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TIDA-00445: Input common mode

Thiago Moraes
Prodigy 30 points
Part Number: TIDA-00445
Other Parts Discussed in Thread: OPA2376, AMC1200

Hi,

I went for the TIDA-00445 after these specs caught my eyes:

So I thought it would provide some sort of shunt based AC mains measurement.

But after reading the document, I found the authors did not address the input common mode - unless I missed something. From the block diagram, we can see the OPA2376 is directly connected to the shunt over an AC line:

The authors discuss the differential voltage, but not the common mode (CM) voltage. The OPA2376 cannot withstand the proposed 1200V CM.

Figure 4.a shows CM was not accounted for:

On the test setup:

"The DC input voltage was supplied by a Keithley 2450 source meter. Accuracy of the design was
calculated for every 1 mV in the input voltage over a range of –15 to 25 mV.
A similar test was conducted with AC input voltage and current wherein the design was tested for input
ranging from 1 to 17 mVRMS with 1-mV steps."

So it is my understanding that this design was not tested with a real AC line, CM up to 1200V. Am I correct?

If so, this design would have some very bad results if implemented and used as described.

  • 0
    TI__Intellectual 1620 points

    Hi Thiago,

    Thank you very much for reaching out to us on this question.

    We are checking with the author of the design / document and will update to you at here ASAP (it may be in two or three days).

    Best regards,

    Jerome Shan

  • 0 in reply to Jerome Shan
    Prodigy 30 points

    Thank you so much for the quick reply.

  • 0 in reply to Thiago Moraes
    TI__Intellectual 1620 points

    Hi Thiago,

    The AMC1200 is used in this design to provide galvanic isolation between the high and low voltage sides. It can handle up to 4000 Vpeak between the high voltage and low voltage sides, and was selected here mainly because of its very high transient immunity (10 kV/μs min), very low non-linearity (0.075 % max), and very low offset error. Please refer to section 3.1.2 of the Design Guide (Shunt-Based 200-A Peak Current Measurement Reference Design Using Isolation Amplifier) for more details.

    In this design, the sensed current is the system's input current. With an inductor in serial, we can think the sensed current will not vary very fast. To curb the influence of high frequency noise, a RCR filter with a time constant of 7.92uS (18Ohm*0.22uF*2) is used at the input of the analog amplifier stage. The time response test results (refer to section 6.3 of the design guide) of 'the measured rise time of the step input is 16.76 μs and the corresponding rise time of output is 29 μs' when the AMC1200 is fed to with a step input of 250mV, which reaches its positive full sense range. So we think the AMC1200 can handle a common mode voltage of up to 1200Vpk without impact the sensing accuracy in this application, if properly designed.

    Wish this explains the point of your concern.

    Best regards,

    Jerome Shan

  • 0 in reply to Jerome Shan
    Prodigy 30 points

    Hi Jerome,

    Thank you for your reply, very interesting.