9.3.2.1 AC-Coupled Signal Path Considerations for Single-Ended Input to Differential Output Conversion

When the signal path is ac coupled, the dc biasing for the LMH5401-SP 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 they can both 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 to the required ADC input common-mode. The feedback path must always be dc-coupled. 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. If the output path is also ac coupled, simply letting the V_OCM control pin float is usually preferred in order to obtain a midsupply default V_OCM bias with no external 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 positive or 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 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 output swing of this device. Going to the device input pins side, because both the source and balancing resistor on the non-signal input side are dc blocked (see Figure 59), 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. The minimum headroom for the input pins to the positive supply overrides the headroom limit for the output V_OCM because the input V_ICM is the output V_OCM for ac-coupled sources. Also, the input signal moves this input V_ICM around the dc bias point, as described in the Resistor Design Equations for Single-to-Differential Applications subsection of the Fully-Differential Amplifier section.