When the signal path can be ac coupled, the dc biasing for the THS4541-Q1 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 Vocm control voltage uses the ADC common-mode reference to directly bias the FDA output common-mode to the required ADC input common-mode. In any case, the design starts by setting the desired Vocm. When an ac-coupled path follows the output pins, the best linearity is achieved by operating Vocm at midsupply. The Vocm 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 Vocm control pin float is usually preferred to get a midsupply default Vocm bias with minimal elements. To limit noise, place a 0.1-µF decoupling capacitor on the Vocm pin to ground.

After Vocm is defined, check the target output voltage swing to confirm that the Vocm plus the positive or negative output swing on each side does not clip into the supplies. If the desired output differential swing is defined as Vopp, divide by 4 to obtain the ±Vp swing around Vocm at each of the two output pins (each pin operates 180° out of phase with the other). Check that Vocm ±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 7-1), 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 Vocm voltages. Because the input Vicm is the output Vocm 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 Vocm. Also, the input signal moves this input Vicm around the dc bias point, as described in the Section 8.4.1.3 section.