5V ground reference point by having two 1K resistors between the arduino's +5v and ground.
Then connect one of your lines of the AC to this point. The other line should be connected to the analogue input. Providing you have no more than 5V peak to peak this will work.