I am using a Si4685-A10 chip to receive FM and DAB. I want to match the receiver's input to an antenna with 50 Ohms source impedance and have some questions to which I could not find answers in datasheet, and application notes (AN651 and AN851).


1) Which impedance (x kOhm || x pF) does the chips's VHFI pin itself have?
I looked at the datasheet (Si4685-A10Rev1.1.pdf), and in

  • Table 7 "Analog FM Receiver Characteristics"
  • Table 9 "DAB Receiver Characteristics"

I only found values like sensitivity and IP3, but no input impedance of the VHIF pin (pin 10).


And even more important for practical circuit design:
2) Which input impedance does the application circuit have?

I found two different application circuits:
1) Figure 6 "Typical Si4685 Application Schematic" in the Si4685 datasheet:


2) Figure 1 "Si468x QFN Schematic Design" in AN851 Rev. 0.2 "Si468X SCHEMATICS AND LAYOUT GUIDE":


There are some differences reagarding the FM / DAB input circuit between them. Which one is the recommened one - regarding an input impedance of 50 Ohms?

And, In AN651 - under "3. FM Analog Radio Testing" - I read this:
"In the case of Si468x FM tuner, input impedance is high; therefore, to convert the value displayed on the signal generator to EMF, double the voltage (add 6 dB)."
So is the application circuit's (and Si468x Evaluation board's) antenna input high-ohmic and not 50 Ohms?


Best regards,

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  • Did you find answers to your questions? I'm struggling with the same, using the SI4684.


  • kruskal wrote:

    Did you find answers to your questions? I'm struggling with the same, using the SI4684.

    Unfortunately not. Robot Sad


    Best regards,


  • What I got back then (October 2015) was this:


    50 Ohms FE network


    But it unfortunately did not yield the desired results.


    Best regards,