Mapping Common-Mode Headroom Through a Differential ADC Driver

A differential signal can have the correct amplitude while one of its physical pins operates outside an allowed range. Review the common-mode level and individual pin voltages through the driver and ADC interface. A differential waveform alone can hide the limiting condition.

Separate input and output common mode

The THS4551 data sheet discusses input and output headroom and explains how the external feedback network affects the amplifier’s input common-mode voltage. Setting the output common-mode control does not remove the need to check the input pins.

Draw the source offset, input resistors, feedback resistors, output common-mode source and ADC inputs. Label the actual supply rails at both devices. Include any series resistance or filtering between the amplifier outputs and converter pins.

Calculate the physical pin excursions

Express the two output voltages as common mode plus and minus half the differential voltage for a balanced signal. For illustration, 2 V peak-to-peak differential swing centered at 1.5 V means each output swings 1 V peak-to-peak, nominally from 1.0 V to 2.0 V.

Compare those endpoints with the amplifier’s output capability under the actual load and with the ADC’s allowed input range. Then calculate the amplifier input-pin voltages using the selected resistor network and source offset. Repeat for both extremes of the source signal and supply tolerance.

Follow the bias through the interface

Identify the device that establishes output common mode. Check whether that source is available during startup and whether it can drive the connected load. A converter’s common-mode output may have specific loading and decoupling requirements that differ from a general reference supply.

Account for DC drops in series components and any input current described by the ADC manufacturer. Do not assume that the voltage at the amplifier output is identical to the voltage at the sampling pins under all conversion rates.

Measure more than the difference

Capture both ADC input voltages and their calculated common-mode and differential components. Use matched probing and sufficient bandwidth for the intended observation. Check startup, maximum signal excursion and relevant sample-rate settings.

Record the source range and offset alongside the measured pin limits. A useful approval table shows the minimum and maximum voltage of each important node, rather than only a differential full-scale value. Revisit the table if the supply, gain, ADC reference or source offset changes. These changes can move a hidden common-mode boundary even when the nominal differential amplitude remains exactly the same. Record the component tolerances used for each limiting calculation.