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ADS131A04: Calculation of Effective Resolution

Rakhi Rajalekshmi54
Prodigy 50 points
Part Number: ADS131A04

Hi

How is the Vrms voltage calculated/measured to determine Effective Resolution as shown in the datasheet pages 21-22? The Delta sigma evaluation GUI calculates two parameters ENOB and NFB. Why do we have three different parameters? How are these related to Effective resolution as mentioned in the datasheet?

thanks

Rakhi

  • 0
    TI__Mastermind 38745 points
    Hello Rakhi,

    All of these parameters come down to a matter of semantics. There are different schools of thought within the industry as to how each parameter should be calculated, but at the end of the day, everything is referring to "dynamic range" a.k.a. the ratio of the largest signal you can measure to the smallest signal you could measure.

    In the EVM, the parameter "ENOB" is the same as "Effective Resolution" (some may dispute that they are equivalent in theory, but my explanation will be based on how we calculate them).

    Effective resolution (or ENOB in the EVM) is the ratio between a full scale RMS signal and the RMS noise that is measured from the device. Noise that is measured from the device is measured using the "input short" method where the inputs are shorted together, data points are collected for some time, and the standard deviation of the points is calculated. "Vrms" is the RMS noise. Looking at tables 1-4 in the datasheet, you can plug the noise values under "uVrms" into equations 1 and 2 to calculate the other values in the tables.

    Noise free bits is similar to effective resolution, except the peak-to-peak noise value is used in the calculation rather than the rms value. I do not know the exact formulas that the GUI uses, but I can find out for you. Keep in mind that the EVM will calculate the RMS of the data and use it in its calculation for "ENOB" and "NFB" even if you do not have the inputs shorted together.

    Brian
  • 0
    TI__Mastermind 38745 points

    Rakhi,

    The formulas for how the parameters are calculated in the EVM are as follows:

    ENOB = log_2(2^24/Vrms)

    NFB = log_2(2^24/Vpk-pk)

    Sorry for all the various ways of calculating similar numbers. Hopefully this alleviates some of the confusion.

    Brian

  • 0 in reply to Brian Pisani
    Prodigy 50 points
    Thanks Brian. I would expect a voltage parameter at the numerator (of the ENOB formula) since we are taking the ratio of full scale value to the RMS voltage.

    If I understand correctly, the ENOB in the GUI is the same as Effective Resolution in the datasheet?
  • 0 in reply to Rakhi Rajalekshmi54
    TI__Mastermind 38745 points
    Rakhi,

    They are almost the same. They are calculated slightly different between the two. In the datasheet, the "full-scale" signal is calculated as an RMS sine wave. This can be thought of "ac dynamic range". The ENOB in the EVM is calculated using the entire code range of the ADC. This is "dc dynamic range". The EVM's results should look slightly "better" that what you would calculate using the datasheet's formula, but it is really just the difference between calculations rather than difference in performance.

    Brian