The silicon piezoresistive - bridge pressure transducer (SPPT) is a dominant technology in automotive, industrial, medical, and environmental pressure sensor applications. All SPPTs share a similar architecture in which a thin (5 to 200μm) micro machined silicon diaphragm incorporates an implanted piezoresistive Wheatstone-bridge strain-gauge. Applied pressure bends the diaphragm, imbalances the strain gauge, and thereby produces a differential output signal proportional to the product of pressure time bridge excitation voltage. SPPTs must be supported by appropriate signal conditioning and calibration circuits. Finite elasticity limits the SPPT diaphragm to relatively small deflections which generate only ±1% modulation of the bridge resistance elements and low signal output levels, creating the need for high gain, low-noise, temperatures DC amplification.
The detailed circuit of Figure below, the A2 circuit provides for the precision adjustment, via DCP1, of any transducer initial null offset error. To accomplish this, the bridge excitation voltage is programmable attenuated by the R2, R3, R4, R5 network and applied to DCP1. The range for the zero adjustment voltage is from +22mV to –22mV. The resolution is 172μV and proportional to the bridge excitation voltage, thus improving the temperature stability of the zero adjustment. Boosting the ~10mV/psi bridge signal by 100x to a convenient 1V/psi output level is the job of the A3 non inverting amplifier via its feedback and calibration network consisting of R7 through R9 and DCP2. The gain of A3 can be varied from 75 to 125 with a resolution of 0.2. Bridge bias is provided by the A1 circuit which uses voltage reference D1 and current sense resistor R1 to generate a constant-current bridge drive of I = 1.225V / 2kΩ = 612μA.
The Figure;
The net result of the combination of transducer and the Figure circuitry is a signal conditioned precision pressure sensor that is compatible (thanks to DCP1 and 2) with full automation of the calibration process, is very low in total power draw (<>
Source; www.intersil.com
