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Published in 2014-08-25 12:26:13       

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Design Considerations

CPAPor Continuous Positive Airway Pressure is a method of respiratory ventilation used mainly for the treatments of sleep apnea at home. During sleep, the muscles tend to naturally relax causing the upper airway to become narrow. This reduces the amount of oxygen in the blood and causes arousal from sleep.



Pressure sensors play an important role for respiration equipment by converting physical values such as airway pressure and flow into a differential signal. The air and flow sensors generate signals to help the microprocessor regulate the motor to adjust/maintain the desired pressure as the person inhales or exhales. Often, the sensors are very cost-effective with large offset and offset drift causing the signals to be off scale, temperature variant and non-linear. Amplifiers with low offset voltage and drift over time and temperature are ideal for signal conditioning.



The actual controlling of the DC Motor can be done by monitoring at least two of the three current phases and the DC bus voltage feeding the motor drive bridge. For the phase currents, two approaches can be used: high-side or low-side. Direct phase measurement or high side, requires high speed difference amplifiers or current shunt monitors and is generally more accurate. The low-side approach measures near the half bridge ground connection and uses more simple amplifiers which can be less costly but also less precise. The DC Motor is driven by discreteFETs. TI's DRV family offers an integrated driver and bridge with thermal protection that is smaller, more precise and much more efficient that is highly recommended.



The microprocessor performs multiple operations including sampling the pressure signals and computing a desired airway pressure and flow level to communicate with the motor. To achieve these operations efficiently and in real-time, a high-speed, low-power, highly-integrated microprocessor should be used. A high-quality DSP can be used for such applications and will also provide the patient with an ultra quiet operation.



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