How to set LTC5594 ADRF6520 AD9684 Common mode voltages ?

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    •  Analog Employees 
    Jun 8, 2018 +1 verified

    In addition to setting LTC5594's Vcm pin to 1.375V, you also need to properly set the output ground return resistors, R3-R8. The LTC5594 has current-mode IF outputs that require a termination resistance…

  • 0
    •  Analog Employees 
    on Jun 8, 2018 8:24 PM

    ADRF6520's input common mode voltage is 1.375V, while output common mode is 1.65V.  There is no on-chip mechanism to change these common-mode voltages, so you must match the pre- and proceeding stages to the ADRF6520's common-mode. 

    The LTC5594's IF Amps' output common-mode voltage is 0.9V by default, but can be change from 0.5V to 2.0V via the Vcm pin.  I'd drive Vcm = 1.375V to match that of the ADRF6520.

    You might have to implement a level shifter between the output of the ADRF6520 and the input of the AD9684, to shift from the 1.65V to 2.05V on the AD9684's input.

    Is DC coupling a must for your application?  With the larger signal BW's in the Receive chain, it is often unnecessary to DC couple given one uses large enough AC coupling caps.

  • +1
    •  Analog Employees 
    on Jun 8, 2018 9:13 PM

    In addition to setting LTC5594's Vcm pin to 1.375V, you also need to properly set the output ground return resistors, R3-R8. The LTC5594 has current-mode IF outputs that require a termination resistance to create output common-mode voltage. The optimum current is around 18mA per output pin. The internal feedback adjusts the current to match the output common-mode voltage to what is set on Vcm.

  • Hi Jdobler,

    Thank you for your help but I already know everything written in datasheets. I need an implemented demonstration applicable for Zero IF. As written on the top of my image shared, it is a Zero-IF application, so DC coupling is a must.

    Please send me a level shifter for that apllication specific case.

  • Hi Shsiao,

    Thank but what must be the R3-R8 resistors value for 1.375V ?  

  • Can you have a look at the below VCMs which are feeded from ADR4520 voltage ref IC? Is The curent supplied from ADR4520 enough or NOT? AND What are you suggestions If I use those components as design below? Does it work or NOT?