System-level design trade-offs for truly wearable wireless medical devices

Power and current management in emerging wearable medical devices, intended to continuously monitor physiological signals, are crucial design issues. The overall size of the electronic part of these systems is generally going to be dominated by the size of the batteries. Unfortunately, the options of smaller batteries do not only come at the expense of a lower capacity and hence shorter operation time. It also significantly constrains the amount of available current that can be used by different electronic blocks, as well as their operating power supply voltage. This paper discusses all the typical power and current management system level issues in the design of a typical miniature wearable wireless medical device. The discussion is illustrated with experimental results obtained with two devices built using two of the currently most popular low power commercial transceivers in the market, the Texas Instruments (TI) CC2500 and the Nordic Semiconductor nRF24L01+. The numbers presented can be used as a more realistic guidance of the energy per bit required in a real system implementation, as opposed to the ideal figures normally quoted by the manufacturers. Furthermore the analysis in this paper can also be extrapolated to the design of future wireless monitoring wearable devices with further optimized radio transceivers.