Oscillometric Blood Pressure Measurement
An automated oscillometric blood pressure device offers blood pressure measurement in an office or home setting. Cuff pressure is lowered below the systolic pressure and as the blood starts to flow through the artery, the walls of the artery begin to vibrate. In turn, these vibrations give rise to small vibrations or fluctuations in the cuff pressure. The onsets of the pressure fluctuations indicate the systolic blood pressure. Diastolic pressure measurement involves a complex, typically proprietary, algorithm that identifies a change in the slope of the amplitude of the pressure fluctuations. Oscillometric blood pressure devices must be capable of running on a single 3 V battery supply and must offer enough memory and processing power to run the blood pressure algorithms. Because these devices are distributed to the consumer market, low cost is key.
Analog Devices offers several high performance ICs that address the demands of these blood pressure monitoring devices.
The AD627 is an integrated, micropower instrumentation amplifier that delivers rail-to-rail output swing. It provides excellent ac and dc specifications while operating at only 85 µA maximum.
The ADuC7022 is a 12-bit analog I/O, ARM7TDMI microcontroller that offers low operating power and is capable of running on 3 V. It supports both memory-intensive and processing-intensive algorithms.
The AD8617 micropower CMOS R-R I/O operational amplifier features low offset voltage as well as low input voltage and current noise. It is capable of running off of a single rail. In a blood pressure measurement design, it can function as a 4-pole band-pass filter for arterial vibrations.
The ADuM2402 is a 4-channel digital isolator that provides superior performance characteristics to alternatives such as optocoupler devices. By avoiding the use of LEDs and photodiodes, iCoupler® devices remove the design difficulties commonly associated with optocouplers, including such challenges as uncertain current transfer ratios, nonlinear transfer functions, and temperature and lifetime effects. iCoupler products offer simple digital interfaces and stable performance characteristics. The need for external drivers and other discretes is eliminated. Furthermore, iCoupler devices run at one-tenth to one-sixth the power of optocouplers at comparable signal data rates.