**Part Number:**AFE7225EVM

**Other Parts Discussed in Thread:**TRF370333, AFE7225

Can you explain the rationale behind the filter chosen between the AFE7225 and the TRF370333? I've referred to SLA399, section 4.1.3, and it has a substantially different solution. I've also used the SLA399 included spreadsheet (Matched_Impedance, for example) and get different values. I'm hoping to understand the methodology so I can increase the order of the filter and understand if there are impedance implications when routing the board.

- What is the purpose of the 37.4 serial resistors? (R117, R125, R130, R140).
- What is the purpose of the 4.7pF to ground right before IN? (C95, C96, C108, C109) Are these to provide a bypass path to ground at the driven end after the filter?
- Doing a little reverse engineering, the 2nd order filter inductor and capacitor values chosen would imply a 50 ohm filter input SE impedance, and a 260 ohm filter output impedance, but R155 doesn't match. Also, those are kind of...unique...values. What is the expected input and output impedance for the filter?

Using SLA399, 4.1.3 (which has a nice explanation of deriving various values). I would expected to eliminate the 37.4 resistors, the 4.7pF to ground capacitors, and change R154/R155 to 200 ohms. A differential 2nd order Butterworth with input impedance of 100 ohm (50 ohm SE) and output of 200 ohm would have inductor values of 215nH and a capacitor of 5.6pF (using rf-tools.com/lc-filter and converting for differential).

Any insight would be much appreciated. Thanks!