Hello, and welcome to TI's Precision Labs, an online video curriculum for engineers. This Precision Lab video will provide a brief introduction to isolated interfaces. For more topics on isolation, you can visit www.ti.com/precisionlabs.

This video will answer the following questions. What are isolated interfaces, and why are they used? How do I isolate RS-485 interfaces? How I dislike like CAN interfaces, and how to isolate I squared C interfaces?

What are isolated interfaces? Isolated interfaces are the integrated combination of isolation technology with a communication interface. There are many integrated isolated interfaces available, but this video will focus on the use case of CAN, RS-485, and I squared C as primary examples with general considerations for isolating a communications interface.

In addition to high voltage protection, isolation of communication interfaces is typically used to prevent ground loops and noise coupling or for connection of buses at different ground potentials where risk of damage to adjacent circuits may exist. Regardless of which interface is being isolated, the decision of whether to use a discrete or integrated solution is a first step in the evaluation of design trade-offs.

A typical discrete solution for isolated interfaces will include a digital isolator replace between the MCU or CPU and the transceiver or interface device on the bus line, as well as an isolated power supply solution to maintain the integrity of the isolation barrier. In many cases, an LDO or a line regulator and some additional protection circuitry will be needed. This will maintain the signal integrity on higher voltage lines.

When determining the best isolated solution for your design, there are some basic aspects of device selection beyond choosing the right communication interface to consider. Isolation ratings, package options for minimum creepage and clearance requirements, primary and secondary power supply requirements, signal data rates, over-voltage protection, and emissions performance will all be standard design considerations to include in your selection process. Integrated isolated interface solutions may include the integration of communication interfaces, signal isolation, and in some cases, even integrated power.

Isolated CAN devices integrate both the isolation barrier in the CAN or control area network. The CAN bus is a multi-master differential wired interface used for message broadcast networking. CAN is most commonly used in safety critical applications because of the features defined in its protocol, such as priority-based messaging, bit-wise arbitration to handle bus contention, and error detection and recovery. Isolating a CAN port is a common design challenge encountered in many industrial and automotive applications.

The CAN standard, ISO11898-2(2016), requires plus and minus 12-volt common mode range support for compliant CAN transceivers. This means a CAN receiver needs to tolerate up to plus or minus 12-volt common mode voltage on the CAN lines with respect to bus side ground and still be able to consistently replicate differential voltage transitions on the bus.

When the communicating nodes in a CAN network have larger ground potential differences that are higher than the supported common mode voltage range of the transceiver, isolating the CAN node becomes necessary. The isolation barrier also acts as an impedance to common mode noise transients that are common in industrial environments, such as ESD, EFT, and Search.

When building discretely, a typical isolated CAN solution includes a digital isolator placed between the MCU or CPU to isolate the CAN interface transmission lines. A CAN transceiver, as well as isolate power supplies to maintain the isolation barrier, will also be used. The CAN standard places strict timing requirements on total loop delay and on pulse with distortion for FT data rates of two megabits per second or five megabits per second. Discrete solutions will need to account for PCB parasitics that exists in the signal path to ensure timing is compliant to the CAN standard for bit-wise arbitration and signal integrity. This additional challenge makes the integrated solution a simpler option for many designers.

Integrated solutions combine the CAN transceiver and digital isolator into a single package. The key design considerations to evaluate when selecting and isolating a CAN transceiver are data rate, CAN FD replaces classical CAN to allow for faster communication in long and heavily-loaded networks. Bus fault protection. Higher bus fault protection prevents damage when high voltage is applied to bus pins.

Withstand and working voltage levels. End equipment that operates in the presence of higher voltages requires higher withstand voltage and higher working voltage to ensure reliable system protection over the equipment lifetime. Emissions and immunity for signal integrity. Low emissions and high immunity help to maintain signal integrity and compliance to industry standards for automotive and industrial applications.

And ESD, ESD protection is a critical consideration. If a system does not have the correct ESD protection, the high voltage of an ESD strike through an interface connection, for example, the CAN bus, can cause a large current spike to flow directly into the transceiver device causing damage. Consideration for proper ESD protection will play a role in evaluating your CAN solution, whether integrated or discrete.

Isolated RS-485 integrates both the digital isolator and the RS-485 transceiver. RS-485 as a bi-directional half-duplex standard with multiple drivers and receivers in which each driver can relinquish the bus to one of the other drivers. The RS-485 receiver has a high input impedance, which allows for multiple devices to be connected to the line and a wide common mode range of negative 7 to 12 volts, which enables data transmission across long cable links.

These features make the RS-485 ideal for use on factory floors and in industrial environments. And today, the RS-485 interface is one of the most commonly used interfaces selected when more than one master or driver is required. End applications use the RS-485 interface coupled with protocols such as Profibus, Modbus, or BACnet, which are system descriptions common to the industrial environment that uses the RS-485 standard, each with a unique set of system requirements based on differential output voltage, maximum load, and data rate.

Galvanic isolation is commonly used with RS-485 when high ground potential differences or noise coupling on the bus lines becomes an issue. The TIA/EIA-485-A standard requires RS-485 compliant transceivers to operate with a plus or minus 7-volt ground potential difference. The common mode voltage on the receiver bus pins includes contributions from ground potential differences, driver output common mode voltage, and any common mode noise coupled onto the bus pins.

As the communication difference between the nodes increases or the environmental noise increases, higher ground potential differences and noise coupling compounds, which can disrupt common mode voltage on receiver bus pins. This moves the bus out of its recommended operating condition and causes data corruption or damage to the transceiver. Isolators mitigate these issues by introducing significantly higher impedance to the bus lines and can manage the high ground potential differences while managing signal integrity across the isolation barrier.

Modern discrete implementation includes the use of digital isolator in RS-485 transceiver. In the solution, the enables transmit and receive signals or isolate it using a digital isolator between the MCU and the RS-485 transceiver. One key advantage of the discrete solution is the flexibility to choose the best transceiver for the specific application. However, the discrete solution does require additional board space because of the multi-chip solution. Typical design considerations for isolated RS-485 interfaces include the selection of half or full duplex, IEC-ESD requirements, use of Profibus, and the data rate of the isolator, including consideration of slew rate and timing requirements.

What is isolated I squared C? The inter-integrated circuit, or I squared C bus is a two-wire bus, half-duplex communication protocol primarily used for short distance communication between lower speed devices. The I squared C bus uses an open drain, open collector with an input buffer on the same line, which allows a single data line to be used for bi-directional data flow and communication between a master or multiple masters and a single or multiple slave device. The flexibility allows many different peripherals to share a single bus, which is extremely useful for connection between lower speed devices in a system like microcontrollers, EPROMs, ADD converters, and I/O interfaces with other peripherals on the system.

At the time of this video, I squared C clock runs up to 3.4 megahertz. And the I squared C implementation requires two wires, SDL and SDA and a pull-up to supply voltage VDD. For a deeper explanation of the I squared C protocol, the application report "Understanding the I2C Bus," is available at www.ti.com.

Galvanic isolation of I squared C bus is often introduced for protection, but also to break ground loops and ground potential differences that can create noise and disrupt signal communication. Digital isolators are uni-directional. This method of isolating a I squared C using standard digital isolators requires splitting the half-duplex line into separate transmit and receive paths and converting the isolators push-pull outputs into open collector outputs.

After the bi-directional data is separated into uni-directional signals, the digital isolator will modulate the input signal for each channel and pass the signal across the isolation barrier before de-modulating the signal at the output. Implementation for full duplex or multi-channel systems can be achieved by increasing the channel count of the isolators. You can read about the full detail of implementing a discrete isolated I squared C solution in the TI white paper "Designing a Reinforced Isolated I2C Interface by Using Digital Slaters" found at www.ti.com.

The integrated I squared C solution uses a similar approach with the use of isolated buffers and a comparator circuit. The bi-directional serial data line signal from the I squared C bus is internally separated into two unidirectional signals that are isolated using the channels of the digital isolator. The isolated I squared C devices are designed to interface with low capacitance nodes on the primary side and maximum loading of the I squared C bus on the secondary side. Isolated I squared C devices are commonly available with options for single and multiple masters.

Each solution for isolating the signal in an I squared C system does have trade-offs. Choosing a discrete digital isolator offers more options in packaging and isolation reading, but does require significantly more boards place in the complexity of additional circuitry for optimized functionality.

This concludes a brief introduction to isolated interfaces. We introduced isolated interface components and that they are most commonly used to provide voltage protection, to break ground loops, reduce unwanted noise coupling, and enable communication between multiple line potentials. We shared that isolated interfaces will either be discrete or integrated solution. And typically a digital isolator placed between an MCU or CPU and the transceiver interface device on the bus lines, as well as an isolated power supply solution, and occasionally additional level-shifting circuitry.

We introduced three common isolated interface examples including CAN, RS-485, and I squared C, which will require the additional considerations of isolation readings, package options for minimum creepage and clearance, primary and secondary power supply requirements, data rates, over-voltage protection, emissions performance, and ESD ratings. Please continue watching to take the online quiz.

The RS-485 standard requires compliant RS-485 transceivers to operate with plus and minus 7-volt ground potential differences. What are the sources of ground potential difference that cause isolation to be needed? The common mode voltage on receiver best pins includes contribution from ground potential differences, driver output common mode voltage, and any common mode noise coupled onto the best pins.

In the noisy environment of the factory floor, as communication distance between nodes increases or environmental noise increases, higher ground potential differences and noise coupling increases. And this can disrupt the common mode voltage on receiver best pins, moving the bus out of its recommended operating conditions. Isolation introduces a significantly higher impedance to the bus lines to help manage ground potential differences.

Thank you for watching. You can learn more about TI's isolation solutions at www.ti.com/isolation.