Query about Front-End design of AD9253

Hi all,

My customer has a question about differential transformer design on analog input of AD9253.

In the example design within the AD9253's datasheet below

Is the 49.9ohm resister mandatory ?

If the 49.9ohm resister is eliminated, is there any problem ?

Thanks.

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  • 0
    •  Analog Employees 
    on Jan 6, 2017 10:23 AM

    Hi JHShin,

    Thank you for your interest in AD9253.

    Figure 59 in the AD9253 datasheet is a conceptual diagram. Many signal generators have an output impedance of 50Ohms. If you are maintaining a 50Ohm environment up to the ADC, you are best off terminating the receiving end with 50Ohms (or 49.9Ohms as shown in the figure) to avoid reflections. The input impedance of the AD9253 is relatively high so the 50Ohm resistor will dominate. This termination resistor (and transformer) should be placed as close to the ADC inputs as is reasonably possible.

    If you remove the 50Ohm resistor your system could still work but you will have increased reflections and your signal will be degraded. The effect will be worse as input frequency increases. This all assumes you are maintaining a 50Ohm environment.

    Thank you.

    Doug

Reply
  • 0
    •  Analog Employees 
    on Jan 6, 2017 10:23 AM

    Hi JHShin,

    Thank you for your interest in AD9253.

    Figure 59 in the AD9253 datasheet is a conceptual diagram. Many signal generators have an output impedance of 50Ohms. If you are maintaining a 50Ohm environment up to the ADC, you are best off terminating the receiving end with 50Ohms (or 49.9Ohms as shown in the figure) to avoid reflections. The input impedance of the AD9253 is relatively high so the 50Ohm resistor will dominate. This termination resistor (and transformer) should be placed as close to the ADC inputs as is reasonably possible.

    If you remove the 50Ohm resistor your system could still work but you will have increased reflections and your signal will be degraded. The effect will be worse as input frequency increases. This all assumes you are maintaining a 50Ohm environment.

    Thank you.

    Doug

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