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ADS8568: ADC Interfaced to Isolation Amplifier

Adi Selimovic
Intellectual 560 points
Part Number: ADS8568
Other Parts Discussed in Thread: AMC1301, , MSP430FR5994, OPA2277, TIDA-00810, TIDA-00835, AMC1200

If the isolated current measurement is provided via an isolation amplifier (AMC1301, sigma-delta modulator) and afterward connected to an ADC (e.g. ADS8568 - 16 bit SAR), which accuracy should be expected in the end taking into account that ENOB is not given for devices with analog output (AMC1301)?

Best regards,
Adi

  • 0
    TI__Expert 3175 points
    Hi Adi,

    We're looking into this and will get back to you.
  • 0 in reply to Evan Sawyer
    Intellectual 560 points

    Hi Evan,

    I hope to get answer as soon as possible.

    Best regards,
    Adi

  • 0 in reply to Adi Selimovic
    TI__Mastermind 24940 points

    Hi Adi,

    The accuracy of the system will be limited by the worst performing device in the system.

    Are you buffering the output of the AMC1301? This device has a limited bandwidth that cannot drive the ADS8568 at full throughput.

    The Analog Engineers Calculator has a lot of helpful tools, I would suggest looking at the ADC + Signal Chain Noise under the Data Converters Tab.

    Regards

    Cynthia

  • 0 in reply to Cynthia
    Intellectual 560 points
    Hi Cynthia,

    To be honest, I am not common with the term "buffering the output" of AMC1301. Could you clarify it?
    What is the worst resolution to expect? ADC is not less than 16 bit and MCU is MSP430FR5994.
    In general, I will use it while implementing 3-phase current and voltage measurement.

    Thank you also for this TOOL.

    Best regards,
    Adi
  • 0 in reply to Cynthia
    Intellectual 560 points

    Hi Cynthia,

    Meanwhile, I have done the following:
    I have applied filter in front of AMC1301 to suppress noise. Furthermore, Low pass filter is also applied after the AMC1301, so I ended up with the total noise of 55 uV at 20 kHz which is a cut-off frequency of the low pass filter.
    Then I used the tool you recommended (ADC + Signal Chain Noise). Parameters for the ADC are 2,5 full-scale voltage and 87 dB of SNR. The final result was 82 dB.

    ENOB = (82 dB – 1.76)/6.02 = 13.3 bits

    Is the previous procedure correct?

    Best regards,
    Adi

  • 0 in reply to Adi Selimovic
    TI__Mastermind 24940 points

    Adi,

    The AMC1301 cannot drive the input of the ADS8568 properly if running the ADS8568 at full through put because it does not have enough bandwidth. I would suggest looking at the OPA2277
    Or, you can buffer the output of the AMC1301 to the input of the ADS8568. this would mean adding another op am with enough bandwidth to support the ADS8568 input.
    Are you only simulating this, or are you building the circuit on a board?

    Regards
    Cynthia

  • 0 in reply to Cynthia
    Intellectual 560 points

    Hi Cynthia,

    I am implementing the circuit and doing simulation as well.
    I am not sure whether I understood you. In this case, the limiting factor is minimum input bandwidth of ADS8568 - 6.7 MHz and AMC1301 output is 100 kHz?

    I have also seen several reference designs where AMC1200 or AMC1301 are directly connected to certain ADCs - both SAR and sigma-delta (TIDA-00810 or TIDA-00835).

    Best regards,
    Adi