Design Ideas: March 17, 1994

A programmable reference diode, such as the Motorola TL431A, can supply constant-current bias for a silicon pressure-sensor bridge (Fig 1). This circuit is simpler than using an op amp and separate reference diode or than using a current diode, which requires temperature compensation.

The TL431A produces a V_REF of 2.5V over a current range of 1 to 100 mA. The value of V_REF/R₂ sets the necessary bias current for the bridge sensor, as specified by the sensor manufacturer. The reference diode current, set by R₁ and the supply voltage V_S, usually equals the bridge current. As an example, for a supply of 12V, a reference diode and bridge current of 1 mA, and a bridge impedance of 5 k Ohm, R₁ should equal 2250V, and R₂ should equal 2500V. The bridge output has a common-mode voltage equal to V_REF plus one-half times the voltage across the bridge.