ISO7841: Carrier frequency and GSM immunity

AndyP

Part Number: ISO7841
Other Parts Discussed in Thread: ISO7341C

Hi all,

I would like to know the carrier frequency used in ISO7841 and similar digital isolators. This relates to some issues we encounter with close proximity GSM antenna transmissions, that possibly corrupts isolated I2C communication (isolated via ISO7841). Immunity problems occurs when working on 2G 900MHz band and in weak GSM signal strength conditions (the GSM bursts have up to 2W of power in such situation). If you have some other useful information regarding this ISOxxxx family isolators and its immunity to high frequency interference please share.

Best regards,
Andy

over 3 years ago

0 Koteshwar Rao over 3 years ago

TI__Guru 52530 points

Hi Andy,

Thanks for reaching out and sorry to hear about the issue.
The frequency of the carrier used in ISO7841 and other devices using On-Off Keying (OOK) architecture is ~450MHz. There is a possibility of interference when there is an RF module mounted close to ISO7841 on the same PCB. Please see below article that discusses more in this regard and also provides some solutions address the issue.

https://www.ti.com/lit/slyt742

Alternatively, if you do not require reinforced isolation, you could also consider using digital isolators that employ edge-based architecture which do not have any internal high frequency carrier. ISO7341C from ISO73xx family is one of such devices that I can recommend. Thanks.

Regards,
Koteshwar Rao

0 AndyP over 3 years ago in reply to Koteshwar Rao

Expert 1756 points

Thanks for the tip, despite the application note mentioned describes somewhat different problem. My problem may be related to disturbance of ISO7841 by GSM transmission, not the other way around (disturbing RF receiver by digital isolator). This is only hypothesis as I'm not sure what is causing my I2C problems. Still need to investigate.
Best regards,
Andy

0 Koteshwar Rao over 3 years ago in reply to AndyP

TI__Guru 52530 points

Hi Andy,

Thanks for the clarification that the issue you are facing is possibly related to immunity to external radiations and not radiated emissions of the digital isolator. If this is indeed the issue, then using a stitching cap as described in the document that I shared should still solve your issue.

Alternatively, you could also experiment with a Y-cap of value >100pF connected directly across the GND pins of ISO7841. Please do try this out and let us know your results, thanks.

Regards,
Koteshwar Rao

0 Robert White over 3 years ago

Prodigy 20 points

Hi there,

We have also ran into the same problem with the ISOxxxx isolators. I have created another post with some of our findings.

https://e2e.ti.com/support/isolation/f/isolation-forum/994815/iso7841-interreference-from-2g-3g-4g-modem-causing-data-corruption

Hopefully now that two customers are reporting the same behaviour that this thread receives some more attention.

Kind regards

0 Koteshwar Rao over 3 years ago in reply to Robert White

TI__Guru 52530 points

Hi Robert,

I have responded to your other post, we can continue our discussion on that post. Thanks.

Regards,
Koteshwar Rao

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ISO7841: Carrier frequency and GSM immunity