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AMC1301: AMC1301 Output Ripple Issue

Vincent Li00
Intellectual 2035 points
Part Number: AMC1301

Hi Guys

AMC1301 is being used to sense AC input voltage. After filtering, Vac1_LINE and VAC1_NEUTRA are directly input to MCU, and AGND is connected to AC1_SGND. R159 and R160 are not soldered, and when C156 and C157 are 0.1uF, Vac1_LINE and VAC1_NEUTRA have large ripples. After removing C156 and C157, the ripple disappears. After replacing it with 4700pF, the ripple becomes smaller but still exists.
From the AMC1301 datasheet, it can be seen that the output of the isolated op amp has no capacitance to ground. Are these two grounding necessary? What is the reason for the ripple?

Test waveform:

1.Remove C156, C157, CH1: Vac1_LINE and VAC1_NEUTRA       CH2: Resistance string sampling Vac

2. C156, C157 4700pF , CH1: Vac1_LINE and VAC1_NEUTRA CH2: Resistance string sampling Vac

3.C156, C157: 0.1uF  CH1: Vac1_LINE and VAC1_NEUTRA CH2: Resistance string sampling Vac


 

Thanks

-Vincent

  • 0
    Guru 140200 points

    Hi Vincent,

    due to too high capacitive loading (ruining of phase margin) the output OPAmps of AMC1301 become instable and oscillate :-(

    It's not recommended to mount caps from the outputs to signal ground.

    If you want to enhance the common mode filtering, you could do it as discussed here:

    Kai

  • 0 in reply to kai klaas69
    TI__Guru 51081 points

    Hi Vincent,

    As Kai said, this is most likely due to capacitive loading of the outputs. Common-mode filter capacitors should not be necessary, but if they are to be used limit them to be one decade smaller than the differential capacitor, C431. 

    Thanks Kai!