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Employ SoCs for portable medical equipment

Posted: 07 Feb 2013 ?? ?Print Version ?Bookmark and Share

Keywords:Portable medical electronics? blood pressure monitor? Glucose Meters? Pulse Oximeters?

Portable medical electronics has seen remarkable growth and adoption in the recent years. More equipment variants are being introduced in the market by an increasing number of companies. The need of the hour is better mass producible designs which are low in complexity and provide acceptable performance so as to keep the cost of the device low. To achieve this, designers need to consider power efficiency, cost, form factor, and Food and Drug Administration (FDA) certification of components, among other factors.

A typical portable medical electronic system comprises components like analogue front-ends for data acquisition, amplifiers and filters for signal conditioning, analogue-to-digital converters (ADCs) for signal and sensor data acquisition, buttons to accept user feedback, an MCU to execute algorithms, and a variety of interfaces such as an LCD display, USB port and so on. Traditional design methodologies bring together all of the needed functionality onto a PCB by way of individual components. This method increases the overall system BOM, PCB complexity, and design cycle. Using individual analogue components also reduces analogue IP protection as the system can be reverse engineered easily.

Portable medical equipment design and manufacture is also regulated by the FDA. This means that their design and construction must follow precisely documented processes, and performance must meet stringent documentation, development testing, production testing, and field maintenance requirements. One FDA regulation requires that the components used in a medical device have to be guaranteed to be available in production for the next five years. This provides an incentive for developers to reduce the overall number of components used to make FDA certification simpler.

Figures 1 and 2show a typical blood pressure monitor (BPM) and a non-contact digital thermometer built using traditional approach.

Figure 1: Blood pressure monitor in traditional design approach.

Figure 2: Non contact digital thermometer in traditional design approach.

Traditional approach
A typical BPM uses a differential pressure sensor to measure cuff or arm pressure. As the output of this sensor lies within a few micro volts (30-50?V), the output pressure signal has to be amplified using a high-gain instrumentation amplifier with a good common mode rejection ratio (CMRR). Usually the gain and CMRR need to be around 150 and 100 dB respectively. The frequency of oscillatory pulses in the pressure signal lies between 0.3-11Hz with an amplitude of a few hundred microvolts. These oscillations are extracted using band-pass filters with gain around 200 and cut-off frequency at 0.3-11Hz. A 10bit ADC with a speed of 50Hz is used to digitise the pressure sensor and oscillatory signal. Two timers are used to calculate the heart rate and implement safety timer functionality. A safety timer regulates the pressure kept on a subject's arm for a certain period of time. This safety timer is a part safety regulation in AAMI standards. A microcontroller core calculates the systolic and diastolic pressures values using an oscillometric algorithm. The cuff is inflated and deflated using motors driven by PWMs.

A typical non-contact digital thermometer uses a transducer, also called a thermopile, consisting of a micro machine embedded membrane with thermocouples to measure thermocouple temperature and a thermistor to measure ambient temperature. The thermocouple generates a DC voltage corresponding to the temperature difference in its junctions. The output of the thermocouple is on the order of a few?V. The signal from the thermocouple is amplified using a low-noise precision amplifier. A voltage divider is constructed with the thermistor and external precision voltage reference. This voltage divider converts the change in thermistor resistance with respect to temperature to change in voltage. Voltages from the thermocouple and thermistor are used to calculate the thermocouple and ambient temperatures. The temperature is obtained from voltages using a polynomial function given by the sensor manufacturer or through a look-up table with pre-stored readings. The ambient temperature is added to the thermocouple temperature to get the final temperature measurement.

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