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Questions loss of ADS4249?

Henry Jung
Prodigy 220 points
Other Parts Discussed in Thread: ADS4249

DEAR. ALL

The Loss of the ADS4249 input frequency varies by IF.

  1. 142.5Mhz -2dB
  2. 237.5Mhz -4dB
  3. 332.5Mhz -6dB

 Analog Input matching circuit designed in the same way to the 0dB  LOSS of IF?

Calculation method that can minimize the Loss by changing the value of 100 ohms and 5 ohnms? ?

  • 0
    TI__Genius 17310 points

    Hi Henry,

    There are a couple things that can be degrading the ADC BW.

    1. The input to the ADC for the most part looks like a capacitor (sampling cap). This capacitor will form a pole at 1/(2*pi*R*C). C is the sampling cap capacitance, roughly 4 pF differential. R is the total impedance looking back into the input network from the ADC input pins. For the diagram shown in your post, this would be 2*5+(200//200) = 110 Ω. The pole from the sampling capacitor will then be at ~361.7 MHz. This would roughly explain the drop that you're seeing.
    2. The transformer itself will have a certain bandwidth associated with it. Please check the transformer datasheet for the insertion loss at those frequencies.

    To improve the BW you have two options. First, choose a transformer with a flatter frequency response. Second, lower the total impedance seen by the analog input by using a 1:1 transformer and changing the termination resistors to 25Ω or change to a 1:2 transformer with termination resistors of 50Ω. You can also play with the 5Ω series resistors, but it doesn't seem that they should be greatly effecting the bandwidth.

    Regards,
    Matt Guibord