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ADS127L11EVM-PDK: The reason why the noise floor gets worse when shorting the inputs

Yoshikazu Kawasaki
Intellectual 751 points
Part Number: ADS127L11EVM-PDK
Other Parts Discussed in Thread: ADS127L11

Hello,

My customer uses ADS127L11 EVM.  The noise floor when they used it as single end input, it was around -120dB, but when they shorted the inputs by a terminal resistor, it became around -10dB or so.  Would you please tell me the reason why it is?

Best Regards,

Yoshikazu Kawasaki

  • +1
    TI__Mastermind 41090 points

    Hello Yoshikazu-san,

    The amplitude is displayed in dBC, which is relative to the carrier signal.  In the case of shorting the inputs to ground, there is no 'carrier', but the software finds the highest noise spur in the spectrum and assigns this as the carrier.  Since the noise spur is very low and about the same amplitude as the noise floor, the displayed value is much higher in terms of dBC.

    You can export the data from the EVM GUI into the Analog Engineers Calculator and plot it in dBFS.  The below example is similar to your results in dBC, and then the same data is plotted using the calculator in dBFS.

    Regards,
    Keith Nicholas
    Precision ADC Applications