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AC Current Measurement

I need to add current measurement to an existing product, as part of a re-design.   The Input signal limits are: 85-265V 47-63Hz of 0-20A (Line Power).

For the conversion to voltage, I have narrowed it down to either a current transformer (1V on the secondary for 20A on the input, although from what I understand, this could be adjusted somewhat without cause too much change in the frequency response, although I don't know if that would even be an issue.)  The other option is a halls sensor from allegro micro, their ACS712 (output 0.-4.5V for -20-20A with a 2.5V center).  I would prefer the transformer due to simplicity, unless there is a reason to use the halls sensor.

Assuming I use the Current transformer, and I want to be able to sense the difference between 0A and 100mA, taking into account that there is probably going to be noise on the power lines (estimated flat 0-30% of the signal (random), I have estimated that a bare minimum of 10bit ADC is needed, and that's right on the edge (5-25% error @ 100mA) is what i'm seeing, by my calcs), so I am looking for a 12bit (3-10% error @ 100mA)

I see that all the energy measurment ic's use delta-sigma converters, so I would guess that is what is most suited, and after reading a bit, they seem to have quite a bit of advantage for faster sampling and noise issues.

Now having said all that, I cannot seem to settle on a specific adc, and would like suggestions or comments on the matter.   Thanks for any help you can provide.

Parents
• yes, bipolar +/-1V, although, i can use a simple divider to place the center @ any point.

I currently need +12V and +5V on this board.

I only need to sample one channel at a time, calculate the rms current, then switch to the next, calculate the rms current.  Nothing needs to be simultaneous.

Sampling rate, I don't know, although I am thinking that 250 sps should be enough to accurately get the rms current off a 60hz signal, but I haven't tried it yet, so suggestions are welcome.

spi would be preferred, i2c would be a distant second.