LMH32404RHFEVM: Balun connection on the LMH32404 EVB

Hello Sariel,

Good question!  Baluns & transformers can be confusing, especially when deciphering a schematic from another person or company.  While intuition may suggest that baluns should always be wired up with a 'vertical' orientation (see Voltage Balun in Figure 3 of linked article), there are cases such as the LMH32404RHFEVM where the 'horizontal' orientation used does make sense for the circuit application.

As the signaling needs require a transformation from differential to single-ended signal (or balanced to unbalanced), the transformer is placed in-line to the differential output signals.  This may appear counter-intuitive, as there would "appear" to be a "short" of the output signals directly to the SMA connectors.  However, what is actually happening is this: the transformer (balun) is being used in a current balun configuration.  You can read a bit about current baluns in the article below; I will leave it to you to explore the theory in detail.

Essentially the current-balun configuration is most appropriate for the LMH32404RHFEVM; the impedance matching is handled by selected resistors and allow for a differential to single-ended conversion for the OUT+ SMA connector.  If differential output signaling is desired, the transformers T1, T2, T3, and/or T4 can be removed and shorted across the output lines.  Be careful to also desolder R2, R7, R16, or R27 depending on which transformers are removed.  These resistors short OUT- signals to GND.

https://blog.minicircuits.com/demystifying-transformers-baluns-and-ununs/

Best,

Alec