adc analog input resistance (ad7274/ad7276)

Hello,

I need to calculate the effective noise figure for the adc's shown above. Therefore I need the analog input resistance to determine full scale power level but it's not given in the datasheet (for other devices, like ad9467, it is). Can anybody help me with this problem?

Thank you in advance

  • 0
    •  Analog Employees 
    on Dec 8, 2020 1:12 PM

    Hansiblue,

    For precision ADCs like the AD7274 the converter noise tends to be dominated by KT/C versus the input resistance of the ADC.  The easiest method I've found to estimate NSD is to use the transition noise plot (Figure 22) which shows a histogram plot of the code output with a DC input.  What we see here is that the peak-peak noise distribution is approximately 1LSB (and let's assume that VREF = VDD = 5.0V).

    So for the AD7274/6, 1 lSB = 5.0/(2^12) = 1.22mVpk-pk which if we assume the distribution is gaussian places the RMS noise around (1.22mV/6) = 200uVrms.  This equates to 170nV/rt Hz in the Nyquist bandwidth of 1.5MHZ. 

    Hope that helps.

    Sean