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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.

  • Hi-

    I moved this question to the Precision ADCs community.  Please continue the discussion here.



    EngineerZone Community Manager

  • Hi,

    Can you give additional information on the output of the CT, is this a bipolar +/- 1V signal swinging around ground?

    What supplies have you available on your board?

    Is it only a single measurement that you want to make, i.e. single channel ADC  vs multichannel vs simultaneously sampled ADC?

    What sampling rate do you want to run the part at?

    What digitial interface do yo want to use to communicate to the part?

    This will help us to narrow down the search.



  • 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.

  • Hi

    For 12 Bits, single channel SAR, look at AD7920, the AD7910 is the 10 bit version of this part.

    If you need more channels then look at AD7912 (2 channel) AD7923/7 (4 channel/8 channel).

    For a 12 bit sigma delta look at the AD7170.

    These devices are unipolar so you would need to level shift the +/- 1V signal.

    For bipolar devices look at the AD7321.

    These devices accept the bipolar input signal but will require a negative rail.

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