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ISO7641FM: RFI disturbing ISO7641 operation

David Hofer
Prodigy 100 points
Part Number: ISO7641FM

We are using ISO7641 to isolate SPI signals going to low side drivers powered by field wiring. We are able to see the outputs of the ISO7641 stop functioning (not transitioning hi/low) when a radio transmitting in the UHF band ( ~ 461.5 – 468.5MHz) is brought in close proximity ( ~ 0.5m) to the device. The power is 5W. I don't know the gain of the antenna so I can't calculate field strength. We have tested this design to IEC 61000-4-3 @ 10V/m without issue so I'm assuming the field strength is much more than that.

Is there any detailed information on the frequency of the oscillator used in the ISO7641 available? We've tried different filtering methods but we suspect that the oscillator may be running at a harmonic of the radio frequency. Any guidance would be appreciated.

  • 0
    TI__Expert 6195 points

    Hi David, 

    Thanks for asking your question on E2E. 

    We are trying to determine the internal frequency of the oscillator now, but I do not think I will have that information for you by the close of business today. We will get back to you on that detail on Monday. Since this is an internal specification of the part, we are typically hesitant to include this kind of detail on the public forum. May we contact you using the email you used to sign up for E2E? 

    One option to consider here would be adding a Y-cap. When correctly implemented, this may increase your system level RI performance. 

    Best regards, 

    Dan

  • 0 in reply to Dan Kisling
    Prodigy 100 points

    Hi Dan,

    Thanks for the reply.

    Yes, please use my email on record to contact me if you prefer.

    Our system uses industrial type 24VDC power supply as the mains input. These supplies typically have RFI filtering in them so I don't think adding a Y-cap will help in this case. 

    --Dave

  • 0 in reply to David Hofer
    TI__Guru 53835 points

    Hi David,

    Dan has spoken to me about the issues you are facing, sorry to hear about the issue.

    ISO7641 employs an edge-based architecture for transmitting data across the isolation barrier and it doesn't involve any high frequency carrier unlike on-off keying (OOK) architecture devices (which usually have a carrier in 100s of MHz). The ISO7641 does have an internal oscillator for other functions but this is usually going to be around 1MHz. Hence, there isn't any internal frequency or its harmonics to get impacted.

    Based on your PCB size and its radiation pick-up characteristics (where the PCB acting as an antenna), it is still possible that the PCB picks-up some radiations at some suitable frequencies. This noise that is picked-up, if gets into device, can impact different blocks differently, especially the ones on the receiver channel.

    One way to improve immunity of ISO7641 to such radiations is to use a Y-cap across the isolation barrier of ISO7641 (between GND1 & GND2). Dan also meant to ask you to add this Y-cap across the isolation barrier of ISO7641 and not across the power supply. Please do try adding a Y-cap of >100pF between GND1 & GND2 and see if this resolves your issue. Please do let us know your observations, thanks.

    Regards,
    Koteshwar Rao

  • 0 in reply to Koteshwar Rao
    Prodigy 100 points

    Hi Koteshwar,

    Thanks for clarifying. I was able to greatly decrease the influence of RFI by adding 330pF cap across the isolation barrier as you suggested. We have used this device on many previous designs and have not seen this phenomenon but those PCB's had different layer stackups, etc. Apparently this time we found the proper stackup / size to make the perfect antenna @ 430Mhz!

    Thanks again.

    --David