Adding temperature measurement functionality to the energy collection design of wireless temperature sensors

　　Temperature monitoring plays an important role in a wide range of applications. For electronic systems, temperatures above or below specifications can affect the nominal performance of circuits and systems. Beyond these traditional thermal management applications, temperature measurement has shifted from an occasional system monitoring function to a core function for applications such as the Internet of Things (IoT). Here, wireless temperature sensors rely on energy harvesting technology to provide power for sensor data measurement and wireless transmission. For these low-power designs, engineers can find integrated sensor ICs from companies such as ADI, Maxim Integrated, Microchip Technology, and Texas Instruments.

　　For general temperature measurement applications, engineers can choose from various temperature sensors, including thermocouples, RTDs, thermistors, and IC sensors. Thermocouples are typically used for high-temperature sensing; RTD is suitable for lower temperature ranges; and thermistors are the preferred sensors for precise detection within a narrow temperature range. Each type can provide sufficiently accurate measurements for most applications, but engineers face a series of challenges in generating reliable and accurate temperature data.

　　Temperature measurement

　　For designers, implementing sensor applications requires building signal conditioning circuits to provide appropriate data for downstream applications. Typically, signal conditioning circuits need to include amplifiers, filters, comparators, voltage references, and ADCs in the signal path. Additionally, depending on the sensor type, designers need to address cold-temperature compensation, provide current or voltage excitation sources, and manage look-up tables for linearization (Figure 1).

　　Adding temperature measurement functionality to the energy harvesting design of wireless temperature sensors Figure 1: Using traditional temperature sensors in design, engineers need to meet sensor excitation and loading requirements and build a signal chain capable of converting nonlinear sensor values into precise temperature data (provided by Maxim Integrated). Although these devices are widely complex and can be used for complex system-level temperature monitoring operations, engineers can find more basic temperature sensor ICs. These devices are specifically designed for temperature measurement, simplifying design by combining on-chip temperature sensors with integrated signal conditioning circuits, eliminating the need for designers to solve key details of signal conditioning and data conversion in simple sensor applications. These integrated devices offer either analog or digital output, including all the signal processing functions needed to produce precise linear output over a wide temperature range. These devices typically reduce the overall power consumption requirements of sensors and typically provide ultra-low power modes required for wireless sensor designs using energy harvesting technology.

　　The Texas Instruments LM74 temperature sensor integrates a bandgap temperature sensor and a 12-bit ΔΣ ADC, with related control logic, registers, and an SPI-compatible three-wire serial interface (Figure 2). By default, the device powers on in continuous switching mode,

　　Consume 265μA (typical value) to add temperature measurement functionality to the energy harvesting design of wireless temperature sensors. Figure 2: IC temperature sensors simplify the design of temperature sensing applications by integrating sensors, regulation, and conversion circuits on the chip (provided by Texas Instruments). However, since temperature sensing applications require periodic sampling, engineers can set the LM74 to low-power power-off mode, with power consumption below 10μA (typical value for DSBGA packages at 3.3 V, and 8μA for SOIC packages at 5 V). In this mode, the serial interface remains active, and the device retains the latest temperature readings in its internal registers. Therefore, engineers can call up the LM74, complete temperature readings, and restore the device to shutdown mode. At any time, including during shutdown mode, individual MCUs can use serial interfaces to collect the latest temperature data.

　　Diverse configurations

　　Engineers can find a wide variety of IC temperature sensors that integrate complete signal chains with different zones (see Figure 1 again), as well as signal chains that provide additional functions. ADI's AD22100 provides a complete analog signal chain without additional analog circuits for fine-tuning, buffering, or linearization. With this type of device, engineers must provide separate conversion capabilities, often relying on MCUs with integrated ADCs.

　　AD22100 provides proportional output, with the output voltage proportional to the device's supply voltage temperature: when the device is powered by a single +5.0 V supply, it swings from 0.25 V at -50°C to +4.75 V at +150°C. Using ratio sensors simplifies ADC usage because the same power supply can serve as a reference for the ADC without the need for a separate, expensive precision voltage reference (Figure 3).

　　Adding temperature measurement functionality to the energy harvesting design of wireless temperature sensors Figure 3: ADI's AD22100 is the proportional temperature sensor IC, allowing the same +5 V power supply for both AD22100 and ADC reference voltages without the need for a separate precision voltage reference (provided by Analog Devices). Small changes in power supply voltage have little impact because both the AD22100 and ADC use the power supply as a reference. For typical energy harvesting applications based on integrated MCUs, engineers can similarly use MCU-integrated ADCs without needing precise voltage references, although a simple RC filter may be needed to provide immunity to high-speed spikes. MCU ADC input pin.

　　Similarly, the Microchip Technology MCP9700 series offers a simple solution for temperature measurement. Based on Microchip's linear active thermistor technology, the sensor IC series relies on the temperature dependence of internal diodes to generate temperature-dependent output voltage levels. The temperature coefficient of the internal diode causes the output voltage to be related to the relative ambient temperature between -40° and 150°C. For MCP9700, voltage changes within this temperature range can be adjusted to a temperature coefficient of 10.0 mV/°C (typical value). While highly complex thermal management ICs can be used, most offer functions for monitoring large systems, beyond the scope required by typical wireless sensor designs. However, even simple temperature sensing applications may carry the risk of operating with temperature deviations beyond the design limits. For these applications, designers can choose temperature sensor ICs such as the Texas Instruments (TI) LM75A, which provide thermal monitor functionality without the overhead of more complex thermal monitoring equipment.

　　Engineers can use devices like the LM75A to measure temperature, but sensitive circuits are disabled in case of overheating. Similarly, the Microchip Technology TCN75A not only allows designers to measure temperature, but also monitors alarm output signals triggered when the temperature exceeds a set threshold.

　　Temperature sensor ICs can significantly simplify the implementation of temperature measurement applications. On the other hand, they use on-chip temperature sensors, meaning that if the optimal thermal path passes through their pins, the device's measurements ultimately reflect the temperature of the PCB it is mounted on (or even the chip itself). Therefore, manufacturers usually recommend using components encapsulated in plastic, as plastic acts as a more effective thermal insulator between the sensor and the PCB. For further isolation, engineers can install sensor ICs in a sealed thermal conductive enclosure and place them in environments of interest.

　　For applications requiring complete isolation of thermal measurement, engineers can still find devices that integrate a complete signal chain but rely on external sensors. Maxim Integrated MAX6682 and MAX6674 use external thermistors and thermocouples to generate digital temperature data, respectively. Designers only need to connect the device's input to the appropriate temperature sensor and connect the device's SPI-compatible three-wire output to the MCU to achieve a complete temperature sensor (Figure 4).

　　Adding temperature measurement functionality to the energy harvesting design of wireless temperature sensors Figure 4: Applications that cannot use integrated temperature sensors can be turned to integrated ICs, such as Maxim Integrated MAX6682 and MAX6674, which integrate a complete signal chain but rely on external thermistors and thermocouples respectively (provided by Maxim Integrated). » Summary: Temperature sensor ICs provide a simple, low-power solution for basic temperature measurement applications. By integrating on-chip temperature sensors with the analog or even digital level of the complete signal chain, these devices can treat temperature measurements as voltage output or final digital values. With available integrated sensor ICs, engineers can easily add temperature measurement capabilities to low-power wireless sensor designs that use energy harvesting technology.