ADA4961 input RLC model


I am looking to do a simulation of the ADA4961 input driver circuitry using a SPICE simulation tool.

I am only able to find the S parameters for the ADA4961 on the ADI website.

I unfortunately do not have access to any software packages that can convert the S parameters to a SPICE model (RLC equivalent circuit).

1) If possible, please can you an RLC equivalent of the ADA4961 input stage that could provide a good approximation of the frequency response?

The  reason I want to simulate the ADA4961 input stage is that I am using the same input stage as the ADA4961 evaluation board. This input stage consists of a TCM2-43X+ RF transformer to convert 50R single ended to 100R differential. I am concerned that this 100R impedance combines with the input capacitance of the ADA4961 (1.3pF single ended) and any parasitic capacitance of the board to create an RC low pass filter. This will cause roll-off of the signal before the 3GHz bandwidth.

2) Do you have a plot of the frequency response of the ADA4961 evaluation board from 350MHz to 3GHz?


  • Hi, Gavint,

         The early rolloff of the ADA4961 is due mostly to, as you suggested, the input mismatch.  The attached plots shows that with matching the rolloff is only 0.4 dB to 2.5 GHz.  I am still working on the equivalent circuit model, and I'll upload it here in the next couple of days.  Please note also that there is some gain peaking below 30 MHz, due to internal caps in the feedback network.  Smaller output inductors can be used to cut down this peaking.