input configuration using Back to back Balun shown in evaluation module of ADS5474

Back to back connection on two transformers means the secondary of the 1^st transformer connects to the secondary of the 2^nd transformer, or primary of the 1^st transformer connects to the primary of the 2^nd transformer. The connection between these two transformers is "cross" link. Back to back connection on two transformers mainly is for improving differential output balance due to balance limitation (cause by parasitic between windings) of single transformer. It improves even harmonies like HD2, but has more insertion loss than using single transformer.

Regards,

Qing

Hi,

i just noticed that this last question was not addressed - because it was added to a posting thread that was marked as 'answered' it can escape notice.

But i do know where those resistor values came from, and it has to do with the choice of the balun on the EVM.  The input circuit for this EVM was copied from the board that we made for the bench characterization, and the characterization engineer chose this balun that is described by its manufacturer as a 75-ohm balun.  And at the time he used a network analyzer on the board to check for the termination values that best terminated the input circuit and the two 39 ohm resistors were the best choice.  (That makes a differential termination of 78 ohms, so i expect that if we had two 37.5 ohm resistors then that would have been marginally better.)

For another choice of transformer or balun, the best termination resistor value might be different.  For a single ended 50 ohm input to a 1:1 balun or transformer we usually use two 25 ohm termination resistors for a differential termination of 50 ohms.  If we use a 1:4 impedance ratio transformer for some voltage step-up, then we would terminate with two 100ohm termination resistors for a differential termination of 200 ohms because of the impedance ratio of the transformer.

Regards,

Richard P.