Design technology and solutions for blood pressure monitors

Introduction

A blood pressure monitor, or sphygmomanometer, uses an inflatable cuff and a listening device to measure blood pressure by listening; it can also use a pressure sensor to measure blood pressure. This monitor can be implemented in two ways: manually inflate the cuff and use a stethoscope to listen to the sound of the arterial wall (auscultation); or embed a pressure sensor inside the sphygmomanometer to measure the vibration of the arterial wall (oscillometric method).

Upper arm sphygmomanometer

Wrist Blood Pressure Monitor

Automatic electronic blood pressure monitor

There are two types of automatic blood pressure monitors: upper arm blood pressure monitors and wrist blood pressure monitors. Upper arm blood pressure monitors contain an air bag that is wrapped around the upper arm during measurement. The inflatable cuff is connected to a monitor placed near the arm by a leather tube. Wrist blood pressure monitors are smaller and the entire device can be wrapped around the wrist, so the size requirements for this type of product are more stringent. Some upper arm blood pressure monitors require manual inflation, but most upper arm and wrist blood pressure monitors on the market today are fully automatic.

Measurement technology

Automatic electronic blood pressure monitors first inflate the cuff to sufficient pressure to block the flow of blood in the local aorta. As the pressure is gradually released, when blood begins to flow through the artery, the pressure value measured is the systolic pressure of the heart. The pulse rate can also be monitored. When blood flow is restored, the value measured at this time is equivalent to the diastolic pressure of the heart. This completes a complete measurement cycle, which is completed by the pump, inflatable cuff, electronic valve and pressure sensor in the device.

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The signal from the pressure sensor is conditioned by an operational amplifier or instrumentation amplifier and fed into an analog-to-digital converter (ADC). Systolic pressure, diastolic pressure, and pulse rate are then calculated in the digital domain using appropriate methods for different types of monitors and sensors, and the results are displayed on a liquid crystal display (LCD), and the data and corresponding time/date are usually stored in nonvolatile memory.

Data Interface

Some blood pressure monitors can upload data to a computer for more in-depth analysis and post-measurement tracking. Data can be transferred via a USB interface. You can choose a discrete USB transceiver or a microcontroller with such a transceiver built in.

Audio Prompts

The blood pressure monitor's audio prompts can be simple beeps or advanced voice prompts. Simple buzzers can be driven using one or two pulse-width modulation (PWM) output port pins of the microcontroller; for advanced voice prompt function circuits, audio digital-to-analog converters (DACs) and speaker amplifiers can be selected to process the audio signal. Power Management Upper arm blood

pressure

monitors are usually powered by 4 AAA (1.5V) alkaline batteries, while wrist blood pressure monitors are mostly powered by 2 AAA alkaline batteries. The blood pressure monitor pump and analog circuits require 5V or 3.3V power supply, and the digital circuits choose 3.3V or 1.8V power supply depending on the specific measurement technology used. Therefore, a typical upper arm blood pressure monitor requires a buck-boost switching power supply to provide 5V power for the pump/analog circuits, and a low-dropout linear regulator (LDO) to provide 3.3V power for the digital circuits. A typical wrist blood pressure monitor uses a boost switching power supply to provide the 3.3V power required for the pump/analog circuits, and uses an LDO to convert the input voltage to 1.8V to power the digital circuits.

To extend battery life, the blood pressure monitor can be powered off when not in use, while only the real-time clock (RTC) is maintained and the blood pressure monitor can be quickly restored to work.

Display and backlighting

Most blood pressure monitors use simple segment LCD displays with 100 segments or less, and the LCD driver is usually integrated into the microcontroller. One or two white light LEDs (WLEDs) or electroluminescent (EL) lamps are used to provide display backlighting. Wrist blood pressure monitors can use a switching power supply topology to provide WLED driving, while upper arm blood pressure monitors can use a linear regulator to provide WLED driving.

Electrostatic Discharge

All blood pressure monitors must pass the electrostatic discharge (ESD) test requirements of IEC 61000-4-2. They can use devices with built-in ESD protection or use ESD protection lines to protect the circuits exposed at the interface.