Digital Signal Isolation for a Better Design

Design engineers need peace of mind. You need to feel secure that your design is rock solid. This will enhance creativity (and can make you a much nicer person to be around).

Using digital isolation on all signal lines in your design might be a great aid to finding this peace of mind. If all your I/O is digital, these isolators will eliminate conducted noise and prevent the always terrible ground loops from wreaking havoc on your measurements. You can find single- to six-channel digital isolator ICs that are a snap to design in. Application areas include factory automation, electric vehicles, the electric grid, robotics, and scientific instrumentation.

You could even A/D convert analog I/Os before the digital isolation barrier so they are super clean also. Complete galvanic isolation between two power domains will also protect circuits from high common-mode transients, which may occur during solar storms or lightning strikes. The isolators can enhance safety if you have any high voltages present.

Digital isolators also throw in signal-level translation for free. Using them is simply good engineering practice and may in fact make the rest of your circuit easier to design. You will need to have two separate power supplies, of course, but that's often easy to accomplish from power transformer taps. The tiny, high-efficiency switching regulators available now help a great deal in this task.

Available isolator ICs yield up to 5kV of isolation, support data rates up to 150Mbps, and require only a few milliamps of supply current. You can select general-purpose unidirectional or bidirectional isolators or transceiver type isolators for RS-232/485, CAN, LVDS, USB, or SPI signal lines.

All these circuit design advantages don’t come free, of course. We can figure around $2.50 for a four-channel isolator IC and a few more cents for some associated discreet parts, plus some additional power supply circuitry. But that’s not much to improve your system’s reliability and robustness – and your state of mind.

Some of the important specifications to evaluate for these ICs include working voltage, common-mode transient immunity, maximum data rate, propagation delay, and supply voltage/current.

A Digital Isolator IC Example

The MAX14934–MAX14936 is a family of four-channel, 5kV_RMS digital isolators using a proprietary process technology for the isolation barrier. For applications that need only 2.75kV_RMS isolation, you can use the MAX14930–MAX14932 and for applications requiring bidirectional channels, such as I²C, refer to the MAX14937. 

The MAX1493x family transfers digital signals between circuits with different power domains via a galvanic separator. The devices are certified to VDE V 0884-10: 2006-12 for basic insulation, with a maximum working voltage of 848V_RMS. They meet the UL1577 safety requirement.

The ICs perform at ambient temperatures up to 125°C and use as little as 0.58mW per channel at 1Mbps with a 1.8V supply. They have an isolation rating of 5kV_RMS for 60 seconds and common-mode transient immunity is a very robust 25kV/µs. You can use 1.71V to 5.5V supplies on each side of the isolator.

The chips offer all three possible unidirectional channel configurations. Each of the four channels only passes data in one direction, as indicated in the functional diagram (see Figure 1), and each channel operates independently. Data rates can be selected from 1Mbps, 25Mbps, or 150Mbps, depending on the version. The output driver of each channel is push-pull and can drive both TTL and CMOS logic. Versions are available with the default output at high or low levels. Input capacitance it just 2pf typical and rise and fall times are typically 2ns. Propagation delay varies from 8ns on the fastest versions to 55ns on the slowest (1MHz) versions.

Figure 1: MAX14934, MAX14935, and MAX14936 functional diagrams.

With 3.3V supplies, the chips take only 2.6mA/channel typical at 1Mbps or 7.1mA/channel typical at 100Mbps. The 16-pin wide-body SO package meets creepage and clearance requirements for equipment powered by 220VAC. The MAX14483EVKIT is a full evaluation kit..

Figure 2: The 16-pin wide body (10.3mm x 7.5mm) SOIC package of the MAX1443x. (Photo courtesy of Digi-Key)

A Six-Channel Isolator For SPI

Another device, the MAX14483, provides six channels, with four forward uni-directional data channels and two reverse channels designated for Serial Peripheral Interface (SPI) bus control signals SDO and Fault. All six unidirectional channels can have up to a 100MHz clock, 200Mbps data rate. The six signal channels are individually optimized for SPI and include very low 10ns propagation delay on the SDI, SDO, and SCLK designated channels (Figure 2), along with 2ns maximum pulse-width distortion. The IC conforms to the IEC 60747-5-5 standard.

Figure 3: MAX14483 functional diagram.

An auxiliary channel is used for passing timing or control signals from the master side to the slave side, and there are also power monitors for both power domains. The IC has an isolation rating of 3.75kV_RMS for 60 seconds and comes in a 20-pin SSOP package with 5.5mm of creepage and clearance. The package material has a minimum comparative tracking index (CTI) of 400V, which gives it a group 2 rating in creepage tables. It is specified for ambient temperatures of -40° to 125°C and a maximum working isolation voltage of 400V_RMS.

The IC withstands ±10kV surge between A and B side with 1.2/50μs waveform and has ±4kV ESD HBM withstanding. There are 1.71V to 5.5V supplies on each side of the isolator. The IC requires only 1.53mW/channel with a 10MHz clock and a 3.3V supply and just 0.77mW/channel with a 1.8V supply.

Specification Checks

Digital isolator specifications can be confusing because of various specifications. They sometimes state an isolation test voltage relevant to product standards (IEC 60950-1, etc), when the specifications should highlight component standards (VDE 0884-10, UL 1577) and working voltage. A designer should ways verify that an isolator has VDE and UL certifications. The best way to make sure the isolator meets the safety rules is to obtain a copy of the VDE and UL certificates.

Avoiding Design Headaches

Digital isolator ICs are easy to use and can save a design from causing a lot of headaches, especially in electrically noisy environments like industrial or with electrical equipment such as in factories, electric vehicles, electric grid, and scientific instrumentation.