Input matching of switched capacitor ADC (AN-742)

Application Note AN-742

One page  4 of AN-742 the design example uses AD9236 input impedance values taken from a graph Fig.6 on page 3 (or tabulated S parameter data) and it appears to be interpretted as elements of a parallel equivalent circuit.

However AN-827 which elaborates on the method and displays exactly the same graphs states that the R &C elements in the plots are SERIES equivalents and gives a method to convert to the parallel equivalent circuit better suited for matching circuit design.

So although the general design outline in AN-742 is sound, the parameter values used  appear to me to be inconsistent with the interpretaion given in AN-827.

Any comment on this greatly appreciated.

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  • 0
    •  Analog Employees 
    on Jul 20, 2011 6:38 PM

    Hi 6j5,

    The parallel equivalent in AN-742 is shown because we are looking at the input differential into the ADC. I also think you are referring to an older revision of this app note. So I attached the latest.

    In all the ADC impedance tables that are provided, the series and parallel equivalents are given so you can calculate your filter or matching circuit in either case.

    Regards,

    Rob

    AN-742.pdf
Reply
  • 0
    •  Analog Employees 
    on Jul 20, 2011 6:38 PM

    Hi 6j5,

    The parallel equivalent in AN-742 is shown because we are looking at the input differential into the ADC. I also think you are referring to an older revision of this app note. So I attached the latest.

    In all the ADC impedance tables that are provided, the series and parallel equivalents are given so you can calculate your filter or matching circuit in either case.

    Regards,

    Rob

    AN-742.pdf
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