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TIDA-00080. Filter.

Other Parts Discussed in Thread: OPA211, AMC1304M25, TMS320F28377D, DAC8564, LMV7219


I've looked at the "TI Design" - TIDA-00080- Shunt-Based AC/DC Current and Voltage Sensing for Smart Grid Applications with Reinforced Isolation

In the proposed schematic between AMC1304M25 (U5-U10) and TMS320F28377D on the Filter Module board is used  active filter at OPA211. I slightly puzzle, as I was sure, that digital filter provided by TMS320F28337 should be enough.

I understand when we use such filter before  modulator (AMC1304) but what reason to use it whith  "digital" signal.

Could anyone explain why we need additional filter after comparator?

  • Hi Oleksiy,

    Perhaps I am not looking at the same schematic you are but the OPA411 devices ahead of the AMC1304's are 'not installed' and the ones on the filter board are after the DAC8564 devices found on the DSP module. They are there to provide an monitor output of the input current and voltage.
  • Hi, Tom,

    Thanks for  reply.  May be we are looking at  different schematic, because I don't find any OPA411 and any OPA  after DAC.  I'm looking at schematic in the file

    Yes there is no installed OPA ahead  of AMC1304 (p.84), but we could put it if we would like to improve the input signal. This is clear.

    But at the page 97 after LMV7219M5X I see the OPA211. If I understand correct the output of the active filter (_ADCIN) is connected to the Delfino internal SAR ADC.

    By another words  LMV7219M5X and OPA211 work as demodulator and form analog signal which could be measured by internal SAR ADC. Thus un this case we have possibility either use data from SDFM (sigma-delta filter modules) either SAR ADC.  Am I correct?

  • Hi Oleksiy,

    Thank you for the clarification.  Yes, you are correct.  The LMV7219 and OPA211 in those circuits are just secondary measurement channels.  They were used to compare/contrast the performance of the SD Filter Module against the internal SAR ADC.  As you noted, the filter is needed to demodulate the bit-stream output so that the SAR can sample the signals properly.

  • Hi Tom,

    This mean that the LMV7219 - OPA211 demodulator is used as demo only and there is no need to use it in the final design , like in the May be I miss something and there is a sense to use such demodulator except demo in current application, isn't it?
  • Hi Oleksiy,

    That is correct, that additional circuit is there just to facilitate an alternative solution in the event that your preferred micro/DS controller does not offer a sigma-delta filter peripheral.
  • Hi Tom,
    Thanks for your inputs.