Following on from previous discussions on the problems of analog signal integrity and isolation, I came across the Linear Technology LTM2886 for isolation of SPI or I2C interfaces. This is one of a range of isolated transceivers from Linear Technology covering RS422, RS485, RS232, SPI, I2C and USB. A clear advantage of digital rather than analog isolation is that some distortion of the signal can be tolerated with a digital system and, provided it is within the noise immunity tolerance of the digital system, the data will be 100% accurate. With analog systems, any distortion will directly affect the final signal. Analog Devices also have isolated CAN and LVDS transceivers such as the ADM3052 and ADN4650. NVE Corporation also make an isolated CAN transceiver – the IL41050TA. The LTM2886 generates power for one side of the interface across the isolation barrier although other isolation interfaces don’t necessarily have that feature.
The LTM2886 is actually a module rather than a simple IC. Providing 10kV isolation across the barrier dictates that there must be more than one IC to do the job. Also, there are some decent sized capacitors (2.2µF) which again, are not likely to be fabricated on an IC. The 10kV is only an ESD (human body model) rating – the actual working isolation rating is 560V peak with a dielectric rating of 2500V for 1 minute. The size of the device would preclude 10kV operation anyway – it is 15mm x 11.25mm overall and less than 4mm thick with around 9mm between the connections across the isolation barrier. Assuming you want to have a PCB layout which meets IPC2221 you would need to use a coating at high altitudes to be able to work at 560V peak. The built-in power supply across the isolation barrier should be capable of supplying your remote circuitry because it can supply +/-5V at 100mA as well as 3V to 5.5V at 100mA.
NVE Corporation make an interesting range of isolation interfaces including straightforward isolated digital interfaces, i.e. not tied to a specific interface type. So, instead of isolating RS422 or CAN interfaces, you can simply isolate any digital signals. Examples would be the IL716 and IL516.
While they look similar except for a large difference in maximum data rate, the IL516 is “DC-Correct”. The IL716 relies on differentiating the digital input signal to make the output state change. There is therefore an upper limit on the rise time of the input signal of 1µs. The output data is presumably latched because there is no minimum data rate, but there is an indeterminate state on power up. The datasheet recommends toggling the data inputs on power up to initialize the outputs to be in the correct state. The IL516 takes care of that for you with the outputs guaranteed to match the inputs within 9µs of power-up. There is a significant difference in the speed between the two devices though – 2Mbps for the IL516 and 100Mbps for the IL716.
Optocouplers are a more common method of isolating digital signals and can make sense for lower speed signals of a few kilohertz or less. However, they can take quite a bit of power. Taking the Fairchild Semiconductor HCPL0637 dual channel optoisolator as an example, current on the transmit side is recommended to be a minimum of 6.3mA per input and the current on the output side is 15mA to 21mA depending on the output state.
You must include a series resistor to limit the current on the input side because you will be driving the internal LEDs directly. For a comparison of power consumption with something such as the NVE devices you would need to look at, the IL511 or IL711 for two channel isolators. With the NVE devices the input circuitry just takes microamps, with the significant power consumption being on the output side. On the IL711 that is typically 2.4mA at 3.3V. So, current consumption is quite a bit lower than with an optocoupler which may be a consideration. The speed may also be an issue. Optocouplers are typically used in low-speed applications although some high speed devices are available. The HCPL0637 is limited to 10Mbps which is enough for lower speed SPI interfaces and I2C and as quick as the LTM2886. The Avago HCPL2430 is rated at 20Mbps (40Mbps typical) and is one of the fastest digital output optocouplers I could find. While the LEDs only need 8mA each, the receive side takes around 34mA at 5V, so they are quite power hungry. The IL711 has a dynamic current consumption of typically 140µA/Mbps per channel at 3.3V. So at 20Mbps on two channels the current would be 5.6mA plus the static current of 2.4mA making a total of 8mA at 3.3V. So, they are a less power hungry solution and would be more efficient at lower speeds due to the significant dynamic current component.
The simplest optocouplers consist of a LED and phototransistor such as the Sharp PC123 series. You would be limited to a few kHz though, depending on the phototransistor load resistor used.
The use of such devices is primarily for simple, isolated digital switches. The output side can usually withstand 30V or more (70V in the case of the PC123) and drive tens of milliamps, although you need to watch out that you don’t exceed the maximum power dissipation, and also make sure the phototransistor is fully turned on when being used at high current. They are not really a solution for digital communications signal isolation due to the low speed. If you need isolated communications then one of the specialist chips which match the interface you are using is likely to be the best choice (CAN, I2C, SPI etc) such as those from Linear Technology, Analog Devices or NVE Corporation.
How to maintain analog signal integrity, by Chris Francis, EEWorld and Power Electronics Tips