Medical Electronics

Quickly integrate clinical-grade temperature sensing into portable, wearable medical

By Majeed Ahmad, Digi-Key International I

n 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

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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 2: The MAX30208 digital temperature sensors are targeted at medical thermometers and wearable body temperature monitors. (Image source: Maxim Integrated)

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.

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.

Components in Electronics

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.

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

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