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ADS1262: ENOB and resolution

Part Number: ADS1262

Tool/software:

I am using this ADC to measure low voltage for a particular detector. In my design, the configuration is AVDD 2.5V, AVSS -2.5V, External Ref 1.25V, Sync 4, 32 Gain.

With help of Mr, Keith Nicholas, I could gain a lot of confidence in my design and could conclude that

ENOB = 23.4, Noise Free Bits = 21.5 and Resolution = 4.7nV

The minimum detectable/measurable voltage = 27nV P-P (7nV RMS)

Now I would like to clear out two more confusions related to this

1) If noise-free bits are 21.5, then what is the significance of specifying ENOB as it is of no use? I am curious to know,  can we improve NFBs to reach ENOBs by doing any further offline processing like filtering or coherent averaging?

2) Table 8.2 of the datasheet of ADS1262, whether or not it is with Chop mode on? If Table 8.2 is with chope mode off, and if we enable it, will the noise be further reduced by a factor of 1.4, as mentioned in section 8.8 Noise Performance in the datasheet?

Seeking comments to resolve the above two queries.

  • Hello Praveenlal,

    ENOB (technically, these noise figures are effective resolution) are more useful when measuring AC signals, since the input is not fixed over time.  For DC measurement applications, noise-free resolution is often times more useful.  Both noise specifications describe the 'same' noise, just in different terms depending on how you are using the ADC.

    1.  Yes, you can further improve noise-free resolution by averaging multiple readings in your MCU/processor.  If you want to maintain your data rate, a moving average filter can be used, otherwise, you can simply average a block of data and the averaged result is the new reading.  Since the internal noise of the ADS1262 is fairly flat over frequency, you should be able to get a reduction in noise voltage by a factor of 1/SQRT(n), where (n) is the number of samples that are averaged together.  As an example, taking 4 samples and averaging will reduce noise by a factor of 2, resulting in an increase in Noise-free bits of 1b.

    2.  Table 8.2 is with Chop mode OFF.  Yes, turning CHOP mode on will further reduce noise by a factor a 1.4, but your data rate will be reduced quite a bit to allow for settling of the internal digital filter.  You can get the same improvement in noise by simply averaging 2 readings in your processor, without the reduction in data rate.

    Regards,
    Keith Nicholas
    Precision ADC Applications

  • Thank you, Keith, for the nice explanation. Now I will be able to conclude with my data analysis part.