The Low-Power Blood Pressure Meter design uses a microcontroller with Low Power technology, suitable for battery-operated medical devices.
A blood pressure meter, known as a sphygmomanometer, is an essential medical tool for monitoring blood pressure. It typically includes an inflatable cuff to constrict blood flow and a pump for inflating the cuff. Modern digital versions of these meters employ the oscillometric method for measuring blood pressure, which utilizes a piezoelectric pressure sensor to gauge both systolic and diastolic pressures accurately. The Low-Power Blood Pressure Meter reference design was developed by Microchip using the PIC24FJ128GC010 system-on-a-chip (MCU). This microcontroller unit is equipped with eXtreme Low Power (XLP) technology, optimizing it for energy efficiency. Notably, it consumes only 18 nA in Deep Sleep mode and 240 nA when operating the Watchdog Timer (WDT) in the same mode. Its Run mode power consumption is 180 µA/MHz, making it a nice choice for battery-operated medical devices.
The technical capabilities of the MCU are impressive, encompassing a wide range of features to support complex medical applications. It includes a 16-bit delta-sigma Analog-to-Digital Converter (ADC) with two differential channels, a 12-bit pipeline ADC capable of 10 Msps across 50 channels, and two 10-bit Digital-to-Analog Converters (DACs) that operate at 1 Msps. Additionally, the MCU is equipped with two operational amplifiers, three comparators, three voltage references, a Charge Time Measurement Unit (CTMU), and USB support for device, host, and OTG configurations with active clock tuning that eliminates the need for a crystal. A segmented LCD driver with a charge pump enhances the device’s display capabilities, rounding out its comprehensive feature set.
The device’s primary power source is either four 1.5V AAA alkaline batteries or the USB’s VBus line. The MCP1802 Low Dropout (LDO) regulator then steps down the 5V or 6V main input voltage to a 3.3V VDD output. The firmware includes a 2-pass moving average filter with a cutoff frequency of 3.5 Hz, which processes the raw ADC samples collected from each ADC channel.
The design can measure both blood pressure and pulse rate while the cuff is inflating. This Measurement While Inflating (MWI) technique shortens the overall measurement duration and minimizes the discomfort associated with prolonged cuff pressure.
In operation, the meter’s motor initially rapidly inflates the cuff to 30 mmHg. Following this, the motor shifts to a slow mode, gradually and linearly inflating the cuff. As the cuff pressure increases to a level that begins to restrict blood flow, the arterial pulse becomes detectable by the pressure sensor, allowing for accurate readings.
Microchip has tested this reference design. It comes with a bill of materials (BOM), schematics, assembly drawing, printed circuit board (PCB) layout, and more. The company’s website has additional data about the reference design. To read more about this reference design, click here.
