There are two typical ways to use the LMH5485-SEP with an AC-coupled differential source. In the first method, the source is differential and can be coupled in through two blocking capacitors. The second method uses either a single-ended or a differential source and couples in through a transformer (or balun). Figure 9-1 shows a typical blocking capacitor approach to a differential input. An optional input differential termination resistor (Rm) is included in this design. This Rm element allows the input Rg resistors to be scaled up while still delivering lower differential input impedance to the source. In this example, the Rg elements sum to show a 200 Ω differential impedance, while the Rm element combines in parallel to give a net 100 Ω, AC-coupled, differential impedance to the source. Again, the design proceeds ideally by selecting the Rf element values, then the Rg to set the differential gain, then an Rm element (if needed) to achieve a target input impedance. Alternatively, the Rm element can be eliminated, the Rg elements set to the desired input impedance, and Rf set to the get the differential gain (= Rf / Rg).

The DC biasing here is very simple. The output V_OCM is set by the input control voltage. Because there is no DC current path for the output common-mode voltage, that DC bias also sets the input pins common-mode operating points.

Transformer input coupling allows either a single-ended or differential source to be coupled into the LMH5485-SEP, which also improves the input-referred noise figure. These designs assume a source impedance that must be matched in the balun interface. Figure 9-2 shows the simplest approach where an example 1:2 turns ratio step-up transformer is used from a 50 Ω source.

In this example, this 1:2 turns ratio step-up transformer provides a source and load match from the 50 Ω source if the secondary is terminated in 200 Ω (turns-ratio squared is the impedance ratio across a balun). The two Rg elements provide that termination as they sum to the differential virtual ground at the FDA summing junctions. The input blocking cap (C1) is optional and included only to eliminate DC shorts to ground from the source. This solution often improves the total noise figure compared to using just the FDA, as it allows for the noise gain of the amplifier to be reduced.