Friday, February 23, 2024

Low Power Blood Pressure Meter Reference Design

By Akanksha Sondhi Gaur

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Extreme power efficiency, integrated sensors, and smart algorithms ensure accurate and comfortable readings. 

A sphygmomanometer, commonly known as a blood pressure meter, is a medical instrument to gauge an individual’s blood pressure. This device consists of an inflatable cuff designed to temporarily restrict blood flow and a pump responsible for inflating the cuff. In the case of digital blood pressure meters, they employ an oscillometric detection method and rely on a piezoelectric pressure sensor to accurately measure both systolic and diastolic pressures. A digital blood pressure meter allows easy monitoring of blood pressure at home, in a doctor’s office or in a hospital. Today’s blood pressure meters must meet the demands of measuring, storing and transmitting blood pressure, pulse and other medical data. 

Microchip’s Low-Power Blood Pressure Meter showcases the PIC24FJ128GC010 MCU, renowned for its versatility as a system-on-a-chip. With features like eXtreme Low Power (XLP) technology, it excels in power efficiency, consuming a mere 18 nA in Deep Sleep mode and operating the Watchdog Timer (WDT) at just 240 nA in the same state. It maintains a power consumption of 180 µA/MHz in active mode, ensuring efficient performance. The MCU has a 16-bit delta-sigma Analog-to-Digital Converter (ADC) with two differential channels and a 12-bit 10 Msps pipeline ADC supporting 50 channels. It also boasts two 10-bit 1 Msps Digital-to-Analog Converters (DACs), two operational amplifiers, three comparators, three voltage references, and a Charge Time Measurement Unit (CTMU). Additionally, it offers USB functionality with support for device/host/OTG and active clock tuning, eliminating the need for an external crystal.

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The blood pressure meter efficiently captures systolic and diastolic pressures while measuring the heart rate, attributing its cost-effectiveness to high integration levels that reduce the overall Bill of Materials (BOM) cost. Operating efficiently through eXtreme Low Power (XLP) mode, it prioritises patient comfort with the Measure While Inflating (MWI) algorithm to minimise discomfort during measurements. The device leverages the MCU’s integrated Pulse-Width Modulation (PWM) module to control the air pump’s motor, facilitating fast and slow speed modes. The integrated LCD controller generates necessary data and timing controls for driving static or multiplexed LCD panels. An incorporated high-pass filter using one of the MCU’s built-in operational amplifiers ensures accuracy. At the same time, the MCP1802 Low Dropout Regulator (LDO) efficiently converts the primary 5V or 6V input voltage to a stable 3.3V VDD supply.

Energizer Application Support rigorously tested the Low-Power Blood Pressure Meter Demo, estimating a 700-use battery life with standard AAA alkaline batteries. Comparable tests on five non-microchip blood pressure meters yielded battery life ranging from 75 to over 500 uses. Energizer’s Ultimate Lithium AAA batteries boosted the demo’s usage to nearly 1,200 uses. Microchip has thoroughly tested this reference design, which includes a design guide, block diagram, notes and more. Please visit the company’s website for further information on this reference design. To explore this design in greater detail, click here.

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