Choosing the appropriate temperature sensor for a specific application depends on the temperature range to be measured and the required accuracy. System accuracy depends on the accuracy of the temperature sensor and the performance of the analog-to-digital converter (ADC) that digitizes the sensor's output. In most cases, a high-resolution ADC is required because the sensor signal is very weak. ΣΔ ADCs offer high resolution and typically include built-in circuitry required by the temperature measurement system, such as an excitation current source. This article primarily introduces the available temperature sensors [thermocouples, resistance temperature detectors (RTDs), thermistors, and thermistor diodes], the circuitry required to connect the sensors to the ADC, and the performance requirements of the ADC. Thermocouples Thermocouples consist of two different types of metals. When the temperature is above zero degrees Celsius, a thermoelectric voltage is generated at the junction of the two metals, the magnitude of which depends on the temperature deviation from zero degrees Celsius. Thermocouples are small in size and have a wide operating temperature range, making them ideal for measuring extremely high temperatures (up to 2300°C) in harsh environments. However, the output of a thermocouple is in the mV range, therefore requiring precise amplification for further processing. Different types of thermocouples have varying sensitivities, typically only a few mV per degree Celsius, thus requiring a high-resolution, low-noise ADC. Figure 1 shows a thermocouple system utilizing a 3-channel, 16/24-bit AD7792/AD7793 ΣΔ ADC. Its on-chip instrumentation amplifier first amplifies the thermocouple voltage, then the ADC performs analog-to-digital conversion on the amplified voltage signal. The voltage generated by the thermocouple is biased near ground. An on-chip excitation voltage source biases it within the amplifier's linear range, allowing the system to operate with a single power supply. This low-noise, low-drift, on-chip bandgap reference voltage source ensures the accuracy of the analog-to-digital conversion, thereby guaranteeing the accuracy of the entire temperature measurement system. Resistance Temperature Detector The resistance of a resistance temperature detector changes with temperature. Common materials for resistance temperature detectors are nickel, copper, and platinum, with platinum resistance temperature detectors (RTDs) having a resistance between 100Ω and 1000Ω being the most common. Resistance temperature detectors are suitable for temperature measurements with near-linear response over the entire temperature range of -200℃ to +800℃. A resistance temperature detector (RTD) consists of 3 or 4 leads. Thermistors The resistance of a thermistor also changes with temperature, but its accuracy is not as high as that of an RTD. Thermistors typically use a single current supply. Similar to RTDs, a precision resistor is used as a reference voltage source, and a current source drives both the reference resistor and the thermistor, meaning a ratio configuration can be achieved. This also explains why the accuracy of the current source is not critical, as temperature drift from the current source affects both the thermistor and the reference resistor, thus offsetting the drift effect. In thermocouple applications, thermistors are typically used for cold junction compensation. The nominal resistance of a thermistor is typically 1000Ω or higher. Thermistors Temperature can also be measured using thermistors: the temperature is calculated by measuring the voltage across the diode (typically a transistor's base to emitter). Two different currents are passed through the thermistor, and the voltage from base to emitter is measured in both cases. Knowing the current ratio allows for accurate temperature calculation by measuring the difference in base-emitter voltage under two different current conditions. For example, we set the excitation current source of the AD7792/AD7793 to 10mA and 210mA (other values ​​can also be chosen). First, we pass a 210mA excitation current through the diode and measure the base-emitter voltage using the ADC. Then, we repeat the measurement using a 10mA excitation current. This means the current is reduced to 1/21 of its original value. The absolute value of the current is not important in the measurement, but the current ratio must be constant. Temperature measurement systems are typically low-speed (maximum 100 samples per second), so narrow-band ADCs are suitable; however, the ADC must have high resolution. The requirements of narrow band and high resolution make a ΣΔ ADC ideal for this application. In this configuration, the analog input is continuously sampled at a frequency significantly higher than the useful bandwidth. The ΣΔ regulator converts the sampled input signal into a digital pulse train, where the density of "1"s includes digital information. The ΣΔ regulator also performs noise shaping. Noise shaping shifts noise within the useful bandwidth to the unwanted frequency range. Higher-order regulators have a more pronounced noise-shaping effect within the useful bandwidth. However, higher-order regulators are more prone to instability. Therefore, a trade-off must be made between regulator order and stability. In narrowband ΣΔ ADCs, second- or third-order regulators are typically used, offering good device stability. A digital filter following the regulator samples the regulator output, providing a valid data conversion result. This filter also removes out-of-band noise. The digital filter image frequency appears at multiples of the master clock frequency; therefore, using a ΣΔ structure means the only external component required is a simple RC filter to eliminate the digital filter image frequency at multiples of the master clock frequency. The ΣΔ structure enables a 24-bit ADC to have a peak-to-peak resolution of 20.5 bytes (stable or flicker-free bytes). Gain Typically, signals from temperature sensors are very weak. For small temperature changes of a few degrees, the corresponding analog voltage changes produced by temperature sensors such as thermocouples and resistance temperature detectors are at most a few hundred mV. Therefore, the typical full-scale analog output voltage is only in the mV range. Without a gain stage circuit, the full-scale range of an ADC is typically ±VREF. To optimize ADC performance, most of its analog input range should be used. Gain is exceptionally important when using this type of sensor to measure temperature. Without any gain, only a small portion of the ADC's full-scale range is utilized, resulting in a loss of resolution. Instrumentation amplifiers allow for the development of low-noise, low-temperature-drift gain stage circuits. Low noise and low temperature drift are critical, ensuring that voltage changes due to temperature variations are greater than the noise voltage of the instrumentation amplifier. The AD7793's gain can be set to 1, 2, 4, 8, 16, 32, 64, or 128. Using the maximum gain setting of 128 and the resulting reference voltage source, the AD7793's full-scale range is ±1.17 mV/128 mV, or approximately ±10 mV. This ensures that the ADC's high-resolution characteristics can achieve optimal results without any external amplifier components. The ADC's built-in digital filter is highly effective at suppressing out-of-band quantization noise and other noise sources, particularly at 50Hz/60Hz frequencies . One source of noise is the frequency generated by the power grid supply system. When the power grid supplies power to devices, it generates power grid frequencies of 50 Hz and its harmonics (in Europe), or 60 Hz and its harmonics (in the United States). Narrowband ADCs primarily use sinc filters. The AD7793 has four filter options, and the ADC can automatically select the required filter type based on the update rate. A sinc3 filter is used at an update rate of 16.6 Hz. As shown in Figure 2, the sinc3 filter has grooves in the spectrum. When the output word rate is 16.6 Hz, these grooves can be used to suppress frequencies of 50 Hz or 60 Hz simultaneously. Chopper Systems always have adverse factors such as offset voltage and other low-frequency errors, and temperature measurement systems are no exception. A chopper is an inherent feature of the AD7793 and can be used to eliminate these error signals. The chopper works by alternately inverting (or clipping) the phase at the ADC's input multiplexer. Then, an analog-to-digital conversion is performed on each chopped phase (positive and negative phase). Next, the results of the two conversions are averaged using a digital filter. This eliminates any offset errors within the ADC and, more importantly, minimizes the effect of temperature on offset drift. Low Power Consumption Many temperature sensing systems are not electrically powered. In some industrial applications, such as temperature monitoring in factories, the entire temperature system, including the sensor, ADC, and microcontroller, is housed on a separate circuit board, using a 4 mA to 20 mA loop power supply. Therefore, the maximum current budget for this separate circuit board is 4 mA. In applications like portable systems that use battery power, low power consumption is important, but high performance is also crucial.