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ADS54J20EVM: How do i calculate the R&C value in analog input network when the input frequency changed

Part Number: ADS54J20EVM

Hi jim,

    For sampling the low frequency input i replace the balun,can i need to recalculate the R&C value of the analog input network?If i need,is there have some calculate formula for this?It is best to be able to provide document about the calculation。


  • Suy,

    Figures 133 and 134 in the data sheet show the Rin vs Frequency and Cin vs frequency values. You do not have to calculate these values.



  • Yes i  saw this,but the Rin&Cin are inside chip,which means the input RC off-chip is the same as inside?If it is,just only need to confirm the highest frequency of the balun and do not care the lowest frequenc,And  in the curve RC vs frequency looks like not match the vaule of RC input network in ADS54J20EVM。

    Another question,if the input signal driver strenth is weak,so it need to add the differential op AMP,which parameters need my attention for match the ADC chip。

  • Suy,

    Use the impedance trace you plan on using along with the parameters of the highest frequency you plan on using for your calculations.

    The ADS54J20EVM uses the two 24.9 Ohms for 50 Ohm termination from the input. The two series 4.99 Ohm resistors are used to dampen any kickback from the ADC, and the last parts are a LPF filter. The design use 100 Ohm differential traces.

    Please send the other question to the high speed amplifier forum. They should be able to assist with this.



  • Jim,

       I still have some doubts,please help me confirm this:

       1, In the section,as long as i choose the frequency of input signal,i can select the corresponding RC value and calculate the ZIN

       2,At this step i’m not sure whether to use the R&C value  to calculating by theR&C off-chip and ZIN

      For example,if the requirement of design  is 100Ohm input and the frequency is 100MHz,then we can  confirm the RIN is 900Ohm and CIN is4.75pF,so the ZIN equal to RIN || CIN and the value is 244Ohm。

      Therefore in order to get the 100Ohm input impedance the R value is 169Ohm(100Ohm = 169Ohm || 244Ohm),is that correct?