Circuit Topology
Excitation of resistive sensor networks through a fixed amperage source maintains a stable signal output regardless of lead wire resistance changes. The constant current bridge forces a specific quantity of electrons through the sensing element, ensuring that the voltage drop across the active arm remains proportional to the resistance change alone. This configuration is particularly effective for remote sensing where long cables introduce parasitic resistance that would otherwise degrade the measurement.
It contrasts with voltage driven bridges which suffer from sensitivity loss as cable length increases.
Linearity Gain
Output responses for single element resistive sensors show improved proportionality when driven by a current source. In a standard Wheatstone bridge, the relationship between resistance change and output voltage is non linear. Utilizing a current drive linearizes this transfer function, simplifying the digital correction required in the downstream processor.
Thermal Management
Power dissipation within the sensing element must be carefully controlled to prevent self heating errors. Because the power equals the square of the current multiplied by the resistance, even small increases in drive current lead to notable temperature rises in the gauge. Such heat shifts the baseline resistance of the sensor and introduces a thermal offset that is difficult to distinguish from physical strain.
Stable current sources must be tuned to the lowest level that provides an acceptable signal to noise ratio.
Source Stability
Precision of the final measurement is limited by the drift of the current regulator. Any fluctuation in the supplied amperage appears directly as a change in the bridge output, mimicking a change in the measured variable. High grade instrumentation uses temperature compensated references to keep this source drift below five parts per million.
These regulators must also provide high output impedance to ensure the current remains steady even as the bridge resistance fluctuates during operation.
