• Resolved

iso7421: EMI from internal oscillator

Part Number: iso7421

What is the frequency of the internal oscillator used for low frequency signals? I am putting 24 of these in a piece of equipment and don't want issues passing FCC EMI tests. Thanks,

  • Hi Jonathan,

    The internal oscillator of ISO7421 is really low in power and it shouldn't really show-up in FCC or CISPR emissions tests even if you use 24 of them. Do you have more details on which emissions standard are you planning to comply with?

    Regarding the selection of digital isolator, ISO7721 would be a much better option over ISO7421 for your application. ISO77xx family of devices have superior performance in many aspects compared to the older ISO7421 devices and still be competitive in price. You also have varied package options to choose from across the family (SOIC-8, WB SOIC-16, QSOP-16). The family also includes 6-channel devices, ISO776x.

    Koteshwar Rao
  • In reply to Koteshwar Rao:

    Thanks for your answer. As for the standard we want to comply with, it's a bit more complicated than that. Our equipment is used as a load for testing a 48 port customer device which itself is being tested FCC Class A or B (we don't know which). Running 48 cables outside the test chamber is a royal pain, so the customer wants our unit to be inside the chamber but not add significant emissions. I'll look at the 7721.
  • In reply to Koteshwar Rao:

    Forgot to ask - can you give me an upper bound on the internal oscillator frequency? We use switchmode power supplies at low enough frequencies that we don't have to worry about emissions since FCC Part 15 mask starts at 30MHz.
  • In reply to Jonathan More:

    Hi Jonathan,

    Thank you for providing more details regarding your application and emissions requirements. Please note that the absolute power levels of switching in power supply and data switching in digital isolators will be many folds different. Hence emissions from digital isolators are generally much lower than what you see from power supply switching. The internal oscillator, especially, would be further lower in power levels and local to their respective die (Die1 for VCC1, Die2 for VCC2).

    Unfortunately I do not have emissions test data for ISO7421 to share with you. I do have CISPR 22 Class B emissions test result of ISO77xx, please find the results pasted below. From the test results, you can see that there is no contribution from ISO77xx for emissions and you only see the noise floor. As mentioned earlier, this is due to the low power levels of data being processed inside device. Even if you use 48 devices of ISO7721, it shouldn't be a problem.

    ISO77xx-Q1 is also being successfully used in automotive applications whose emissions requirements are much more stringent than industrial emissions requirements.

    I hope that answers your questions, thank you.


    Koteshwar Rao