How should I drive an LVDS signal into the LO port of a mixer when the data sheet specifies a 50 ohms differential input impedance and the eval board has 50 ohm traces to ground?

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Accepted Answer

The LO input port has a differential input impedance of 50 ohms. The evaluation board was designed to offer various LO configuration options, including differential input, single external input to balun to differential input, and single-ended input with ac-coupling capacitors. This presents challenges for the PCB trace design, whether and which traces should be 25 ohms or 50 ohms. The evaluation board was designed with 50 ohm traces, which suits single-ended drive through an ac-coupling capacitor with LOIN ac-coupled to ground. If you want to drive the LO port differentially with an LVDS signal, which has a characteristic impedance of 100 ohms, you would ideally use two side-by-side 50 ohm traces, a 2:1 balun, and two 25 ohm traces into LOIN/LOIP. A simpler approach would be to take one side of the LVDS signal, ac-couple it into LOIP, and then ac-couple LOIN to ground. In this case, all of your traces would be 50 ohms.

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ADL5801 : FAQ