Quickly Integrate Clinical-Grade Temperature Sensing into Portable, Wearable Medical Designs
This article discusses the basic types of temperature sensors before focusing on digital IC sensors and the core features while also showing how these devices can meet the needs of next-generation systems.
In the wake of global concern over COVID-19, designers of portable and wearable devices for temperature sensing are being challenged to decrease device size, cost, and power consumption, even as they must improve accuracy, sensitivity and reliability. To help meet the challenge, sensors are improving not only in performance but also in overall ease of use to simplify the design and integration process.
This article will discuss the basic types of temperature sensors before focusing on digital IC sensors and the core features for which designers should be on the lookout. It will introduce digital temperature sensor examples from ams and Maxim Integrated, as well as an infrared thermometer from Melexis Technologies NV as an example of non-contact temperature sensing. It will also show how these devices can meet the needs of next-generation systems, and describe related evaluation boards and probe kits and how they can be used to help designers get started.
Temperature Sensor Choices
Of the four common types of temperature sensors designers can choose for temperature sensing—thermocouples, resistance temperature devices (RTDs), thermistors, and temperature sensor ICs—temperature sensor ICs are a good option for contact-based medical and healthcare designs. This is mainly because they don’t require linearization, offer good noise immunity, and are relatively easy to integrate into portable and wearable healthcare devices. For contactless sensing, infrared thermometers can be used.
Key parameters for designers to consider, especially for wearable applications—whether it’s a wrist-worn device, embedded in clothing, or a sticky medical patch—include size, power consumption, and thermal sensitivity. Sensitivity is important because when designing for clinical-grade accuracy, even transient power on the order of microwatts (µW) can heat the sensor and cause inaccurate readings. Another consideration includes the type of interface (digital or analog) as this will determine the requirements of associated components, such as the microcontroller.
How to Achieve Clinical-grade Accuracy
Meeting clinical-grade accuracy, per ASTM E112, starts with the selection of the appropriate sensor. Maxim Integrated’s MAX30208 digital temperature sensors, for example, feature ±0.1°C accuracy from +30°C to +50°C and ±0.15°C accuracy from 0°C to +70°C. The devices measure 2 x 2 x 0.75 millimeters (mm) and come in a thin 10-pin LGA package (Figure 1). The ICs operate off a supply voltage ranging from 1.7 to 3.6 volts and consume less than 67 microamps (µA) in operation and 0.5 µA in standby.
Figure 1. The MAX30208 digital temperature sensors offer clinical-grade measurement accuracy of ±0.1°C for battery-powered devices such as smartwatches and medical patches. (Image source: Maxim Integrated)
As mentioned, a critical challenge when designing to clinical-grade accuracy is to ensure that the sensor’s own temperature doesn’t influence the measurement reading of a wearable device.
The sensor IC’s heat, which travels from the pc board through the package leads to the sensor die, can affect the accuracy of temperature readings. In a temperature sensor IC, this heat is conducted through a metal thermal pad located on the underside of the package, resulting in parasitic heating. This, in turn, can cause thermal conduction in and out of other pins. Inevitably, this interferes with temperature measurements.
To counter parasitic heating, designers can employ a number of techniques, starting with the use of thin traces to minimize thermal conductivity away from the sensor IC. Also, instead of using the thermal pad on the underside of the package, designers can measure the temperature at the top of the package, as far away as possible from the IC pins. In the case of the MAX30208CLB+ and other MAX30208 digital temperature sensors, the temperature measurement is taken at the top of the package.
Another mitigation technique is to place other electronic components—which can contribute heat to the temperature monitoring system—as far away from the sensing element as possible to minimize their impact on the temperature measurement data.
System-to-user Thermal Design Considerations
While ensuring thermal isolation from heat sources, designers must also guarantee a good thermal path between the temperature sensing element and the skin of the user. The location underneath the package makes it challenging for the pc board to route metal tracks from the point of contact with the body.
So, first and foremost, the system should be designed such that the sensor is as close as possible to the target temperature to be measured. Second, as enabled by the MAX30208 sensors, wearable designs and medical patches can use flex or semi-rigid pc boards. The MAX30208 digital temperature sensors can be connected directly to a microcontroller using a flat flexible cable (FFC) or flat printer cable (FPC).
When using these cables, it’s essential to place the temperature sensor IC on the flex side of the pc board, which reduces the thermal resistance between the surface of the skin and the sensor. Also, designers should minimize the thickness of the flex board as much as possible; a thinner board can flex more efficiently and enable better contact.
Digital temperature sensors are typically linked to microcontrollers via an I2C serial interface. Such is the case with Maxim’s MAX30208CLB+, which also uses a FIFO for temperature data, allowing a microcontroller to sleep for extended periods to conserve power.
Figure 2. The MAX30208 digital temperature sensors are targeted at medical thermometers and wearable body temperature monitors. (Image source: Maxim Integrated)
The MAX30208CLB+ digital temperature sensor uses a 32-word FIFO to create a temperature sensor setup register offering up to 32 temperature readings, each comprising two bytes. These memory-mapped registers also allow sensors to offer high and low threshold digital temperature alarms.
There are also two general purpose I/O (GPIO) pins: GPIO1 can be configured to trigger a temperature conversion, while GPIO0 can be configured to generate an interrupt for selectable status bits.
Factory Calibrated Temperature Sensors
Many digital temperature sensors are now factory calibrated, eliminating the need to 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.
Figure 3. The AS621x sensors provide a complete digital temperature system with factory calibration. (Image source: ams)
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.
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.
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.
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, 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 from MikroElektronika. 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).
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)
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.
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.