When the signal path can be ac-coupled, the dc biasing for the THS4551 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. The input can be ac-coupled and the output dc-coupled, or the output can be ac-coupled and the input dc-coupled, or both can be ac-coupled. One situation where the output can be dc-coupled (for an ac-coupled input), is when driving directly into an ADC where the V_OCM control voltage uses the ADC common-mode reference to directly bias the FDA output common-mode voltage to the required ADC input common-mode voltage. 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 VOCM at midsupply, which can be easily delivered by floating the VOCM pin. The V_OCM voltage must be within the linear range for the common-mode loop, as specified in the headroom specifications (approximately 0.7 V greater than the negative supply and 1.3 V less than the positive supply for the full –40°C to +125°C operation). If the output path is also ac-coupled, simply letting the VOCM control pin float is usually preferred in order to obtain a midsupply default V_OCM bias with minimal elements. To limit noise, place a 0.1-μF decoupling capacitor on the VOCM control pin to ground.

After V_OCM is defined, check the target output voltage swing to ensure that the V_OCM plus the positive and negative output swing on each side does not clip into the supplies. If the desired output differential swing is defined as V_OPP, divide by 4 to obtain the ±V_P (peak voltage) swing around V_OCM at each of the two output pins (each pin operates 180° out of phase with the other). Check that V_OCM ±V_P does not exceed the absolute supply rails for the rail-to-rail output (RRO) device. Common-mode current does not flow from the common-mode output voltage set by the VOCM pin towards the device input pins side, because both the source and balancing resistor on the non-signal input side are dc blocked (see Figure 9-5). The ac-coupled input path sets the input pin common-mode voltage equal to the output common-mode voltage.

The input pin positive headroom requirement (1.2 V) is less than the V_OCM positive headroom (1.3 V). If the V_OCM is in range, the input pins are also in range for the ac-coupled input configuration. This headroom requirement functions similarly for when the output V_OCM voltage approaches the negative supply. The approximate minimum headroom of 0.6 V to the negative supply on the V_OCM voltage is greater than the input pin voltage headroom of approximately 0 V for the negative rail input design. The input common-mode voltage is also in range if the output common-mode voltage is in range and above 0.6 V from the negative supply because the input common-mode voltage follows the output V_OCM setting for ac-coupled input designs.

The input pin voltages move in a common-mode manner with the input signal, as described in Section 9.3.3. Confirm that the V_OCM voltage plus the input V_PP common-mode swing also stays in range for the input pins.