Medical Electronics Catch here

Figure 3: The AS621x sensors provide a complete digital temperature system with factory calibration. (Image source: ams)

be calibrated in the field or recalibrated once a year, as is the case for many legacy temperature sensors. Moreover, factory calibration bypasses the need to develop software to linearize the output, as well as simulate and fine-tune the circuit. It also eliminates the need for a multitude of precision components and minimizes the risk of impedance mismatches. For example, the AS621x family of temperature sensors from ams is factory calibrated and comes with integrated linearization (Figure 3). It also has eight I2C addresses to allow designers to monitor temperature at eight different potential hot spots using a single bus.

The serial interface with eight I2C addresses also makes prototyping and design verification easier for health-related monitoring system developers. To help match the sensors to their specific application requirements, the AS621x sensors are available in three accuracy versions: ±0.2°C, ±0.4°C, and ±0.8°C. For health-related monitoring systems, accuracy within ±0.2°C is sufficient, making the AS6212-AWLT-L a suitable option. All AS621x devices have 16-bit resolution to detect small variations in temperature over their full operating temperature range of -40°C to +125°C. The AS621x measures 1.5 mm2 and comes in a wafer-level chip-scale package (WLCSP) to make it easier to integrate into a healthcare device. It operates off a supply voltage of 1.71 volts and consumes 6 µA during operation and 0.1 µA in standby mode. The tiny footprint and low power consumption make temperature sensors such as the AS6212-AWLT-L particularly suited to battery-powered mobile and wearable device applications.

Contactless temperature sensors Unlike temperature sensor ICs which require some physical contact, infrared thermometers perform non-contact temperature measurements. These contactless sensors measure two parameters: ambient temperature and the temperature of an object.

Such thermometers detect any energy above 0 Kelvin (absolute zero) emitted by an object in front of the device. The detector then converts the energy into an electrical signal and passes it to a processor to interpret and display the data after compensating for variations caused by ambient temperature. For example, the MLX90614ESF-BCH-000- TU infrared thermometer from Melexis comprises an infrared thermopile detector chip and a signal conditioning chip integrated into a TO-39 package (Figure 4). A low noise amplifier, 17-bit analog-to-digital converter (ADC), and digital signal processor (DSP) integrated into the MLX90614 family ensure high accuracy and resolution.

Development with temperature sensors

The MAX30208 line of sensors is supported by Maxim Integrated’s MAX30208EVSYS# evaluation system, which includes a flex pc board to hold the MAX30208 temperature sensor IC (Figure 5). The evaluation system comprises two boards: the MAX32630FTHR microcontroller board and the MAX30208 interface board, which are connected via headers. Designers only need to connect the evaluation hardware to a PC using the provided USB cable. The system will then automatically install the necessary device drivers. Once those are installed, the EV Kit Software needs to be downloaded.

multiple MAX30208 temperature ICs can be connected via I2C addresses in a daisy-chain arrangement to a single battery and host microcontroller. Here, each temperature sensor is polled by the microcontroller regularly to create a profile of both local and whole-body temperature. For the MLX90614 infrared sensor, medical device developers can get started with the compact MIKROE-1362 IrThermo Click board fromMikroElektronika. This links the MLX90614ESF-AAA single- zone infrared thermometer module to the microcontroller board via either the mikroBUS I2C line or PWM line (Figure 6). MikroElektronika’s 5 volt board is calibrated for a temperature range of -40°C to 85°C for ambient temperature and -70°C to +380°C for object temperature.


Figure 4: The MLX90614 infrared thermometer has a standard accuracy of 0.5°C at room

temperature. (Image source: Melexis)

The MLX90614 infrared thermometers are factory calibrated for a temperature range of -40°C to 85°C for ambient temperature, and -70°C to 382.2°C for object temperature. They feature a standard accuracy of 0.5°C at room temperature. These contactless temperature sensors provide two modes of output: pulse width modulation (PWM) and SMBus via a two-wire interface (TWI) or I2C link. The sensor comes factory calibrated with a digital SMBus output and can serve the entire temperature range with a resolution of 0.02°C. On the other hand, designers can configure the 10-bit PWM digital output with a resolution of 0.14°C.

Figure 5: Designers can connect the evaluation hardware to a PC with the provided USB cable. The necessary device drivers are automatically installed. (Image source: Maxim Integrated)

It’s also worth mentioning here that a mobile or wearable device can measure body temperature at multiple locations. For example, in a sports garment,

Designers are being challenged to make clinical-level temperature sensing more available to the mass market, despite challenges such as power, size, cost, reliability, and accuracy. Contact and contactless sensors, supported by evaluation kits, are now available to help them meet this demand, quickly and efficiently. As shown, these sensors not only come with the performance characteristics required for clinical temperature measurement, they also come with the factory calibration and digital interfaces needed to make them easier to integrate into next-generation designs.

Figure 6: The MIKROE-1362 IrThermo Click board can be used to get started on development with Maxim Integrated’s MLX9016 sensor. (Image source: MikroElektronika)

Components in Electronics October 2020 13

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