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Simplifying sensor design for medical equipment

Simplifying sensor design for medical equipmentOf the four common types of temperature sensors – thermocouples, resistance temperature devices, thermistors and temperature sensor ICs – temperature sensor ICs are a good option for contact-based medical and healthcare designs. Principally, they do not require linearisation, they 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 are size, power consumption and thermal sensitivity. The last is important for clinical-grade accuracy because even transient power (µW) can heat the sensor and cause inaccurate readings. Another consideration is the type of interface (digital or analogue), which will determine associated components, such as the microcontroller.

Clinical-grade accuracy

Meeting clinical-grade accuracy, per ASTM E112 (standard test methods for determining average grain size), starts with 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. (Maxim Integrated was acquired by Analog Devices in August 2021.) The devices measure 2x2x0.75mm and are in a thin 10pin LGA package (Figure 1). The ICs operate from a supply voltage of 1.7V-3.6V and consume less than 67µA in operation and 0.5µA in standby.




It is important that the sensor’s own temperature does not influence the measurement reading of a wearable device. The sensor IC’s heat, which travels from the PCB through the package, leads to the sensor die and can affect accuracy. In a temperature sensor IC, this heat is conducted through a metal thermal pad on the underside of the package, resulting in parasitic heating. This can cause thermal conduction in and out of other pins, interfering with temperature measurements.

There are a number of techniques to counter parasitic heating. Thin traces can be used to minimise thermal conductivity away from the sensor IC. Designers can measure the temperature at the top of the package, as far away as possible from the IC pins, rather than use the thermal pad. This is the case for the MAX30208CLB+ and other MAX30208 digital temperature sensors.

Another option is to place other electronic components as far away from the sensing element as possible to minimise their impact on the temperature measurement.

Thermal design considerations

To ensure thermal isolation from heat sources in wearable devices there must be 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 PCB to route metal tracks from the point of contact with the body.

The system should be designed so that the sensor is as close as possible to the target temperature to be measured. Using MAX30208 sensors, wearable designs and medical patches can use flex or semi-rigid PCBs. The MAX30208 digital temperature sensors can be connected directly to a microcontroller using a flat flexible cable or flat printer cable.

It is essential to place the temperature sensor IC on the flex side of the PCB, which reduces the thermal resistance between the surface of the skin and the sensor. Designers should also minimise the thickness of the flex board for efficient flexing and better contact.

Digital temperature sensors are typically linked to microcontrollers via an I2C serial interface. Maxim’s MAX30208CLB+, for example, uses a 32-word first in first out to create a temperature sensor setup register offering up to 32 temperature readings, each comprising two bytes. This allows a microcontroller to sleep for extended periods to conserve power (Figure 2). The memory-mapped registers also allow sensors to offer high and low threshold digital temperature alarms.

A general purpose input/output pin can be configured to trigger a temperature conversion and another configured to generate an interrupt for selectable status bits.

Factory calibration

Many digital temperature sensors are factory-calibrated, eliminating the need for recalibration once a year, as is the case for many legacy temperature sensors. This bypasses the need to develop software to linearise the output, as well as simulate and fine-tune the circuit. Additionally, it eliminates the need for multiple precision components and minimises the risk of impedance mismatches.

The AS621x family of temperature sensors from ams is factory calibrated and has integrated linearisation (Figure 3). It also has eight I2C addresses for temperature monitoring at eight potential hot spots via a single bus. The serial interface and multiple I2C addresses make prototyping and design verification easier.

Versions accurate to ±0.2°C, ±0.4°C and ±0.8°C are available. For health-related monitoring systems, accuracy within ±0.2°C is sufficient (the AS6212-AWLT-L). All AS621x devices have 16bit resolution to detect small variations in temperature over the -40°C to +125°C operating range.

The AS621x measures 1.5mm2 and comes in a wafer level chip scale package. Supply voltage is 1.71V with 6µA consumption during operation and 0.1µA in standby, making the AS6212-AWLT-L particularly suited to battery-powered applications.

Contactless temperature sensors

Infrared thermometers perform non-contact temperature measurements of 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 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.

An example is the MLX90614ESF-BCH-000-TU infrared thermometer from Melexis. It comprises an infrared thermopile detector chip and a signal conditioning chip integrated into a TO-39 package (Figure 4). There is also a low noise amplifier, 17-bit analogue to digital converter and digital signal processor for accuracy and resolution.

The infrared thermometers are factory calibrated for a temperature range of -40°C to 85°C (ambient) and -70°C to 382.2°C for object temperature. Standard accuracy is 0.5°C at room temperature.

The sensor is factory calibrated with a digital SMBus output and has a resolution of 0.02°C. Alternatively, designers can configure the 10bit pulse width modulation (PWM) digital output with a resolution of 0.14°C.

Development support

The MAX30208 sensors are supported by the MAX30208EVSYS# evaluation system, which includes a flex PCB to hold the MAX30208 temperature sensor IC (Figure 5). The MAX32630FTHR microcontroller board and the MAX30208 interface board are connected via headers. The evaluation hardware can be connected to a PC using the provided USB cable. The system will then automatically install the necessary device drivers ready for the EV kit software to be downloaded.

For measuring body temperature at multiple locations, MAX30208 temperature ICs can be connected via I2C addresses in a daisy-chain arrangement to a single battery and host microcontroller. Each temperature sensor is polled by the microcontroller regularly to create a profile of both local and whole-body temperature.

Developers can use the Mikroe-1362 IrThermo Click board from MikroElektronika for use with the MLX90614 infrared sensor. 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).

The 5V board is calibrated for -40°C to 85°C ambient temperature and -70°C to +380°C object temperature ranges.

About The Author

Bill Walsh is product manager, Digi-Key Electronics