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TPS734: SLYY076 - Noise Measurement, RBW

Markus Kuebler
Intellectual 430 points
Part Number: TPS734

Tool/software:

Hello for a new design I have to do a noise measurement. I found the application note SLYY076, "How to measure LDO noise" that is a great help for understanding.

Nevertheless, I have a question on the measurement. The spectrum analyzer that I use provides the measurment data in dBµV, the value is not yet normalized to 1Hz. As far as I know, I have to divide the measured value by the square root of the measurement bandwidth. Now I don't know whether I have to convert from dbµV to µV first and divide with sqrt(RBW) then, or vice versa.

Can you help me with this topic?

Best Regards
Markus

  • 0
    TI__Genius 15125 points

    Hello Markus,

    Have you taken a look at equation 5?

    This is how you would convert to uV/sqrt(Hz).

    Page 6 and 7 of the app note provide a great explanation on this conversion.

    Please let me know if you have any additional questions.

    Best regards,

    Hannah

  • 0 in reply to Hannah Grace
    Intellectual 430 points

    Hello Hannah,
    thank you for your fast response.

    Yes I took a look at this equation. But this one tells me how to convert dBµV to µV when RBW is already considered by the spectrum analyzer. Therefore, the unit is µV / sqrt (1Hz).

    Is this correct?

    Page 6 states: "Most spectrum analyzers record the measured noise in dBµV/√Hz."
    => Unfortunately my spectrum analyzer records in dBµV.

    Page 7 states: "Once you have the spectral density in µV/√Hz, you can calculate the RMS noise ..."
    For my measurement I set RBW to 100Hz, so the values are related to 100Hz. I don't know how to do the conversion to get the value related to 1Hz.

    Can you tell me how to do the conversion?

    Best regards;
    Markus

  • +1 in reply to Markus Kuebler
    TI__Genius 15125 points

    Hi Markus,

    The equation is not stating to take the sqrt of just 1 Hz. This value would be the RBW you used, which is 100Hz. 

    Hope this helps!

    Best,

    Hannah