You Passed EMC Testing… But Wait, Is There More?

You Passed EMC Testing… But Wait, Is There More?

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.

Above and Beyond EMC

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.

EMC Extra Credit #1: Common Mode Transient Immunity (CMTI) Testing

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!

EMC Extra Credit #2: Direct Power Injection (DPI) Testing

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:

ADI: The DPI Test - Know Your Digital Isolator is Immune to Noise

Conclusion

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?

Anonymous
  • Hi Alexxx,

    I hope you are well. Thank you for your question. 

    You are correct, our first generation was edge-encoded that used extremely fast pulses to encode the data and transfer the information across the barrier. This system also included a refresh system, such that if no input edges occurred, if data was static high or low, the isolator would send across a refresh pulse to ensure the output still had the right voltage level. This was not a fully differential system, therefore energy could possibly cross the barrier. Just to note edge encoded schemes do have the advantage of being low power. 

    I would recommend our ADuM34x family of isolators. The ADuM341 is our next generation of isolators and the first part numbers were released last week (https://www.analog.com/en/products/adum341e.html)

    This is a On-Off-Keying (OOK) encoding with differential architecture, with centre-tapped transformers that also reduce common mode currents across the barrier while also improving the immunity/robustness of the device. This should reduce the transfer of unwanted noise generated by the isolator across the barrier.

    The isolator design also has back to back-barrier, so that's two layers of protection, one on the isolated and non-isolated sides of the isolator. This helps reduce the input-to-output capacitance by 60% from our first generation to our new generation. I hope this helps with your design.

    With regards the Power solution, I consulted with a colleague of mine who is more familiar with our power catalogue, Going by his suggestion the LT3999, or LT8301 can all achieve +/-15V out with the right transformer.

    If you have any other questions, please reach out

    James 

  •  can you please share your insight here?

  • Hi James,

    with digital isolators I had the problem with parasitic input-output capacitance. When the interfering frequency is high, the grounds isolation is sagging - as the parasitic I/O-impedance gets smaller. Also the older digital isolators uses extremely short pulses inside that can produce common mode disturbance on the ground plane.
    For very low voltage, high resolution digitizeing, what part can you reccomend to isolate the analog section with the ADC from the digital section?
    What is the best way to get (for example) +/-15V over the isolation barrier with minimal ground coupling?
    What strategies can you reccomend?
    Thanks for help.