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ADS127L01EVM: About ADC distortion

Zhonghui Xu
Expert 5445 points

Part Number: ADS127L01EVM
Other Parts Discussed in Thread: ADS127L11, ADS8900B, ADS127L01

Hi team,

 I want to know why HR mode has more harmonic distortion at low frequency than LP mode? Is this related to the MCLK frequency?, And increasing over sampling ratio can reduce THD even further? Our project only needs 20kHz bandwidth, That's enough, Do you have a better selection and recommendation?

Thanks for your time. 

Best regards,

  • 0
    TI__Guru 52000 points

    Hello,

    When operating in LP mode, the maximum MCLK frequency is lower, which results in a longer acquisition period for the input sampling capacitor.  The longer acquisition period leaves more time for the input amplifier and internal sampling capacitor to settle, which results in lower THD. Keeping MCLK frequency the same and increasing OSR will not lower the THD.

    For new projects, I suggest you look at the ADS127L11.  This is the next generation device and includes internal pre-charge buffers for both the analog inputs and the reference.  Using the internal pre-charge buffers reduces the load seen by the external amplifier, improving THD performance.

    Regards,
    Keith Nicholas
    Precision ADC Applications

  • 0 in reply to Keith Nicholas
    TI__Expert 5445 points

    Hi Keith,

    Thank you for your reply.

    I found a precision SAR ADC ADS8900B,which seems to achieve - 132dB THD at differential input. As you said,I have to lower the MCLK frequency to lower the THD,And the bandwidth and SNR will be reduced accordingly. So ADS8900B seems to be a better choice?
    In the ADS127L01 datasheet,I see the FFT chart in LP mode showing a-128dB THD at 1kHz tone,The measured performance is in single ended input or differential input? If we use differential input, can we achieve the same performance as ADS8900B(-132dB at 1KHz tone)?ADS127L11 is the next generation product,I found that SNR has slightly improved,Will the THD also be improved?
    In some ADC datasheet,lower the input level can reduce THD little bit,If we increase VREF to the maximum recommended range,Will THD decrease or increase in the same input level?
    in the same input Voltage!

    Best regards,

  • 0 in reply to Zhonghui Xu
    TI__Guru 52000 points

    Hello,

    ADS8900B also has excellent specifications.  Please keep in mind that it is a SAR data converter, and there are no integrated digital filters inside this device.  The ADS127L11 and ADS8900B have very similar THD.  The SNR for the ADS127L11 can be higher depending on the filter setting of the device.

    SNR and THD specifications for all devices are based on differential inputs.  The ADS8900B can only operate with differential inputs; the ADS127L01 and ADS127L11 can operated with a single ended input, but SNR and THD will be degraded.

    In general, THD will improve with lower input levels for all ADC types, as well as the external amplifiers.

    Regards,
    Keith