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AMC3330: Suggested suitable configurations for current measurement using current transformers

David Nyarko
Expert 1605 points
Part Number: AMC3330
Other Parts Discussed in Thread: AMC3302, AMC3301

Are there any example configurations which are more suitable for the AMC3330 front end when current transformers are used for current measurement.

Thanks

David Nyarko

  • 0
    TI__Guru 51101 points

    Hi David,

    The AMC3330 is primarily intended for voltage measurements due to it's high input impedance and larger input voltage range of +/-1V. 

    The AMC3301 (+/-250mV) or AMC3302 (+/-50mV) may be more appropriate for current measurements due to their smaller input voltage range and the tradeoff between maximizing FSR and power dissipation of the shunt resistor.

    However, for extremely high currents to be measured, a gain stage before the AMC3330 may be used, similar to what is shown in this TIDA: https://www.ti.com.cn/cn/lit/ug/tidubv1a/tidubv1a.pdf 

    Please let me know if you have additional questions.

  • 0 in reply to Alexander Smith
    Expert 1605 points

    Hi Alex,

    I will try the AMC3301 or AMC3302 you suggested  since i prefer the single supply instead of the bipolar needed for the input op-amp in the document you linked to.

    The examples used in the AMC3302 data sheet used a shunt resistor.

    I will prefer to use a current transformer for its' inherent isolation.

    Could i view the shunt resistor is being replaced with my CT burden resistor?

    Thanks

    David

  • 0 in reply to David Nyarko
    TI__Guru 51101 points

    Hi David,

    I apologize but I don't fully understand what you are trying to do. Can you please elaborate on how you would like to use the AMC3302?

    The AMC3302 is primarily used for isolated current measurements using a shunt resistor. The AMC3302 will have isolation qualities similar to the CT and in this case I would expect the AMC3302+Shunt resistor would replace your CT. 

    Higher accuracy can be achieved with the AMC3302 compared to a CT, please see this document for additional details: https://www.ti.com/lit/an/sbaa464/sbaa464.pdf 

  • 0 in reply to Alexander Smith
    Expert 1605 points

    Hi Alex,

    I get you. It looks like using the CT + AMC3302 is an overkill,  since i could just connect the CT burden resistor to my microcontroller

    ADC differential input.

    My reasoning was that if I used the CT + AMC3302, my system design for the voltage and current sections will be quite similar.

    In addition, a CT problem which could cause a high voltage will not damage the microcontroller ADC. And if any damage occured

    it would likely affect jjust the AMC3301 section.

    Thanks

    David

  • 0 in reply to David Nyarko
    TI__Guru 51101 points

    Hi David,

    I see, I think I have a better understanding now. Yes, unless you have a requirement for double isolation, it would be overkill from an isolation perspective. 

    That said, there's no reason that it wouldn't work and it would indeed protect your uC as it's input protection is quite robust. 

    If you are interested in looking closer at this solution I'm happy to help you design. 

  • 0 in reply to Alexander Smith
    Expert 1605 points

    Hi Alex,

    Will appreciate any help on using the CT + AMC3302 configuration.

    Thanks

    David

  • 0 in reply to David Nyarko
    TI__Guru 51101 points

    Hi David,

    You can use a configuration like this, actual values will need to be modified based on CT output and isolated amplifier input range:

    Since you are already using AMC3330 for voltage measurements, you can use that instead of AMC3302 to reduce your BOM count. 

    Out of curiosity, can you share a little bit about your application? What is the end equipment and where are these measurements being performed in the system? 

    Can you share a datasheet for the CT?

  • 0 in reply to Alexander Smith
    Expert 1605 points

    Hi Alex,

    Thanks. Will the "AGND" in the diagram be the connection to "HGND" on the AMC3330/AMC3302?

    Our application uses 3-phase 60hz AC for a  heating process via electrodes for soil cleanup. 

    The power to each electrode is controlled by firing back-to-back SCRs. The control scheme for the SCRs

    are selected as a percentage of a fixed time interval.

    e.g. if the fixed time interval is 10 seconds, a 10% duty cycle means they are on for 1 second and off the remaining 9 seconds of each

    10 second interval. The actual 1 second time could be slightly off since the SCR controllers turn the SCRs on or off only at the zero-crossing of

    the AC signals. Due to the non-steady current, the typical measurement ICs for steady 50/60hz have not worked.

    This approach will be to compute the AC parameters (Voltage and Current) using the sampled values obtained via the microcontroller ADC .

    The CT is the CR8459-2000-N from CR magnetics ()

    Thanks

    David

  • 0 in reply to David Nyarko
    TI__Guru 51101 points

    Hi David,

    Happy to help! I really appreciate the background on your application - very interesting to learn about how this soil cleanup technique works! 

    Yes, thank you for clarifying AGND = HGND. Please let me know if you have additional questions.