Optimize battery life for battery powered medical devices and wireless medical devices: Page 3 of 5

October 21, 2013 //By Mike Hawes
Optimize battery life for battery powered medical devices and wireless medical devices
Mike Hawes explains how the medical device industry is seeing an explosion in the number and type of mobile and wireless medical devices, and is ripe for a new approach to analyzing the energy requirements of your battery powered medical devices (BPMDs).
current spikes up to 3A, and measurement of sleep currents of tens of microamps with resolution of tens to hundreds of nanoamps, within the same digitization pass (Figure 3a). This dynamic range is obtained by seamless ranging between three different SMU current measurement ranges. The traditional AUTORANGE function found in DMMs involves switching in different front-end attenuation for different ranges, which can glitch the measurement system. This obviously distorts the real power consumption by the BPMD. Seamless ranging does not glitch the measurement. Additionally, most AUTORANGE functions aren’t fast enough to detect quick pulses in power and therefore may overload, or completely miss the pulse event. If only one range is used (e.g., 1V range with 1Ω shunt resistor for the blip blood pressure monitor) to capture the peak current and measure sleep current, the measured value of the sleep current is imperceptible in the A/D noise floor (~30µA) for this range (Figure 3b). Unlike the traditional method, this new approach provides accurate measurement of dynamic power transients (turning on actuator, RF transmission), enabling clearer understanding of the impact on battery life.

Figure 3a  Integrated solution using seamless ranging (Average = 53.7µA, Peak-Peak = 5.5µA)

Figure 3b  Traditional method using multiple instruments and Excel (Average = 40.3µA, Peak-Peak = 87µA)

Figure 3  Sleep mode current measurement

For a different analysis perspective on the power consumption of your BPMD, a complementary cumulative distribution function (CCDF) can be used. The CCDF is very useful to determine how much current was drawn during a specific percentage of one’s datalog record (Figure 4). The x-axis is a log scale of current, and the y-axis is a log scale of percent-of-time during the datalog record.

So, if the CCDF line near 690mA moves horizontally, this would suggest changes in the peak current value. Likewise, if the CCDF line near 30% moved vertically, this would indicate changes in the percentage of time the blip was

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