We are trying to get a board designed for a new product prototype, and ran into some questions with the AD8348 I/Q downconverter.

This device states that it has a 200 ohm input and recommends a minimum loss L-pad consisting of a 57 and 174 ohm resistor. It also mentions that the AD8348 does not require the input to be matched and you can use a 60-some-odd ohm resistor to still provide a broadband match to the driving circuit.

However, elsewhere in the datasheet (page 23 under the IF section) it says that this minimum loss L-pad introduces an 11.5 dB loss.

If the impedance is 200 ohms I calculate about 5.7 dB loss thru the pad.

What is the actual impedance here and what is the actual loss using the pad, and is this accounted for elsewhere in the spec?

Input Z is 200 Ohms. The 11.5 dB loss is a power loss. The voltage loss is 5.5 dB (basically the voltage divider between the 200 Ohm input Z and the 174 series R).

If you turn the L-pad around and have a low impedance source driving it and then terminate with 50 Ohms (like how the output of the AD8367 eval board is done), the voltage loss is 17.5 dB. But the power loss is still 11.5 dB.

Does that make sense? I can attach some math if need be.

Joel