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High-Accuracy Thermometer

2017-08-17 12:02  
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This circuit combines a current source and a platinum RTD bridge to form a complete high-accuracy thermometer. Th
High-Accuracy Thermometer

e ground-referred RTD sits in a bridge that is composed of the current drive and the LT1009 biased resistor string. The current drive allows the voltage across the RTD to vary directly with its temperature-induced resistance shift. The difference between this potential and that of the opposing bridge leg forms the bridge output. The RTD`s constant drive forces the voltage across it to vary with its resistance, which has a nearly linear positive temperature coefficient. The nonlinearity could cause several degrees of error over the circuit`s 0?°C -400?°C operating range. The bridge`s output is fed to instrumentation amplifier A3, which provides differential gain, while simultaneously supplying nonlinearity correction. The correction is implemented by feeding a portion of A3`s output back to Al`s input via the 10- to 250-KOhmhm divider. This causes the current supplied to RP to slightly shift with its operating point, compensating sensor nonlinearity to within ?±0.05?°C. A1B, providing additional scaled gain, furnishes the circuit output. To calibrate this circuit, follow the procedure given in the diagram.

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