Designing Multiple Output Power Supplies With Topswitch

Description: This application note describes a digital blood pressure meter concept that utilizes integrated pressure sensor analog signal-conditioning circuitry, microcontroller hardware/software, and a liquid crystal display. The sensing system measures cuff pressure (CP) and extracts pulses for analysis to determine systolic and diastolic pressure. This design employs a 50 kPa integrated pressure sensor (Freescale Semiconductor, Inc. P/N: MPXV5050GP), which provides a pressure range from 0 mm Hg to 300 mm Hg. This method is commonly used by most automated non-invasive devices. A limb and its vasculature are compressed by an inflatable compression cuff. Blood pressure readings for systolic and diastolic values are taken at the parameter identification point. The simplified measurement principle of the oscillometric method involves measuring the amplitude of pressure changes in the cuff as it is inflated beyond the systolic pressure. The amplitude increases significantly as the pulse breaks through the occlusion, closely approximating the systolic pressure. As cuff pressure decreases further, the pulsation amplitude rises to a maximum and then diminishes rapidly. The diastolic pressure index is identified at the onset of this rapid transition. Consequently, systolic blood pressure (SBP) and diastolic blood pressure (DBP) are determined by identifying the regions of rapid increase and decrease in pulse amplitude, respectively. Mean arterial pressure (MAP) is found at the point of maximum oscillation. The cuff pressure is sensed by Freescale's integrated pressure transducer. The sensor's output is divided into two paths for different purposes: one path measures cuff pressure, while the other is processed further by a circuit. Since the MPXV5050GP is signal-conditioned by its internal operational amplifier, the cuff pressure can be directly interfaced with an analog-to-digital (A/D) converter for digitization. The second path filters and amplifies the raw CP signal to extract an amplified version of the CP oscillations, which occur as the subject's arm expands during cardiac systole. The sensor output consists of two signals: the oscillation signal (approximately 1 Hz) superimposed on the CP signal (approximately 0.04 Hz). A 2-pole high-pass filter is designed to block the CP signal before amplifying the oscillation signal. If the CP signal is not adequately attenuated, the baseline of the oscillation will be inconsistent, and the amplitude of each oscillation will lack a uniform reference for comparison.

The digital blood pressure meter circuit comprises several key components for effective operation. The Freescale MPXV5050GP pressure sensor serves as the primary sensing element, converting the mechanical pressure exerted by the cuff into an electrical signal. The output of this sensor is processed through a dual-path configuration. The first path directly interfaces with an A/D converter, allowing for digital representation of the cuff pressure, which can be displayed on the liquid crystal display for user readability.

The second path is critical for extracting the oscillometric signal. This signal represents the pulsatile changes in pressure within the cuff as blood flows through the compressed artery. The design includes a high-pass filter to eliminate low-frequency components of the cuff pressure, ensuring that only the oscillatory signals are amplified. The filter is tuned to allow frequencies around 1 Hz to pass while attenuating the lower frequency component of approximately 0.04 Hz, which corresponds to the static cuff pressure.

Following filtering, the oscillation signal is amplified to a suitable level for further processing. This amplified signal is then analyzed by the microcontroller, which employs algorithms to detect the peak amplitudes corresponding to the systolic and diastolic pressures. The microcontroller processes the data, calculates the SBP, DBP, and MAP, and formats this information for display.

The entire system is powered by a suitable power supply, ensuring stable operation. The microcontroller can also include features such as data logging, user interface options, and connectivity for external devices, enhancing the functionality of the blood pressure meter. Overall, this design exemplifies a modern approach to non-invasive blood pressure measurement, integrating advanced sensor technology with digital processing capabilities for improved accuracy and user experience.This application note describes a Digital Blood Pressure Meter concept which uses an integrated Pressure Sensor Analog signal-conditioning circuitry, Microcontroller hardware/software and a liquid crystal Display The sensing system reads the cuff pressure (CP) and extracts the pulses for analysis and determination of systolic and diastolic pressur e. This design uses a 50 kPa integrated Pressure Sensor (Freescale Semiconductor, Inc. P/N: MPXV5050GP yielding a pressure range of 0 mm Hg to 300 mm Hg. This method is employed by the majority of automated non- invasive devices. A limb and its vasculature are compressed by an encircling, inflatable compression cuff. The blood pressure reading for systolic and diastolic blood pressure values are read at the parameter identification point. The simplified measurement principle of the oscillometric method is a measurement of the amplitude of pressure change in the cuff as the cuff is inflated from above the systolic pressure.

The amplitude suddenly grows larger as the pulse breaks through the occlusion. This is very close to systolic pressure. As the cuff pressure is further reduced, the pulsation increase in amplitude, reaches a maximum and then diminishes rapidly. The index of diastolic pressure is taken where this rapid transition begins. Therefore, the systolic blood pressure (SBP) and diastolic blood pressure (DBP) are obtained by identifying the region where there is a rapid increase then decrease in the amplitude of the pulses respectively.

Mean arterial pressure (MAP) is located at the point of maximum oscillation. The cuff pressure is sensed by Freescale`s integrated pressure X-ducerG. The output of the Sensor is split into two paths for two different purposes. One is used as the cuff pressure while the other is further processed by a circuit. Since MPXV5050GP is signal-conditioned by its internal op- amp, the cuff pressure CAN be directly interfaced with an Analog-to-digital (A/D) converter for digitization. The other path will Filter and amplify the raw CP signal to extract an amplified version of the CP oscillations, which are caused by the expansion of the subject`s ARM each time pressure in the ARM increases during cardiac systole.

The output of the Sensor consists of two signals; the oscillation signal ( ‰ 1 Hz) riding on the CP signal ( ‰¤ 0. 04 Hz). Hence, a 2-pole high pass Filter is designed to block the CP signal before the amplification of the oscillation signal.

If the CP signal is not properly attenuated, the baseline of the oscillation will not be constant and the amplitude of each oscillation will not have the same reference for comparison. Figure 1 shows the oscillation signal Amplifier together with the Filter