ADL5511 impedance matching

Hi , 

Take a closer look at Figure 49. There is a 250 Ohm resistor internally to ground. The parallel combination is 57.7 Ohms. This gets further reduced by capacitive loading, internal to the IC. For lower RF frequencies, input impedance is then about 58 Ohms, but at higher frequencies, the magnitude of impedance gets reduced. The 58 Ohm parallel combination is intentionally chosen to be larger than 50 Ohms, for best wideband input matching. 

thanks for you kind response, perfect ! 

I also have a question .In this ADL5511,the RMS model Is of this chip the same way as the RMS module of AD637?

Hello zai123,

Sorry about delayed response. 

I don't have the requested information. The two designs are likely substantially different, given the vastly different input frequency range of the two devices, AD637 and ADL5511.   -Bruce H.