Trace the Reference Path in a Ratiometric Bridge Measurement

Calling a bridge interface ratiometric describes a relationship between the measured signal and the ADC reference. It does not mean all supply, wiring and temperature errors disappear. Review the actual voltage paths to see which variations are shared and which affect only one side of the ratio.

Identify the ratio being measured

Draw the bridge excitation source, bridge output, ADC inputs and reference inputs. Mark any amplifier between the bridge and ADC. Texas Instruments’ bridge measurement brief describes differential reference inputs for ratiometric bridge acquisition. The implementation must ensure that the reference represents the intended excitation under the actual operating conditions.

As a simple conceptual example, a bridge output proportional to excitation can retain the same output-to-excitation ratio when both change together. A voltage drop in a cable that affects the bridge but is absent from a locally sensed reference breaks that simple correspondence. Draw the sense points before relying on cancellation.

Account for the cable and connector

Identify whether excitation is sensed at the board or at the remote bridge. Include connector and lead resistance in the review. If separate sense conductors are used, trace them through protection and filtering to their destination. Confirm that the sensor wiring and PCB connector support the intended arrangement.

Evaluate open or disconnected sense leads. A nominally correct ratiometric scheme can produce misleading values if a reference input floats or falls outside its valid range. Define how software recognizes invalid conditions and avoids presenting them as a normal load or pressure reading.

Review errors outside the shared ratio

Consider amplifier offset and gain drift, bridge offset, resistor matching, reference-input behavior and thermal effects. A ratiometric connection does not inherently cancel those mechanisms. Check input common-mode limits and PGA range at the maximum bridge offset as well as at nominal zero load.

If excitation is switched, include settling of the bridge, filters, amplifier and reference path in the timing budget. The signal and reference may respond at different rates. Sampling too soon can defeat the relationship that holds after both have settled.

Challenge the assumed cancellation

At a stable applied stimulus, vary excitation within the approved operating range and record both raw code and relevant voltages. Add representative lead resistance or compare cable lengths when those variations are part of the product use case. Investigate residual changes rather than assuming they are random ADC noise.

Keep the reference routing, sensor wiring, filter values and sampling sequence in the verification record. Revisit the ratio when the cable, excitation source or ADC configuration changes. The useful claim is a measured reduction of defined error sources under stated conditions, not a blanket promise of supply-independent accuracy.