8.3.2.1 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 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 midsupply. 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 AC-coupled, letting the V_OCM control pin float is usually preferred 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 61), 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 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. The input signal moves this input V_ICM around the DC bias point, as shown in the Resistor Design Equations for Single-to-Differential Applications subsection of the Fully-Differential Amplifier section.