Inquiry about the design of the AD9258 and the ADA4937-1

Hello,

My customer will use ADA4937-1 and AD9258 at repeater application.

1. Input voltage range of the Vocm pin in the ADA4937-1 is min 1.2V to max 3.8V. However, the VCM output of the AD9258 is 0.9V.

    The voltage ranges between the Vocm pin and the VCM pin do not match each other.

    Is there a problem with the optimum performance of the AD9258 when two pins are connected to each other like the example application below ?

   

2. The figure below shows an applicaton example in ADA4937-1 data sheet. Can we replace the AD9246 with the AD9258 ?

    

    Thanks.

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  • +1
    •  Analog Employees 
    on Jan 16, 2020 2:52 PM 9 months ago

    Hello,

    If you look carefully..............you will see DC blocking caps that are at the output of the ADA4937-1 in the second figure you show.   This allows the ADA4937-1 to operate with a common that is centered about its mid-supply point of VS+/2 (when the VOCM pin is left open) for optimum ac performance and the AD9258's input to operate at its optimum input common mode (using same interface circuit with 200ohm resistors connected to VCM).   Again............this assumes that DC coupling is NOT required in the application which would be the case in a repeater application using IF sampling.

Reply
  • +1
    •  Analog Employees 
    on Jan 16, 2020 2:52 PM 9 months ago

    Hello,

    If you look carefully..............you will see DC blocking caps that are at the output of the ADA4937-1 in the second figure you show.   This allows the ADA4937-1 to operate with a common that is centered about its mid-supply point of VS+/2 (when the VOCM pin is left open) for optimum ac performance and the AD9258's input to operate at its optimum input common mode (using same interface circuit with 200ohm resistors connected to VCM).   Again............this assumes that DC coupling is NOT required in the application which would be the case in a repeater application using IF sampling.

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