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Temperature sensor resistance conversion to binary code using pulse width modulation

The presented researches are related to measurement technologies and can be used to design intelligent resistive sensors. To convert the resistance of resistive sensors into binary code in microprocessorbased systems, bridge circuits are used that interface with analog-to-digital converters (ADC). The bridge circuit of a resistive sensor is the main circuit solution for cases where the task is to ensure high measurement accuracy. However, the problem of eliminating the nonlinearity of the conversion characteristic remains and needs to be solved. As a rule, nonlinearity is eliminated programmatically using conversion tables or selection of an analytical approximating function, which is not the best option. In this paper, the linearization problem of the conversion characteristic is solved by automatically balancing the bridge circuit using the pulse-width modulation method. In some cases, it is necessary to use amplifiers to interface bridge circuits with ADC. These elements introduce additional errors in the conversion result, as well as complicate the hardware. A variant of the circuit solution without ADC and amplifier is proposed. The paper discusses a microcontroller measuring converter (MMC), which allows obtaining a linear dependence of the binary code on the resistance of the temperature sensor. The equations for calculating software-realizable parameters are given, which provide the required resistance resolution. The MMC resolution is 1 Ω, when the sensor resistance changes from 500 to 1500 Ω at a measuring current of 1.23 mA. In this case, MMC conversion time – 4 ms. Experimental studies were performed on the basis of the Atmel’s 8-bit AVR microcontroller. The study results can be applied to design intelligent sensors using other resistive sensors, such as strain gages.