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ADC ENOB - Defino F2837x

Etai
Expert 3400 points


Hi 

My customer would like to get a better than 14bit ENOB with the Soprano ADC, they want an ENOB of 15bits, so if they are willing to sample slower or oversample, is there a way we can get there (with oversampling and filtering in SW)?

Thanks!

Etai

  • 0
    TI__Guru 60865 points
    Hi Etai,

    Let's start with 16-bit mode:
    SNR = 87.6dB
    THD = -93.5dB
    SFDR = 95.4dB
    SINAD = 86.6dB
    ENOB = 14.1dB

    The only thing that we can increase is SNR, and we do this by oversampling. For every 2x oversampling, we gain 3dB of SNR. This process is practically limited by SFDR.

    15-bit ENOB = SINAD of 92dB. With THD = -93.5db, we need an SNR of 97.5dB to get SINAD of 92dB. Since 97.5dB > 95.4dB (SFDR) we can't quite get there. If we say that SNR = 95.4, then SINAD = 91.3dB which is equivalent to ENOB of 14.9. 95.4 - 87.6 = 7.8, so we would need to over-sample by at least 8x (this gives 9 dB gain).

    Note that we spec' SFDR and all the other items for 16-bit with ADC fin = 10KHz. With a slower moving signal, they may be slightly better.

    Overall, if they use 16-bit mode and over-sample at least 8x they will get pretty close.

    For 12-bit mode, ignoring everything but SNR, we start at SNR = 68.8dB. 92 - 68.8 = 23.2. We would need to oversample by at least 256x to get in the right SNR ballpark. 12-bit mode also has much lower SFDR and THD, so even with huge oversampling, you probably wouldn't practically really get there.

    If they need really high ENOB, but with lower sample rate, the way to go is with sigma-delta ADCs. F28x7x devices have a digital sigma-delta filter module than can interface with the analog front-end portion of a sigma-delta ADC.