Let’s say your product survived its visit to the electromagnetic compatibility (EMC) testing facility. Let’s even say it passed with flying colors. First off, congratulations: That’s no easy feat. Technically, your design is ready to enter the real world—but just how ready?
Checking off the EMC compliance box is like getting top marks in a class: It looks great on paper, and you’ve done everything you need to do, but there is always room for extra credit. After all, the more robust your device is in the real world, the happier your customer will be.
EMC testing to standards gives an indication of the noise immunity of your isolated design. However, in the real world, not everything is a nicely specified waveform. That’s where additional tests such as common mode transient immunity (CMTI) and direct power injection (DPI) can be used to further characterize your design.
Think about your favorite movie. Just as digital isolators are created and tested in a lab, films are edited in the vacuum of a studio, where it’s quiet and easy to hear all the dialogue. But the final audience will be watching it in the real world: A theatre, where other moviegoers may be whispering, or at home, where cars may be driving past the house or a dishwasher may be running. How can filmmakers ensure their movie is immune to environmental disruption?
Unfortunately for them, they can’t; that power lies with makers of fancy speakers and noise-canceling headphones. Digital isolation designers, on the other hand, are fortunate: We have tests and standards that can quantify how our designs will perform in the face of outside noise, so you get a better idea of how the design will perform when it goes out into the real world.
Common mode transient immunity (CMTI) testing is a measure of robustness for digital isolators. While it is not an EMC test, it is one way to see how your product will operate in variable conditions. CMTI is one of the key characteristics associated with digital isolators.
A successful isolation design prevents high-frequency transients from crossing the isolation barrier and corrupting sensitive data. CMTI is a measure of that. The higher the CMTI value, the more successfully your design can withstand these corrupting frequencies and keep your data intact.
CMTI uses trapezoidal waveforms to test noise immunity against fixed frequencies. However, trapezoidal waveforms are frequency-limited and decrease in amplitude significantly at higher harmonic frequencies. To ensure the integrity of data as it crosses the isolation barrier, the system must be immune to noise at all frequencies – even up to the Giga Hertz range.
Therefore, CMTI is one extra credit test you should do, but because of its limited frequency content, it shouldn’t be the only one. Be sure to keep an eye out for our future posts on the topic of common mode!
Real-world, harsh environments are tricky to replicate. Testing with direct power injection (DPI) ensures the highest level of noise immunity for your isolated system by testing to failure across the full frequency spectrum. ADI’s iCoupler digital isolators, for example, adapt the IEC 64132-4 DPI (an internationally recognized test) to measure the performance of isolation barriers against a wide range of frequencies that are commonly found in real-world environments.
DPI testing creates a plot of an isolator’s noise immunity at each frequency, providing broader and more granular insights than those offered by CMTI. The higher the passing power level, the better the device’s noise immunity.
The limitations of CMTI and the method of adapting DPI for further characterizing the isolation barrier, along with ADI’s iCoupler superior noise rejection, are demonstrated in this video:
A product that has undergone EMC testing has a known compliance level. It’s a good product—top marks. But designing with digital isolators that also have high CMTI and DPI immunity can create a solution that is better positioned to withstand noise and harsh environments in its real-world application. And who wouldn’t want that extra credit on their report card?