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AMC1200: Output waveform give negative values.

Damith Pavithra
Expert 1125 points
Part Number: AMC1200


I built this circuit to measure AC voltage. In the data sheet, it is shown that the output voltage can be obtained from the output terminals with a common mode voltage of 1.65V, when using a 3.3V supply. That means, peak to peak value should be 3.3V, right?  But i get the  following waveform with both negative voltage and positive voltage. However, if i used one output terminal and the ground of the low side power, the wave form which i got is fully in the positive side. Why this is happening. I want to use this with a microcontroller to measure the voltage. Is it ok to use one output terminal with the ground to get full voltage?    

  • 0
    TI__Guru 55342 points
    Hello Damith,

    Thanks for your post!

    The nomenclature can be somewhat confusing so I will do my best to clarify. When using differential outputs, the signal on the positive channel -(minus) the signal on the negative channel represents the measurement.

    For the AMC1200, the gain is fixed at 8 and the output common-mode voltage is either 2.55V or 1.29V depending on the supply voltage. Since you are using the 3.3V supply, this mean that both signals will swing around the 1.29V common-mode voltage. Looking at the positive waveform in the first picture, the minimum is ~380mV and the pk-pk value is 1.66V. This means that the signal is ranging from ~380mV to 2.04V. Since the gain is fixed at 8, I assume that you are applying ~250mV to the input. This matches the intended behavior of the device.

    If you are using a single-ended ADC to read from the AMC1200, VOUTN should be left floating - not grounded. For full dynamic range, feed the outputs into a buffer amplifier and do a differential to single-ended conversion. Additional information on the differential to single-ended conversion can be found in this tech note: www.ti.com/.../sbaa229.pdf

    Additional threads that may be helpful:
    e2e.ti.com/.../2257045
    e2e.ti.com/.../2064986
    e2e.ti.com/.../649274