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ADS127L01: ENOB and effective resolution

justifice zhou
Intellectual 400 points
Part Number: ADS127L01

In the datasheet of ADS127L01 part 7.1 noise performance: the equation is:

ENOB = In (2 x VREF / VRMS_noise) / In (2)

But isn't it

Effective resolution = In (2 x VREF / VRMS_noise) / In (2)?

ENOB is different from effective resolution according to the E-book below.

Fundamentals of Precision ADC Noise Analysis (ti.com)

And it also confuses me that some ADCs like ADS8588 don't have VRMS_noise or Effective resolution parameters.

So how can we compare the noise performance between 2 different ADCs if one only have  AC parameter (like ENOB, SNR) and the other one only have DC parameter (like effective resolution VRMS_noise)?

  • +1
    TI__Mastermind 41390 points

    Hello Justifice,

    TI previously described delta-sigma ADC noise specifications differently from SAR ADC specifications.  Unfortunately, the term ENOB was used differently for a delta-sigma ADC verses a SAR ADC.  On newer datasheets, we have changed our terminology so that ENOB definition is the same regardless of the ADC.

    The correct definitions are as follows:

    For specifications based on a DC input (usually shorted inputs, or 0V):

    Effective resolution = In (Vfs / Vrms_noise) / In (2)

    Dynamic range (DR) = 20*log(0.7071*(Vfs/2)/Vrms_noise)

    Vfs is the full scale, peak to peak, input range of the ADC.  For a unipolar input ADC with an input range from 0V to Vref, Vfs=(Vmax-Vmin)=(Vref-0)=Vref.  For a bipolar input range ADC with an input range from -Vref to +Vref, Vfs=(Vmax-Vmin)=(+Vref - (-Vref))=2*Vref

    SNR and Dynamic Range (DR) are typically very similar numbers, but DR is based on shorted input noise, and SNR is measured using a sinewave (typically 1kHz or 2kHz), collecting a large number of samples and running through an FFT algorithm to calculate SNR (all noise except harmonics) and SINAD (all noise including harmonics).

    The IEEE definition for ENOB is based on SINAD:

    ENOB=(SINAD-1.76)/6.02 (dB)

    Since SINAD, SNR, and DR are usually very close in amplitude, a good approximation for ENOB is then:

    ENOB~=(DR-1.76)/6.02 (dB)

    If you want to compare noise between ADS8588 and ADS127L01, you can use ENOB per the above definition, or directly compare SNR to DR.

    Selecting the High-resolution, 512ksps, Wideband 1 filter for the ADS127L01, the DR (listed as SNR) is 103.7.

    ADS127L01 ENOB~ = (103.7-1.76)/6.02 = 16.9b.

    Looking at the ADS8588 datasheet, Input range = +/-10V, 1kHz input sinewave, the SINAD is specified as 92.6dB.

    ADS8588 ENOB=(92.6-1.76)/6.02 = 15.1b.

    Also, for most SAR ADCs, there will be a histogram in the Figures that shows noise for shorted inputs.  Figure 13 in the ADS8588 datasheet shows the noise with shorted inputs.  The sigma=0.55 is the noise expressed in LSB-rms.  You can convert this to voltage by first calculating the LSB size.

    1LSB=Vfs/2^n where Vfs = (Vmax-Vmin) = (+10V - (-10V) = 20V and n=16 (ADC resolution)

    1LSB = 20V/2^16 = 305uV

    Vrms_noise = Sigma * 305uV/LSB = 0.55*300uV = 168uVrms

    Regards,
    Keith Nicholas
    Precision ADC Applications