LMH5401: LMH5401 impedance matching using a 1: 1 balum

Hi Art,

I would recommend a 25-ohm series isolation resistor from each of the LMH5401 outputs to the 1:1 balun. This is done so that the overall output load at the LMH5401 is 100-ohms (2x25ohms + 50 ohms). I would remove the two shunt 50-ohm resistors R3 and R5. The C7 and C10 capacitors isolate the DC voltage at the LMH5401 outputs and hence I would keep them.

Best Regards,

Rohit

Hi Rohit!

  Thanks for responding!

The LMH5401 has 10 Ohm resistor at each of it's differential outputs.  In order to get series 25 Ohm resistors as you stated, and at the same time, in order to prevent reflection from going back to the LMH5401 is it necessary to add a 20 Ohm resistor in parallel with the differential outputs?  So, it would be something like this:

  Will this configuration cause problems with overloading the LMH5401 ( 20 Ohm // 100 Ohm)?

Thanks

Art

Hi Art,
Remove both R3 and R5. Use 15 Ohm external resistors to add to the two [10] Ohm on chip resistors. (fixed typo)

Matching is required for a length of transmission line, not a very short interconnect trace. If the trace is less than 1/10 of the shortest wavelength of your signal, then you don't need matching. This is a very common mistake.

Your transformer needs to see 50 Ohms on both sides. The amplifier was not designed for a 20 Ohm load.

Regards,
Loren

Hi Rohit!

    Thank you so much!

I understand what you are saying.

Art