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In the scheme of sphygmomanometer, pulse is detected by blood pressure sensor, and the sphygmomanometer sensor is a pressure sensor based on silicon Piezoresistive effect. Inflatable cuffs are used to block the blood flow of the upper arm artery. Due to the hemodynamic effects of the heartbeat, pressure fluctuations synchronized with the heartbeat, known as pulse waves, will be superimposed on the cuff pressure. When the cuff pressure is much higher than the systolic pressure, the pulse wave disappears. As the pressure on the cuff decreases, a pulse begins to appear. When the cuff pressure drops from above systolic blood pressure to below systolic blood pressure, the pulse wave will suddenly increase. When the average pressure is reached, it then decays as the cuff pressure decreases. Oscillographic blood pressure measurement estimates blood pressure based on the relationship between pulse wave amplitude and cuff pressure. The average pressure corresponds to the maximum pulse wave, while systolic and diastolic blood pressure are determined by the ratio of the maximum pulse wave amplitude, respectively.
For the low-cost sensor scheme of electronic sphygmomanometer, many manufacturers choose constant current source driving mode to obtain temperature output characteristics without calibration. According to the industrial standard of electronic sphygmomanometer, within the temperature range of 5-45 ℃, the error value is ± 2 mmhg, which meets the application requirements.
The piezoresistive silicon pressure chip in the blood pressure sensor adopts advanced sensing technology, which brings smaller chip size, more stable performance, better consistency, and better linearity. The sphygmomanometer sensor generally needs to compensate the bias and sensitivity of the sensor itself through analog or digital methods. For sphygmomanometer manufacturers, after selecting a specific sensor, the sphygmomanometer scheme needs to convert the analog output of the bridge device into digital output.
The analog compensation of the sphygmomanometer scheme increases the constant resistance outside the bridge arm of the silicon pressure sensor to eliminate the bias and sensitivity of the circuit. After determining the circuit in this way, the selection surface of the sensor will be very narrow and can only be selected from products with fixed gears. Digital compensation does not require the selection of specific resistors or variable resistance components to adjust the zero bias and sensitivity of the sensor, thus eliminating the need for two electronic components. This scheme is compatible with sensors with zero bias and sensitivity within the corresponding range.