I want to design a circuit that converts a negative to positive signal into a range suitable for a single supply ADC.
the input is coming from a 120VAC input fed to a step down 10:1 ratio transformer which will outputs 12VAC signal.
is there a circuit that would convert the ±12VAC signal into a 0 to 3.3VAC signal so that it can be sampled by an ADC on a 3.3V microprocessor?
thanks in advance

 

Yes. Reduce the signal to 1.65V peak-to-peak and add 1.65VDC to it.

 

There are various ways to do this. Attached is one way.
This assumes you have a 3.3V supply. If not, let us know.
It also assumes that your A/D input impedance is relatively low, and will therefore need an op amp to drive it.
You may have to adjust the value of R1.
The op amp needs to have rail-to-rail input and output capability, and be capable of running on 3.3V.

 

Yes. Reduce the signal to 1.65V peak-to-peak and add 1.65VDC to it.

I think you intended to say,
Reduce the signal to 3.3V peak-to-peak and add 1.65VDC to it.

 

Sorry, my mistake. I meant 1.65V amplitude.

 

There are various ways to do this. Attached is one way.
This assumes you have a 3.3V supply. If not, let us know.
It also assumes that your A/D input impedance is relatively low, and will therefore need an op amp to drive it.
You may have to adjust the value of R1.
The op amp needs to have rail-to-rail input and output capability, and be capable of running on 3.3V.

Ron H, your output signal looks great! but I do not have any external 3.3V supply! also the micro-controller A/D input impedance is around 200 ohm.

 

Ron H, your output signal looks great! but I do not have any external 3.3V supply! also the micro-controller A/D input impedance is around 200 ohm.

200Ω! Are you sure? That's really low!
Are you saying you don't have a power supply that you can use?

 

I'm not quietly sure, but that's what I understood from the PIC24 datasheet,
"Recommended Impedance of Analog Voltage Source: 200Ω".
my main and only power supply is the 120VAC coming from the outlet and I'm stepping it down to 5VDC to power components like an LCD@5V and a micro-controller @3.3V.
the shifted signal im asking for is to be sampled by that same microcontroller for some power values and measurments.

 

can I use the 3.3V powering the micro-controller as the supply voltage for the shift circuit?
and if yes do you mind giving me a part number for that op-amp used in the shift circuit you gave earlier

 

can I use the 3.3V powering the micro-controller as the supply voltage for the shift circuit?

Yes, that is the supply that I thought you would use.

and if yes do you mind giving me a part number for that op-amp used in the shift circuit you gave earlier

You can use MCP6231. I believe Digikey has them.
Be sure to add a 0.1uF capacitor across the power supply pins, as close to the IC as you can get it. This will minmize the op amp's tendency to oscillate.
The input impedance of your A/D is not 200. That is the maximum recommended source resistance. You might need to add 100Ω between the op amp's output and the A/D input, again to minimize the tendency to oscillate. This may not be necessary.