Constant-Current Drive for Pressure Sensors

Overview

Pressure sensors typically use a resistive bridge output. Excitation is applied across two bridge terminals and the differential output is taken from the other two terminals. Strain-gauge resistors serve as the sensing elements.

Bridge types and excitation

Resistive bridges are commonly implemented as four-resistor, two-resistor, or single-resistor configurations. Excitation can be provided as either constant-voltage or constant-current.

Resistance change under load

When force is applied, the sensing resistances change according to the strain distribution. The bridge output behavior depends on the excitation method.

Constant-voltage vs. constant-current excitation

For a four-resistor bridge, constant-voltage and constant-current excitation produce equivalent results. For a two-resistor bridge, constant-current excitation maintains linearity, while constant-voltage excitation exhibits linearity degradation that depends on ΔR.

Single-resistor sensor simulation

For a single-resistor sensor, simulation data were exported and subjected to linear fitting and residual analysis. The results show that constant-current excitation yields better linearity than constant-voltage excitation. Theoretically, constant-current excitation can provide about twice the performance of constant-voltage in this case.

TL431-based constant-current driver design

A constant-current driver using a TL431 reference is proposed. To facilitate connection to a subsequent amplifier stage, the current source is implemented as a source-type so the sensor can be grounded. In the schematic, R2–R5 represent the resistive bridge of the pressure sensor. Two PNP transistors, Q1 and Q2, form a current-mirror arrangement.

When the circuit is operating, the TL431 (U1) between its A and K terminals regulates the voltage to 2.5 V. This voltage is approximately equal to the voltage across R8, so Q2's emitter current Ie = 2.5/2 = 1.25 mA. Ie is approximately equal to the collector current Ic, therefore the sensor excitation current is about 1.25 mA.

Supply disturbance and stability

Simulation results indicate that even if the supply is unstable, with an 18 V DC output overlaid by approximately 2 Vrms of AC interference, the excitation current remains stable and the sensor output stays essentially unchanged.