By Jonathan Bearfield, Texas Instruments
Whether the monitor is a single or multi-parameter device; targeted capability, power consumption and system versatility are often key requirements. Nowadays, a monitor can move with the patient from the operating room to an intensive care unit, to the hospital room, and even into their home (see Figure 1). This is paramount in today’s world of health care.
The most important features in today’s patient monitors are mobility, ease of use, and effortless patient data transfer. Mobility includes portability as well as the ability to interface with other medical devices such as anesthesia machines or defibrillators. Ease of use can be achieved with touch screen displays and multilevel menu driven profiles that can be configured for the environment as well as the patient’s vital statistics. Data transfer across everything from wireless to RS232 needs to be possible. Hospitals may support a specific infrastructure throughout all areas; however, ambulance, home and other environments may need support for different protocols.
Leveraging a single monitoring system for all of this drives the need for configurable systems. Take apart any portable medical monitoring device and most likely you will find the following five basic building blocks:
Clearly, the specific performance of each block varies from system to system — depending on the specific needs of the patient and environment in which the system is used.
Displays and Touch Screen ControlIn terms of “ease of use,” profile configurability and sheer ability to present data in meaningful ways, advances in display technologies, and touch screen controls have positively impacted patient monitor development. Monitors that display several parameters (i.e., heart rate, systolic, diastolic and arterial pressures, SpO2, ECG readings, etc.) with flexible menu driven screens allow the same monitor to be used in a variety of settings.
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Figure 2. High-level block diagram for medical monitoring device.
Aside from system usability, a technical challenge when working with TSCs is electrostatic discharge (ESD). If the TSC circuit can not dissipate the energy from an ESD strike, the energy can pass through to, and can damage, the central microcontroller/DSP. Other considerations are resolution, conversion type and speed, and overall power consumption.
Another important requirement for displays is visibility under all lighting conditions, which is partially driven by having the right backlighting solution. Whether leveraging inductive or charge-pump-based topologies, backlight solutions with a wide input voltage range reduce the system’s power regulation requirements. Selecting boost or buck/boost solutions that can operate easily and efficiently from multiple battery chemistries provides greater flexibility in the design.
System Power ManagementFor mobile or portable systems, making power decisions at the front end of the design cycle helps to define system-level tradeoffs that may be necessary to meet the portability and run-time targets. Simple systems may use disposable batteries, whereas larger systems leverage various rechargeable battery chemistries and pack sizes. Features such as dynamic power path management available through the bq2403x permits the system to draw power independent of the battery charging path, allowing a device with completely discharged batteries to be used as soon as it is plugged in, versus waiting for the batteries to recharge — which can be life-saving in emergency situations.
Note that battery voltages do not drop off linearly, so voltage tracking doesn’t give a true reading of battery life, especially since the middle third of the voltage scale comprises 60 to 70% of the discharge cycle time. Coulomb counting does not compensate for battery aging, so over time it “assumes” the state of the battery. Impedance tracking allows the system to calculate the remaining run-time to within 1% error over the entire life of the battery.
As system reliability is critical in medical electronics, battery authentication is also important. This is a means of validating that the battery in use meets the original equipment manufacturer’s (OEM) requirements via an encrypted device ID. Using the wrong battery pack can impact system run time, as well as damage the system, or even cause a fire.
Data Transfer InterfacesData integrity and access along with system flexibility and mobility are important to most patient monitoring systems. Newer interfaces such as Ethernet or wireless allow hospitals to network all of the equipment in the building, as well as a patient’s home. Today’s interfaces allow doctors to remotely connect to a patient via a wireless sensor pack worn by the patient, leveraging a hospital’s internal network or links to the patient’s home security system or cell phone. The entire system ties into the Ethernet or a medical call center to receive around the clock monitoring in the privacy of their home. Wireless interfaces such as Bluetooth® and Zigbee® also have a play here. Aside from power consumption, data rate and range are the two key requirements when selecting a wireless interface.
A higher frequency range of 2.4 GHz provides worldwide coverage, a high data rate and duty cycle, with many channels. But lower frequencies increase the signal range. For multi-channel, full-body monitoring, the range could be limited in a fixed location, but the data rate maximized. When monitoring just a few sensors, range may be more important than data rate. Ultimately, the solution choice must fit the system power budget and data transfer requirements.
Sensor Interface and Signal ChainRegardless of the type of biometric data collected, implementing the appropriate signal chain is crucial. Generally, the first stage of the signal chain is an nstrumentation amplifier (see Figure 2). In most cases the target is a microvolt level signal hidden within millivolts of noise. The target signal has AC characteristics, so an amplifier that works well with a high-pass filtering scheme is needed. Leveraging an auto-zero or auto-cal feature can further simplify system compensation requirements.
Typically the second stage in the signal chain is a low-power, wide-bandwidth operational amplifier (op amp) with a rail-to-rail input and output (RRIO) for excellent precision. Features like zero-crossover produce signals with linear offsets over the entire input common mode range. This reduces the need for the microcontroller to compensate for shifts and offsets.
Next is transitioning into a good delta-sigma or successive approximation analog-to-digital converter (ADC). Features such as single cycle filter settling and convert-on-command simplifies the design requirements, improves conversion speeds, and allows larger source impedances. In multichannel systems, global synchronization provides coherent signal acquisition, allowing multipoint signal sources to be compared within the same clocking cycles. With appropriate layout and component selection you feed a clean, accurate and truly meaningful signal into the system microcontroller or DSP.
Microcontrollers and DSPsThese days medical monitoring devices produce endless amounts of raw data. By performing diagnostic-based evaluations on the data, they can even make suggestions on which therapies to administer. Storing the data and processing trends, noting changes, providing feedback, supporting the ability to link up to larger systems, and performing diagnostic algorithms are critical functions of the system controller. Balancing the systems processing needs against the power consumption limitations is a critical decision.
Leveraging newer DSP technologies, power supply topologies, and implementing several power levels and standby modes help systems obtain very high performance at reasonable power levels. A microcontroller can manage a system in standby, sleep and wake up transitions, where a DSP or a series of DSPs drive the overall system performance. Low-voltage DSPs are now available, with idle or sleep modes, and dynamic frequency control to deliver desired performance while optimizing battery life. Development tools also have an impact on system power consumption. For example, the TMS320C55x DSPs are supported by a Power Optimization DSP Starter Kit that allows developers to accurately plan, analyze, manage and optimize real-time power consumption.
If a DSP is active only while processing, average system power consumption remains low with peaks appearing only when the DSP is awake. Given the performance and integration levels of the latest microcontrollers, sophisticated applications needing real-time processing at very low power becomes possible with devices like the MSP430FG461x. Additionally, by integrating a DAC, ADC, op amps and ultra-low power, the MSP430 can be a literal system-on-a-chip for portable applications such as heart rate monitors, blood pressure monitors, pulse oxymeters and other medical products.
ConclusionThe future of medical electronics is an unbounded path that focuses on terms like mobility, flexibility, configurability, and personalization. Systems will be quickly tuned by changing either the hardware or software configuration of the device to meet the demands of any situation. The correct supporting infrastructure, as well as feature set in the monitoring system, is key to enablement. It will be based on the right mix of processing, display, human interface (TSC), data interface, and power management technologies.
Jonathan Bearfield is an end equipment marketing engineer providing complete system solutions for the High Performance Analog team at Texas Instruments.
Sam Nork, Design Center Manager, Linear Technology Maury Wood, Proudct Line Director, GP-DSP, Analog Devices, Inc.