Temperature Measurement

TEMPERATURE MEASUREMENT USING WiWIZ1

 

 

INTRODUCTION

The measurement of temperature is one of the most ubiquitous parameters for measurement and control purposes.  This application is a brief introduction to temperature measurement and how the Wi-WIZ1 and the uSM (wireless micro sensor modules) is an optimum solution to this wireless application.

Description

There are several types of sensors that can be used to measure temperature. The appropriate sensor for the application depends on the temperature range being measured and the required accuracy. The system accuracy depends on the accuracy of the temperature sensor and the performance of the ADC used to digitize the sensor output. In many cases as the magnitude of the signal from the sensor is quite small, a high-resolution Analog to Digital Converter (ADC) is required.  Sigma-delta (?-?) ADCs are suitable for these systems as they are high-resolution devices, and because they often include additional on-chip circuitry, such as excitation currents, that is required in temperature measurement systems. This application note describes the available temperature sensors (thermocouple, RTD, thermistor, and thermal diodes) and the circuitry needed to interface a sensor to an ADC. It also explains the performance required of the ADC.

Thermocouple

A thermocouple consists of two different types of metal. A voltage is generated at the junction of the two metals when the temperature is above zero; its magnitude depends on the deviation of the temperature from zero. Thermocouples are small, rugged, and relatively inexpensive, and they operate over a wide temperature range. They are especially useful for making temperature measurements at extremely high temperatures (up to 2300°C) in hostile environments. However, they produce only millivolts of output, and therefore require precision amplification for further processing. Sensitivity varies for the different types of thermocouples, but it is typically only a few microvolts per degree Celsius, so a high resolution, low noise ADC is required for precision temperature readings. When a thermocouple is connected to the copper tracks on a PCB board, another thermocouple junction occurs at the point where the thermocouple connects to the copper. This results in a voltage being generated which opposes the thermocouple voltage. To compensate for this opposing voltage, another temperature sensor is placed at the thermocouple-copper junction to measure the temperature at this junction. This is known as the cold junction.

RTD

The resistance of RTDs varies with temperature. Typical elements used for RTDs are nickel, copper, and platinum, with 100 ? and 1000 ? platinum RTDs being the most common. RTDs are useful for measuring temperatures from –200°C to +800°C with a near linear response over the complete temperature range. An RTD can consist of three wires or four wires. Figure 2 shows how a 3-wire RTD is connected to the ADC. RL1, RL2, and RL3 are the resistances of the RTD’s leads.

Thermistor

The  resistance of thermistors also varies with temperature, but they are less accurate than RTDs.  canceling its effect. Thermistors are normally used for cold-junction compensation in thermocouple applications. Their nominal resistance is normally 1000 ? or higher.

System Architecture

ADCs

The measurements in temperature systems are typically low speed (up to 100 samples per seconds), so a low bandwidth ADC is suitable. However, the ADC must have high resolution. Their low bandwidth and high resolution make sigma-delta converters an optimum choice.  The WiWiz1 has two(2) Sigma-Delta converters with 24 bits of resolution.  One is a 3-channel unit and the other is a 6 channel device.

Gain

Signals from temperature sensors are, in general, quite small, and the change of a few degrees in temperature cause the corresponding analog voltage generated by temperature sensors such as thermocouples and RTDs to change by a few hundred microvolts at most. This results in a typical full-scale analog output voltage that is only in the millivolt range. The full-scale range of the ADC is normally the voltages that power the device if a gain stage is not present. To optimize the performance of the ADC, most of its analog input range should be used. This highlights the importance of gain when measuring temperature using these sensors. Without any gain, only a small fraction of the full-scale range of the ADC is used, resulting in the loss of resolution.

CONCLUSION

The ADC and system requirements for a temperature measurement system are quite stringent. The components required for each type of temperature sensor differ, but the analog signals generated by these sensors are always quite small. Thus, they need to be amplified by a gain stage whose noise is low so that the amplifiers noise does not swamp the signal from the sensor. Following the amplifier, a high resolution ADC is required so that the low level signal from the sensor can be converted into digital information. ADCs that use a ??? architecture are suitable for such applications because high resolution, high precision ADCs can be developed using these topologies. Along with the ADC and gain stage, a temperature system requires other components such as excitation currents or voltage references. Again, these must be low drift, low noise components so that the system accuracy is not degraded. Initial inaccuracies such as offset can be calibrated out of the system, but the drift of the components with temperature must be low to avoid error introduction. In power battery powered  wireless temperature measurement one of key ingredients for successful operation is very low power consumption.  The Wi-WIZ1 and the uSMs use the lowest power consumption components that are available. 

Temperature Measurement with the WiWIZ1

The WiWIZ1 contains two sigma-delta 24 bit high resolution ADCs, both of which are capable of doing accurate temperature measurement as described above.  The temperature sensor is read by the application microprocessor  and post processing including sample averaging and filtering, if required, can be accomplished locally at the source of the measurement.

uSIM wireless sensor modules

A uSIM module will be available with the same high accuracy 24 bit ADC and Atmel 8 bit RISC microprocessor that is used on the WiWIZ1 wireless board.  All software written for the WiWIZ1  can be transported to the uSIM module using the identical free software development environment.  The WiWIZ1 provides a smooth transition to the tiny uSIM module.

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