Checking Absolute Input Headroom Inside an ADC with a PGA

Increasing an ADC’s programmable gain can make a small differential sensor signal easier to resolve, yet move its internal amplifier outside its valid operating region. Differential full scale and absolute input voltage are separate constraints. A signal can be tiny between the two pins while both pins sit at an unsuitable common-mode voltage.

Check the internal amplifier limits

The TI ADS124S08 data sheet gives gain-dependent PGA input-voltage requirements and distinguishes PGA operation from bypass. Use the exact device equations, including exceptions for digitally implemented gain settings. Do not transfer a bypass input-range specification to an enabled amplifier.

Consider a bridge whose two outputs are centered near half its excitation voltage. The differential signal may be only a few millivolts, but excitation tolerance and wiring faults change the absolute voltage on each pin. Conversely, a ground-referenced sensor with one ADC pin tied to ground may fit the converter’s differential range while violating an enabled PGA’s headroom requirement.

Build a two-pin operating map

For each gain, calculate minimum and maximum voltage of both input pins across sensor output, excitation, supply, and bias tolerances. Include startup while the bridge supply rises before the ADC bias, and include open-sensor diagnostic currents. Use maximum expected differential excursion when the manufacturer’s equation requires it, not only the nominal zero-load signal.

• List analog supply minimums at the ADC pins.
• Bound positive and negative differential sensor output.
• Calculate common-mode movement during normal operation.
• Check diagnostic and calibration connections separately.
• Read any available PGA rail flags together with conversion data.

Verify headroom before calibrating

Sweep common-mode voltage at a constant differential input and observe gain error, distortion, and status. Then sweep differential voltage at the limiting common-mode points. Use an input source capable of controlling both quantities independently. A calibration that forces one saturated operating point to the expected code does not restore linearity over the range.

Reducing gain, changing sensor bias, using a suitable bipolar supply, or bypassing the PGA can create headroom, but each affects noise, power, or allowable input range. Compare the resulting system error rather than assuming maximum gain is always best. Keep instrumentation amplifier headroom principles and ratiometric bridge reference routing in the same design review so analog range and reference behavior remain consistent.