Using SoCs for portable medical equipment

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 (30-50µV), 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 cutoff frequency at 0.3-11Hz. A 10-bit ADC with a speed of 50 Hz is used to digitize 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 µV. 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.

A segment LCD driver, RTC, push buttons, EEPROM and USB are the other peripherals needed in both of the above applications.

The components which are external to microcontroller like the transducer, ADC, LCD driver/controller, USB controller, filter, and amplifiers are the peripheral components. These components interface to the microcontroller through either a GPIO or a dedicated pin. The more external components there are, the more limitations and constraints developers have to account for, such as managing the bill of materials, higher PCB complexity, achieving FDA certification for each and every component, increased design/development time, and reduced analog IP protection.