When the signal path can be AC-coupled, the DC biasing for the LMH5485-SEP becomes a relatively simple task. In all designs, start by defining the output common-mode voltage. The AC-coupling issue can be separated for the input and output sides of an FDA design. In any case, the design starts by setting the desired V_OCM. When an AC-coupled path follows the output pins, the best linearity is achieved by operating V_OCM at mid-supply. The V_OCM voltage must be within the linear range for the common-mode loop, as specified in the headroom specifications (approximately 0.91 V greater than the negative supply and 1.1 V less than the positive supply). If the output path is also AC coupled, simply letting the V_OCM control pin float is usually preferred in order to get a mid-supply default V_OCM bias with minimal elements. To limit noise, place a 0.1 µF decoupling capacitor on the V_OCM pin to ground.

After V_OCM is defined, check the target output voltage swing to ensure that the V_OCM plus the positive or negative output swing on each side does not clip into the supplies. Check that V_OCM ±Vp does not exceed the absolute supply rails for this rail-to-rail output (RRO) device.

Going to the device input pins side, because both the source and balancing resistor on the nonsignal input side are DC blocked (see Figure 8-1), no common-mode current flows from the output common-mode voltage, thus setting the input common-mode equal to the output common-mode voltage.

This input headroom also sets a limit for higher V_OCM voltages. Because the input Vicm is the output V_OCM for AC-coupled sources, the 1.2 V minimum headroom for the input pins to the positive supply overrides the 1.1 V headroom limit for the output V_OCM. The input signal also moves this input Vicm around the DC bias point.