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Linearized Thermometer Circuit

2017-03-02 07:45  
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The circuit is linearized thermometer, the LM392 is used to provide gain and linearization for a platinum RTD in a single supply thermometer circuit which measures from 00C to 500oC with ±1oC accuracy. Q1 functions as a current source which is slaved to the LM103. The constant current driven platinum sensor yields a voltage drop which is proportionate to temperature. A1 amplifies this signal and provides the circuit output. Normally the slight nonlinear response of the RTD would limit accuracy to about ±3 degrees. C1 compensates for this error by generating a breakpoint change in A1′s gain for sensor outputs above 2500C. When the sensor’s output indicates 2500C, C1 “ ” input exceeds the potential at the “-” input and C1′s output goes high. Here is a schematic drawing:

Linearized Thermometer Circuit Linearized Thermometer Circuit

This turns on Q2 whose collector resistor shunts A1 6.19k? feedback value, causing a gain change which compensates for the sensor’s slight loss of gain from 2500C to 5000C. Current through the 220k resistor shifts the offset of A1 so no “hop” occurs at the circuit output when the breakpoint is activated. A precision decade box is used to calibrate this circuit. With the box inserted in place of the sensor, adjust 00C for 0.10V output for a value of 1000 Ohm. Next dial in 2846X (5000C) and adjust the gain trim for an output of 2.60V. Repeat these adjustments until both zero and full-scale are fixed at these points.

Source Circuit : National Semiconductor Application

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