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ADL5902 and highpass frequency (Formula 11)

To whom it may concern,

I doubt that formula 11 in ADL5902 Rev. B datasheet is correct.

For a typical circuit, like Figure 45, in my opinion the resistance determing the high pass frequency is the ADL5902 input resistance and not the global input resistance of 50 Ohm, as assumed in formula 11.

  • The external shunt resistor creates a broad band 50ohm termination load across frequency.  With the 50ohm source, and a 50ohm characteristic transmission line to the input, the impedance looking at the input is about 50ohm(60//2k).  w=1/RC is a approximation of the high pass corner. the exactly value of coupling cap C, is not that critical as long it's big enough to pass the frequency of interest.