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Interfacing AD1248 with PT500/1000 Sensor

Deepak Saraswat
Prodigy 40 points

Dear Joachim,

We still have queries regarding AD1248. In its features, it is written: 24 bits, No missing codes. What exactly it means if we can't access all 24- bits? Or under what conditions it gives the same as output? As per your last reply, you told us that we can achieve only 19-bits from it due to noise. Kindly tell us about this.

Regards,

Deepak Saraswat

  • 0
    TI__Mastermind 44780 points

    Hi Deepak,

    The statement "no missing codes" means that (in the absence of noise) an increase in the input voltage will always result in the increase of the ADC's output code. Non-linearites in the ADC's transfer function can cause missing codes. Missing codes can cause distortion in the measured signal's FFT, and are not good for control systems using feedback loops.

    Therefore, all [(2^24) -1] or 16,777,215 codes are available (subtract 1 because with two's compliment numbers the number b1000...0 is never used because it's 2's compliment is itself). However, because of noise, whenever you sample a DC signal many times you will always see a gaussian distribution of codes centered around the code that actually represents the DC voltage. It is possible to average your results to get a more accurate measurement. The delta-sigma ADC actually does this for you, but you can further average the ADC's output if you wish - ultimately, this will only get you so close to the true DC voltage.

    Best regards,
    Chris