Calibrating a measurement system including bridge circuit

In a measurement system of the type having a sensor the electrical resistance of which varies as a function of a parameter being measured, and a bridge circuit connected to said sensor, wherein a source of a reference voltage is provided to excite said bridge to produce a bridge output indicative of the parameter being measured when said bridge is balanced, and output circuitry is provided for converting said bridge output into a measurement frequency in accordance with an output function, that improvement comprising

• first calibration circuitry for generating a first calibration signal to calibrate said system at one point in a range of values of said parameter,
• second calibration circuitry for generating a second calibration signal to calibrate said system at a second point in said range,
• said calibration circuitries comprising phase locked loop circuitry to phase lock said measurement frequency to reference frequencies respectively corresponding to said range points,
• each said calibration circuitry having a sample and hold circuit including a storage capacitor, and having a sampling mode in which said capacitor is charged to a level corresponding to said calibration signal, and a hold mode in which said charged capacitor provides said calibration signal,
• sequencing circuitry for automatically causing one said sample and hold circuit to enter its sampling mode, then causing the other sample and hold circuit to enter its sampling mode while maintaining the first sample and hold circuit in its hold mode, and then maintaining both said circuits in their hold modes during a measurement cycle of said system, and
• a control for activating said sequencing circuitry.