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AMC3306M05: Measuring AC voltage using Sigma-Delta Modulator ADC IC

DIVYANSHU BANSAL
Prodigy 10 points
Part Number: AMC3306M05
Other Parts Discussed in Thread: AMC3302, AMC3301

Hi Experts,

While selecting components, I got to read up on using this IC to measure bi-directional currents. But I have a doubt on seeing this application circuit.

Since, INN and HGND is shorted together(Although 10ohm resistor provides very low impedance in between still shorting the INN to ground). How is the modulator able to measure the voltage when INP goes below the INN when current is flowing in one direction making absolute voltage at INP with respect to INN(HGND) as negative?

  • 0
    TI__Guru**** 168605 points

    Hi Divyanshu,

    Welcome to our e2e forum!  As you noted, if VINn and HGND are shorted together, the AMC3306 would use that point as the '0V' reference.  Depending on the direction of current flowing through RSHUNT, the voltage on VINp will either be positive or negative relative to HGND.  If current is only flowing in one direction, you would only see positive (or negative) voltage at VINp.  If you look at the Absolute Maximum Ratings in section 6.1 of the datasheet, you will see that both inputs can go below ground

  • 0 in reply to Tom Hendrick
    Prodigy 10 points

    Thank you for replying. Rating specification for negative absolute voltage on input pins are permitted. But my doubt is that, usually in opamp based measurement circuits, the maximum rail and minimum rail voltage defines the output voltage limits. For this case, since VINn is already shorted to ground, when VINp goes negative with respect to ground, why is the output not saturated/limit to minimum rail voltage we have in the circuit. How is it able to measure voltages below minimum rail voltage IC is supplying?

  • 0 in reply to DIVYANSHU BANSAL
    TI__Guru**** 168605 points

    Hi Divyanshu,

    If you look at the AMC3301 or AMC3302 isolated amplifiers, they have the same recommended input configuration, where VINn is tied to the high side ground.  VINp can swing +/- about ground up to the ABS MAX voltage levels, but the voltage sensing range is limited to the saturation voltage levels noted in the respective datasheets.

  • 0 in reply to Tom Hendrick
    Prodigy 10 points

    Hi Tom,

    Yeah. In the AMC3301, as stated in the below image, it shows that differential input voltage supported before clipping is +-320mV.

    As you said, this is still valid when VINn is tied to ground. But, since the minimum voltage in the analog input part of IC is HGND as shown in the below figure. When the VINp goes negative with respect to HGND, would the circuit in red part saturate and not able to sense the differential voltage. If it is not so, then, is there a negative rail generated within the IC to support this operation

  • +1 in reply to DIVYANSHU BANSAL
    TI__Guru**** 168605 points

    Hi Divyanshu,

    The circuit in red will not saturate so long as your input signal is within the linear range of the device.  The VINp pin can go negative without having any issues.

  • 0 in reply to Tom Hendrick
    Prodigy 10 points

    Okay. Thank you for answering the question.