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AD7730L Peak-to-Peak resolution

Hello,

I'm DFAE in Japan, I have received a question about pead-to -peak resulution of AD7730L.

AD7730L is reduced power consumption and used lower clock frequency from AD7730. The customer would like to know reason why is the peak-to-peak resolution of AD7730L lower than AD7730. Does the lower peak-to-peak resolution depend on the clock frequency? What else is affecting the lower peak-to-peak resolution of AD7730L?

Best Regards,

Akira

  • Hi Akira, 

    The pk-pk resolution depends on the selected input range and noise. Pk-pk resolution is calculated as  Peak-to-Peak Resolution = Log2(Input Range/Peak-to-Peak Noise). The Pk-pk noise depends on selected update rate, the lower the update rate the better the noise performance. Then having a higher input range will result to a higher resolution as the ratio of the Input range to noise becomes higher as well. 

    AD7730 have higher resolution with the same update rate for AD7730L due to its higher MCLK.

    So if you take a look at the SF word column this represents the amount of decimation/oversampling happening internally. The higher the decimation/oversampling the better the noise performance (resolution), if you want to understand more about oversampling and how it improved the noise performance you can read this article Fundamental Principles Behind the Sigma-Delta ADC Topology: Part 1.

    This decimation/oversampling will also dictates the output update rate so if you can see the higher the SF word value the lower the update rate which means the better noise performance. So going back to the original question on why a higher MCLK leads to a higher resolution, the reason is because in order to achieve the same update rate as the AD7730L (lower MCLK) you need to oversample more. So for an update rate of 50Hz on AD7730L you have an SF word of 1024 while AD7730 have it with 2048.

    Though what I am expecting is with the same SF value the noise values between the two parts should be quite close so there's other possible reason for this like the other block of the system such as PGA may have a noise performance vs power consumption vs speed tradeoff. This is a very old part but let me check if I can dig dipper more about this topic. 

    Thanks,

    Jellenie

  • Hi Jellenie

    Thank you for your answer. But I'm confusing. Accoding to the following tables, the Peak to Peak Resolution of AD7730L is lower than AD7730, right?

    Thanks & regards,

    Akira

  • Hi Akira, 

    Apologies. I may have also confused with the above question. But I have updated the above statement. 

    Thanks,

    Jellenie

  • Hi Jellenie,

    Thank you for your support. I also received support about this issue form ADI Japan team. And I understand your answer and solved customer's question. 

    Thanks & regards,

    Akira