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ADS8860: About ADC drive design

Part Number: ADS8860

Hi team,

I am learning about ADC drive design, I saw some articles, I have some questions:

1.What does XX mean and what is his calculation principle?

2.What does Ntc mean and what is his calculation principle?

3.Do we have other relevant information to share?

Please explain in detail, I am very interested in this knowledge, thanks!

Best Regards

  • Hello,

    Let me try to answer your questions.

    1.  I am not familiar with this specific application note, so I am not certain what point the author of the article was trying to make.  However, The formula provided gives the maximum rate of change of a sinusoid signal, or dV/dTmax, which is equal to the peak voltage, Vpeak, times 2*3.14*frequency.  In order to calculate how much the input signal has changed during the conversion period, you can multiple this max rate of change by the conversion time, which gives the maximum change in input voltage during the conversion.   Please keep in mind that to properly sample the input, you need to meet the Nyquist sample rate, which is 1/2*Fsample.  So, for a 1Msps conversion rate, the maximum input sine wave frequency will be 500ksps.  However, if you are looking at a step input, then it might be important to understand how much the input signal changes during a conversion period.

    2.  Ntc is the number of time constants.  This equation assumes that the input settling time is dominated by a simple RC network, where the time constant, tc=R*C.  In one time constant, the input will then settle to 63% of the final value.  However, for a 16b converter, we want the final value to settle to within 1/2LSB, or 100*(1-(0.5/2^16)) = 99.999% of the final value.  In this case, we need to multiply the time constant by N to find the total settling time.

    3.  Please view the TI Precision Labs - ADC's videos and presentations.  It goes over much of this material in great detail.

    Keith Nicholas
    Precision ADC Applications