9.1.2 Input and Output Headroom Considerations

The starting point for most designs is to assign an output common-mode voltage. For AC-coupled signal paths, this starting point is often the default midsupply voltage to retain the most available output swing around the output operating point, which is centered with V_CM equal to the midsupply point. For DC-coupled designs, set this voltage considering the required minimum headroom to the supplies listed in the Electrical Characteristics tables for V_CM control. From that target output, V_CM, the next step is to verify that the desired output differential V_PP stays within the supplies. For any desired differential output voltage (V_OPP) check the maximum possible signal swing for each output pin. Make sure that each pin can swing to the voltage required by the application.

For instance, when driving the ADC12D1800RF with a 1.25-V common-mode and 0.8-V_PP input swing, the maximum output swing is set by the negative-going signal from 1.25 V to 0.2 V. The negative swing of the signal is right at the edge of the output swing capability of the LMH5401. To set the output common-mode to an acceptable range, a negative power supply of at least –1 V is recommended. The designed negative supply voltage is the ADC V_CM – 2.5 V for the negative supply and the ADC V_CM + 2.5 V for the input swing. To use the existing supply rails, deviating from the designed voltage may be required.

With the output headroom confirmed, the input junctions must stay within the operating range. Because the input range extends approximately to the negative supply voltage, input range limitations only appear when approaching the positive supply where a maximum 1.5-V headroom is required.

The input pins operate at voltages set by the external circuit design, the required output (V_OCM), and the input signal characteristics. The operating voltage of the input pins depends on the external circuit design. With a differential input, the input pins operate at a fixed input V_ICM, and the differential input signal does not influence this common-mode operating voltage.

AC-coupled differential input designs have a V_ICM equal to the output V_OCM. DC-coupled differential input designs must check the voltage divider from the source V_CM to the LMH5401 CM setting. That result solves to an input V_ICM within the specified range. If the source V_CM can vary over some voltage range, the validation calculations must include this variation.