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Embedded High Performance Analog Simplifies Portable Medical Design

Fri, 08/04/2006 - 7:17am
As more portable electronic medical devices are introduced to monitor and treat patients, manufacturers are seeking technologies with which to reduce the complexity involved in the design of the finished product. This article will explore the use of a low-power MCU combined with an array of high performance peripherals to achieve this simplified result.

By Murugavel Raju
AT A GLANCE
  • Simplifying design
  • Electronic components
  • MCU example
  • Real-world applications
Today, medical electronics OEMs are developing more sophisticated personal healthcare solutions for treating and monitoring common illnesses. These products now greatly improve the quality of healthcare at an affordable cost. Microcontrollers (MCU) play a significant role in a variety of portable medical instrumentation products such as personal blood pressure monitors, spirometers, pulsoximeters, and heart rate monitors. In most of these products, the actual physiological signals are analog and need signal conditioning techniques, such as amplification and filtering, before they can be measured, monitored, or displayed.


Figure 1: A functional block diagram of a blood pressure monitor (click here to enlarge)
Embedding high performance analog peripherals to an ultra-low-power MCU offers a system-on-chip ideal for portable medical electronics and complements long battery life. This article will discuss ways to simplify analog front end design in portable battery operated medical electronics by using embedded high performance peripherals such as opamps, analog-to-digital (A/D) converters, and digital-to-analog (D/A) converters together with a low-power MCU. The MCU offers digital filtering, processing, and displays parametric results of physiological data such as blood pressure, lung capacity, heart rate, and blood oxygen. These features can all be added while meeting demanding power consumption requirements by turning off peripherals for a standby current in the fractions of micro amperes.

One example of a MCU that meets these needs is the MSP430FG4619 MCU from Texas Instruments. The 16-bit RISC CPU delivers the required signal processing and its ultra-low power operation allows years of battery life in such applications. Integrated in this device are peripherals such as operational amplifiers, 12-bit multi-channel A/D converters and dual 12-bit D/A converters, which are integral to the complete analog performance. In addition to the high performance analog, the device also features a 120 KB of Flash memory on-chip and a modern universal serial communication interface (USCI). Following are more details on how these integrated analog peripherals help achieve a single-chip solution for medical products.
Blood Pressure Monitors
Figure 1 shows a functional block diagram of a blood pressure monitor. In this application, a bridge type pressure transducer is typically used as a sensor attached to an inflatable cuff. The transducer is activated via port pins and saves power since it is only activated during pressure measurement. The output from the sensor is proportional to the pressure and is in the microvolt range. This signal needs to be amplified before it can be digitized for measurement by the A/D converter. The amplified signal detects the Korotkoff sounds and determines the systolic and diastolic pressure readings.


Figure 2: The differential amplifier setup using three amplifiers
The three-operational amplifiers in the MCU work well for this purpose. The amplifiers combine to form a high-gain differential instrumentation amplifier block. This setup eliminates the common mode “noise” in this application. Figure 2 shows the differential amplifier setup using three amplifiers. The amplified signal is input internally to the 12-bit A/D converter. The DMA peripheral in the device allows efficient data handling that result in fast algorithms for Korotkoff sound detection and also filters the noise affecting the measurement. The 16-bit CPU provides the required processing power to handle these algorithms with fewer millions of instructions per second.

This device also integrates a 160-segment LCD driver with regulated charge pump for stable contrast that completes the solution using this single chip. The large 120 KB program memory in this MCU is low power Flash-based allowing field software upgradeability. The large 120KB size also serves as a data logger due to in-system programmability of the Flash memory. The USCI serial port in the device allows communication to a PC or PDA to download the logged data. Because of the MCU ultralow power architecture, this solution can operate with less than 3 mA in the blood pressure measurement mode. In the idle mode, this device can run and display a real-time clock with less than 3 microamperes current consumption.

The cuff is inflated/deflated using a DC motor controlled by one of the pulse width modulator (PWM) outputs from the MCU. This is the only part of the application that requires an operating 6V supply used to drive the motor. If not for this power need, the whole application could be run from a single 3 V lithium coin cell. Unfortunately, not many motors available today can operate from this slightly higher impedance coin cell. That said, the application described here uses four commonly available, low-cost AAA alkaline batteries for 6 V and a low dropout operator regulates it to 3.3 V to power the MCU. Considering a blood pressure monitor that is used twice a day, these batteries last up to a couple of years. The MCU can remain in time-keeping mode with an active display because of the low current consumption in this mode. Additionally, the user can browse through stored blood pressure readings without any impact to this low current.

In another variation, the integrated dual-channel D/A converter can be used to generate sine waves with 180° phase shift to excite the transducer to improve performance.
Spirometers
Spirometers, also referred to as pulmonary function testing equipment, are used in medical diagnostics for testing lung capacity. In this application, the measured parameter is air flow over time during exhalation in liters/min. The sensor used for this application is typically a pneumotach transducer which is essentially a differential pressure transducer. This application design is similar to the blood pressure monitor except that an inflation motor is not required. Again, the three MCU operational amplifiers are used as sensor instrumentation amplifiers to measure the flow. The rest of the application is straight-forward, measuring the flow using the 12-bit A/D converter and comparing the measured values against stored standardized values. The Flash memory is useful for storing of a variety of standardized values making the design suitable for use for a variety of situations. Figure 1 can be used as a reference for this application because of the similarity of the transducer that is used in the system. Keep in mind, the motor control section is not required. Again, the low power operation of the MCU offers long battery life and high-integration reduces cost with increased system reliability.
Pulsoximeters and Heart Rate Monitors
There are several different heart rate monitoring and pulsoximetering techniques. This section will focus on non-invasive optical plethysmography. These types of oximeters consist of a peripheral probe combined with the MCU displaying the oxygen saturation and pulse rate. The same sensor is used for both heart-rate detection and pulsoximetering in this application. This technology provides an easy, accurate, and non-invasive way to estimate arterial blood oxygen saturation and heart rate levels. The probe is placed on a peripheral point of the body such as a finger tip, ear lobe, or the nose. The probe includes two light emitting diodes (LEDs): one in the visible red spectrum (660 nm) and the other in the infrared spectrum (940 nm). Figure 3 features this probe.


Figure 3. The light beams from two light emitting diodes pass through the tissues of a finger tip to a photo detector to provide an easy, accurate, and non-invasive way to estimate arterial blood oxygen saturation and heart rate levels.
The light beams pass through the tissues to a photo detector. During passage through the tissues, the light is partially absorbed by haemoglobin in the red blood cells in differing amounts depending on the oxygen saturation level. First, by measuring the absorption at the two wavelengths, the MCU can precisely compute the proportion of haemoglobin which is oxygenated. Second, the light signal following transmission through the tissues has a pulse component resulting from the changing volume of arterial blood with each heart beat.

The two LEDs must be driven with constant current sources to guarantee a stable brightness condition during measurement. The constant current source with automatic gain control feedback can be derived using the internal D/A converter and a simple algorithm running in the MCU. The MCU can select out the absorbance of the pulsatile fraction of blood (i.e., that due to arterial blood) from non-pulsatile venous or capillary blood and other tissue pigments’ constant absorbance. Recent measurement techniques have reduced the interference effects on oxygen saturation calculation. Time division multiplexing, where the LEDs are cycled—red on, then infrared on, then both off—many times per second helps to eliminate background noise. Quadrature division multiplexing is a further advance where the red and infrared signals are separated in phase rather than time and then recombined in phase later. In this way, artifacts from motion or electromagnetic interference may be eliminated since it will not be in the same phase of the two LED signals once they are recombined.

Saturation values are averaged out over 5 to 20 seconds. Depending on the particular monitor, the pulse rate is also calculated from the number of LED cycles between successive pulsatile signals and averaged out over a similar variable period of time.

From the proportions of light absorbed at each frequency, the MCU calculates the ratio of the two parameters. Stored within the MCU’s Flash memory is a series of oxygen saturation values obtained from experiments where volunteers were given increasingly hypoxic mixtures of gases to breath. The MCU compares the absorption ratio at the two light wavelengths measured with these stored values, and then digitally displays the oxygen saturation as a percentage. Typically, the values in the 70% to 100% range are accurate. Below 70%, the data is extrapolated because it is not possible to have data from humans below this oxygenation level.


Figure 4. The MSP430FG461x-based pulsoximeter block diagram (click here to enlarge)
Figure 4 is the MSP430FG461x-based pulsoximeter block diagram. The integrated opamps, and A/D and D/A converters offer a complete analog front-end solution for this application. The D/A converter combined with the on-chip reference help to generate a constant current source for the LEDs. One of the opamps is used as an I to V converter for the sensor photodiode. Automatic Gain Control is provided by adjusting the LED brightness using the D/A converter output and a software algorithm executed by the MCU. The amplified and filtered output is digitized by the A/D converter and averaged out by the software. This data for both the red and infrared sources and their ratio is collected and calculated. This ratio is compared against the stored standard data and the oxygen saturation is accurately determined. The computed oxygen value is displayed on the LCD as a percentage. The A/D conversion values also carry the pulsating heart beat information which is averaged by software for about five seconds and the heart rate is computed. This is displayed on the LCD as well. Additionally, the PWM output of the MCU drives a piezo beeper briefly for every heart beat. The periodic beep serves as an indicator for proper sensor positioning and signal pick-up.
Conclusion
In the above portable medical applications, an ultra-low power microcontroller like the MSP430FG461x offers advantages as a true single-chip solution. The accuracy of the A/D converter easily satisfies the requirements for these measurements. The on-chip opamp and D/A converters greatly help in signal conditioning and automatic gain control. Now that the challenge of choosing the right MCU for these applications is addressed, the next step for designers of these systems is software development. Thanks to the on-chip emulation in this MCU, real-time debug via the JTAG port is a reality. Several compilers and debuggers are available today and the debugger hardware is very inexpensive. The debugger hardware required is a simple logic level shifter connected to a PC parallel port. Conventional ICE is no longer required. The full feature real-time emulation allows break-points to be set in hardware inside the chip and provides full-speed operation while debugging. The device itself offers a great value for the money in system design because of its high integration and ease of code development. The Flash program memory allows instant code refreshing during debug and the time to development is greatly reduced enabling quick time-to-market for the designers using this MCU in their applications. The large 120 KB size also serves as a data logger due to in-system programmability of the Flash memory.
References
Franco, Sergio. Design with Operational Amplifiers and Analog Integrated Circuits. New York: WCB McGraw-Hill, 1998.

Ed. J G Webster. Design of Pulse Oximeters. Institute of Physics Publishing.
ONLINE
For additional information on the technologies and products discussed in this article, see Medical Design Technology online at www.mdtmag.com or Texas Instruments Inc. at www.ti.com.

Murugavel Raju is an applications manager for Texas Instruments Inc., 12500 TI Blvd., Dallas, TX 75243. He was selected as the worldwide winner of the first Texas Instruments Analog Challenge in 1999. He can be reached at m-raju1@ti.com.
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