Reduce emissions and improve noise immunity in isolated CAN systems

“With more and more systems today operating at different voltages, from elevators to electric vehicles and even marine systems, isolated CAN transceivers have become an integral part.

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By Neel Seshan, Texas Instruments

With more and more systems today operating at different voltages, from elevators to electric vehicles and even marine systems, isolated CAN transceivers have become an integral part.

These transceivers combine the priority and arbitration functions of the CAN (Controller Area Network) standard and provide the benefits of isolation (break ground loops, resistance to pressure differentials, common mode transient immunity, etc.) to maintain reliable communication between the two voltage domains in the system.

As with non-isolated CAN systems, the main issue with using isolated CAN systems is the electromagnetic compatibility (EMC) performance of isolated CAN transceivers. EMC performance is measured by two parameters:

device-generated emissions

Immunity to disturbances in the system

emission

Emission is the unwanted release of electromagnetic energy. Ideally, low emissions ensure that a subsystem can operate reliably without affecting the performance of adjacent subsystems.

Depending on the market (industrial or automotive) and application, systems must comply with different emission standards. Although launch testing is performed at the system level, designers typically choose components that meet component-level requirements. This helps ensure that a single device does not exceed its own limits. In addition, system design and board layout also play an important role in the overall launch performance of the system. Among the many transmit tests, the Zwickau standard is a rigorous test for automotive applications, focusing on the transmit performance of CAN transceivers.

Figure 1 is an example of a circuit board for EMC testing. The board features three isolated CAN transceivers connected to the same bus. Transmit measurements were made at the test point (CP1 in Figure 1) with one transceiver transmitting a 50% duty cycle, 250 kHz square wave signal.

Figure 1: Texas Instruments circuit board for EMC testing

Placing a common mode choke (CMC) between the transceiver and the bus will filter out some emissions. Common mode chokes are commonly used in automotive and industrial applications.

Figure 2 shows the transmit data performed at the traditional CAN data rate through the ISO1042 of the EMC test board.

Figure 2: ISO1042 (transmission performed at 500 kbps)

Some certification bodies require data to be collected with the CMC, which also helps keep emissions low. Under the same conditions, ISO1042 outperforms competing devices in terms of emission.

Immunity

Immunity refers to the ability of a device to operate correctly in the presence of interference. To demonstrate the noise immunity of isolated CAN devices, we performed a direct power injection (DPI) test on the same circuit shown in Figure 1, but with a different coupling network. The frequency of the injected noise on the bus is swept, and the difference in transmit and receive patterns is checked by mask testing. The noise signals injected in the test include continuous wave (CW) noise signals and amplitude modulated (AM) noise signals. AM signals are 80% 1-kHz signals. Changes that exceed a certain voltage limit (±0.9 V) vertically or a time limit (±0.2 μs) horizontally are considered failures.

We performed tests under two different conditions:

36 dBm injected noise without common mode choke

39 dBm noise signal with common mode choke

Figures 3 and 4 show ISO1042 plots for conventional CAN under two conditions. In both cases, the isolated CAN performance was above the limit line, indicating that the DPI test was passed. Passing these immunity tests ensures reliable communication and reduces system errors and failures.

Figure 3: ISO1042 DPI test without common mode choke

Figure 4: ISO1042 DPI test with common mode choke

Integrated isolated CAN devices are expected to meet the same emission and immunity specifications as non-isolated CAN devices. Considering low emissions and high immunity in a small package, ISO1042 and ISO1042-Q1 meet the stringent requirements of industrial and automotive applications.

ISO1042 demo video

Please watch the ISO1042 demo video “Interoperability of Isolated FD Nodes.”