The advancement of general-purpose computers has significantly impacted the architecture of the healthcare system as a whole. At the heart of these systems lies the personal computer (PC). By leveraging dedicated software tailored to medical applications, the PC facilitates the configuration of essential functionalities, thereby expediting research and development efforts and lowering expenses. Moreover, this connectivity technology enhances interoperability with other systems and peripherals. However, conventional PC interfaces may present limitations in system applications and prove less cost-effective when compared to USB isolators.
Isolation level or quality related terms for medical systems
- Isolation Rating: The maximum AC voltage that an isolator can withstand instantaneously. The typical specified value is 2.5kV rms for a duration of 1 minute. For equipment requiring higher safety performance, the requirements may be increased to 5kV rms for 1 minute.
- Working Voltage: The continuous AC voltage applied to the isolator during operation. Typical operating voltage values are 400V rms sustained over the device’s operating life.
- Reinforced Isolation: Specifically designed for the application, the specified isolation value is equivalent to two independent isolators. This equivalence is determined by ensuring that the isolation barrier can withstand short-term continuous surge voltages.
- Creepage Distance: The shortest distance between two conductors along the surface of the isolation barrier.
- Clearance: The shortest air distance between two conductors. The safe clearance distance depends on various factors, including implementation standards, type of isolation, and working voltage.
- MOPP: Medical electrical equipment requires patient protection that complies with the regulations of IEC 60601.
- MOOP: Medical electrical equipment necessitates a lower standard of protective isolation for the safety of medical staff.
Isolated Interfaces for Medical Systems
Safety concerns are of paramount importance in the healthcare industry, and the utilization of isolation technologies plays a crucial role in safeguarding healthcare professionals, patients, and equipment systems. The essence of USB isolation is to separate USB data and power lines through isolation devices to achieve galvanic isolation, thereby preventing current or noise from interfering with transmission.
In safety-critical fields, regulatory bodies such as UL and IEC establish standards, which engineers must comprehend and apply in conjunction with their specific applications. For instance, IEC 60601 specifies safety requirements for medical equipment, while IEC 60950 applies to information technology equipment. In many cases, medical devices intended for direct patient use necessitate reinforced USB isolation, operating voltages of 125V rms or 250V rms, and minimum creepage and clearance distances of 8mm.
Regarding isolation strength, the medical system typically determines the division strategy. The provided diagram illustrates the framework of a medical device with multiple interfaces, marking the specific location where USB galvanic isolation is implemented.
Primarily, patient safety demands isolation from the main system, which is achieved at point B, C, or D in the diagram. At point D, where direct connection to the patient is necessary, no USB isolation is required. In certain situations, such as with ultrasonic equipment, point D isolation is provided through insulating materials like plastic casings. Point C, located in the analog domain, requires isolation to maintain accuracy. Overall, galvanic isolation in medical equipment typically occurs at point B, but this solely addresses patient isolation and overlooks the isolation requirements for medical professionals. Therefore, isolation needs to be implemented at other interfaces as well.
To ensure the utmost safety in medical systems, compliance with IEC 60601 is necessary for all interfaces, considering the potential interaction with patients and peripheral equipment. Additionally, any device connected to the patient may be regarded as a peripheral and should be connected to any of the E, F, and G connectors depicted in the diagram above. IEC 60601 also specifies safety considerations for high-voltage defibrillators, requiring the removal of equipment connected to the patient during defibrillation procedures.
Adoption of USB
In the depicted internal interface diagram, points A, B, and C typically employ UART, SPI, and I2C communication protocols. You can choose these protocols based on specific project requirements such as cost, size, and performance. While considering internal interfaces, system architects also need to evaluate options for external connection ports that offer enhanced interoperability.
In previous applications, PC systems heavily relied on RS-232 serial communication. However, as research and development progressed, RS-232 has become less favorable compared to RS-422, RS-485, and Ethernet interfaces. RS-232 exhibits lower transmission speeds, limited cable lengths, significant voltage swings, and bulky connectors. Moreover, it lacks multipoint capability.
In contrast, USB has witnessed rapid development and a gradual increase in market share. This growth can be attributed to the widespread adoption of USB interfaces and the extensive support of peripheral devices. The USB plug-and-play feature significantly reduces application development costs and software requirements. Its integration into medical systems allows patients, even those without professional training, to easily utilize USB functionality. This reduces the strain on the medical system and minimizes hospitalization costs. By employing USB, patients can download necessary data to a memory device and bring it to the hospital for medical personnel to conduct diagnostics. Additionally, USB enables the connection of sensors to the main system, supporting multiple peripherals even with a single port. Currently, up to 127 devices can operate on a single USB bus.
Resistance of USB in Medical Systems
Due to the multitude of advantages and widespread adoption of USB in various industries, it is natural to assume a similar trend in medical applications. However, the utilization of USB in the medical field has not met initial expectations. This discrepancy can be attributed to the distinct isolation requirements specific to the medical domain, which render USB isolation more complex compared to other interfaces.
The USB interface presents challenges for isolation due to its bidirectional nature with differential signals, along with the need for configuring bus speed through pull-up and pull-down resistors. Bidirectional communication necessitates a specialized method to ascertain the direction of data transmission. Within an isolated USB interface, it is imperative that this data direction information permeate through the isolation barrier. Control flow is determined by data structures rather than control signals.
The USB interface consists of four wires: VDD, D+, D-, and VSS. VDD serves as the power supply line for the USB device. VSS denotes the reference ground of the USB interface, providing the circuit with a reference potential. VSS forms a closed circuit with VDD to ensure the stable transmission and operation of signals and power. D+ and D- are the differential data lines responsible for transmitting digital data signals between devices, and they can also be employed for bus status determination. Different USB speed specifications are represented by varying data status and communication protocols. Pull-up and pull-down resistors on the peripheral side of the bus enable speed and idle state configuration of the USB interface. Data can be transferred at three rates: 1.5Mbps (low speed), 12Mbps (full speed), and 480Mbps (high speed).
Since optocouplers are unidirectional devices, the USB signal needs to be converted into a set of unidirectional signals, as depicted in the figure (EEPROMS, although unmarked, is used to store signal conversion codes). In the example, the microcontroller’s D+/D- lines are converted into single-ended, unidirectional SPI signals, which are then isolated and converted back to USB signals using a USB serial interface engine or USB controller. Employing this method incurs additional costs for designers, such as incorporating extra components, wiring, a larger PCB area, increased design time, and higher manufacturing expenses. These factors have contributed to the slow adoption of USB by medical system architects.
As illustrated, there are two options for isolating the USB interface. The left diagram demonstrates the configuration method involving the conversion of D+/D- signals into unidirectional single-ended SPI signals using a microcontroller and a serial interface engine. On the right, a simplified approach is shown, utilizing the ADuM4160 USB isolator, which directly plugs into the D+/D- signal path without necessitating additional signal conversion components. This streamlined solution enables the isolation function of the USB interface.
Optimizing USB Isolation Design
To simplify projects and achieve cost and size advantages in USB isolation, single-package USB isolation is utilized. This approach involves integrating USB isolation functions into a single package or module that can be directly inserted into the D+/D- USB signal path. These isolators offer reinforced isolation up to 5kV rms and support low-speed and full-speed data rates.
Unlike optocouplers that rely on LEDs and phototransistors to transmit data through light, newer technology-based isolators employ planar transformers to transmit data across a 20-μm-thick polyimide insulating layer capable of withstanding 6kV rms voltage. Data transmission occurs through induction from one coil to another.
In comparison, single-package USB isolation offers several advantages. With the use of transformers, data can be transmitted bidirectionally across the isolator. While this method utilizes dedicated transformers for sending and receiving signals, all coils are identical and packaged within a single device, a capability not achievable with optocouplers. Optocouplers require separate devices for communication in each direction.
USB isolators utilizing transformers provide higher speed than optocouplers, enabling support for the higher data rates and shorter propagation delays required by USB. Additionally, the low power consumption of these isolators allows them to meet the stringent standby power requirements of USB.
One key advantage of this isolation technology is its ability to integrate additional functions into isolator products. The integration of these functions results in significant space savings, with USB isolators reducing board area footprint by 75% compared to configurations that employ multiple chips, USB transceivers, and optocouplers.