The MAX30208 is a digital temperature sensor with medical-grade accuracy, from Maxim Integrated. It features an ultra-low power consumption and a compact package size. The thermal sensor itself is located on a top of the die, allowing better contact with the measured surface. Its compact size eliminates the thermal capacity almost completely, especially if mounted on a thin, flexible PCB. Thanks to these features, the MAX30208 is a perfect solution for wearable devices, medical thermometers, IoT sensors, and similar applications with space constraints and power consumption limitations.
However, body temperature measurement is not an easy task, especially when developing a wearable health monitor. Wearables are typically made in the form of a wrist band, but measuring the wrist temperature is not a preferred option, due to exposure of the extremities. This is clearly illustrated by a typical heat signature of a human body, below (Figure 1).
Figure 1. An IR scan of the human body
An ideal solution would be to place several sensors across the chest region. This way, the measurement would be much more accurate. Multiple sensors could be arranged in a grid pattern and placed inside the close-fitting garment (Figure 2). This would allow an accurate multi-point temperature measurement while still providing comfort and freedom of movement.
Figure 2. Garment incorporating multiple MAX300208 sensors
The MAX30208 IC uses the I2C interface for communication. Its slave address can be reconfigured by using two GPIO pins, allowing more than a single device on the same bus. This enables simplified implementation of the multi-point grid arrangement, especially when limited resources are available (Figure 3).
Figure 3. Multi-point temperature sensing with up to 4 I2C addresses
GPIO pins can also be configured to initiate temperature conversion from an external source, and to generate an interrupt signal for a number of different events, including a digital temperature alarm with programmable low and high thresholds. The 32-sample FIFO buffer can be used to reduce data traffic on the communication bus. The output is calibrated in °C so no additional conversion is required. These features can be utilized to reduce the processing workload, lowering the over all system power consumption and extending battery life.