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METROLOGICAL ANALYSIS OF A DIGITAL TEMPERATURE DIFFERENCE METER USING THERMAL RESISTANCE CONVERTERS

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Abstract. A digital temperature difference meter is proposed, in which platinum resistance thermal converters are used. The meter contains a measuring circuit, an analog-to-digital converter (ADC), and a digital indicator. An operational amplifier is introduced into the measuring circuit to create a countercurrent in one of the resistance thermal converters included in the feedback circuit of the operational amplifier. The thermal converters are connected via a four-wire communication line. The line wire resistances do not affect the measurement result and therefore do not need to be adjusted. A push-pull integration ADC is applied. The differential measuring inputs of the ADC receive voltage drop signals from corresponding thermal converters. One of the reference inputs of the ADC receives a voltage drop signal across the reference resistor, in which the measuring current flows through the thermal converter. This makes the output code of the ADC to be independent of the measuring current. The metrological analysis of transformation of the thermal converter resistance difference into the digital code is performed, the revealed sources of errors are the zero-bias voltage of the operational amplifier, inaccuracy and instability of the reference resistor. It is established that the considered scheme of the measuring circuit is suitable for use in narrow-limit temperature difference meters. In this case, the communication lines of thermal converters can have any length and do not need the connecting wires resistances to be adjusted.

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