CN0349 RF interference?

Hi,

I am using the CN0349 as a fully "isolated" conductivity sensor for seawater. Interfacing with the part is fine and it's data output is fine.

However, it causes huge amount of interference (RF?) with other devices on my master PCB board. It can successfully jam my GPS and cause a large reduction in signal quality for my LTE modem. It is apparent because removing it from the header shows a spike in signal quality. Are there any suggestions on how to shield this part from interfering with the rest of the board without turning it off when my gps or modem needs to be used?

Thanks

Parents Reply Children
  • I soldered on the cap to the ADuM5000ARWZ's ground and ground iso pins, however the CN0349's isolation IC's seem to still have a significant effect on the the LTE modem. The capacitance on the X1/Y2 cap is around 5-6nF according to my multimeter. Should I buy a 160pF like the cap in the application note?

  • 0
    •  Analog Employees 
    on Apr 23, 2019 9:57 PM over 1 year ago in reply to jgirgis

    That MIGHT have some effect if the ESL of your 5-6nF capacitor is high, but without a board redesign incorporating internal stitching capacitance in the board itself, we might be grasping at straws.

    Are you just building a few (or one) of these systems based on the CN0349, or are you prototyping a design for which you'll eventually design a PC board?

    If the former, and if you determine that isolation is not necessary, you could just remove the ADUM5000 and connect power / ground to your main circuit.

    If the latter, and if you determine that isolation IS necessary, consider the ADUM5020, it's a newer generation device that addresses this concern. Unfortunately it's not pin compatible with the ADUM5000.

    -Mark

  • Well I think its a mix of both, I am prototyping a PCB to interface with the CN0349. If the AD5934 is operating at high frequency wouldn't that produce considerable noise on the power and ground planes,without isolation? I could just redesign the pcb board to include the ADUM5020 or without isolation but my supervisor was wanting the product to not have to go through another pcb design/wait period. Would it be worth trying a low ESL cap instead?

  • 0
    •  Analog Employees 
    on Apr 23, 2019 10:50 PM over 1 year ago in reply to jgirgis

    Re: AD5934 producing ground noise - not necessarily - if the supply pins are well bypassed, any high-frequency activity inside the part is confined to a small area. And you don't have the dipole situation that you have with the ADUM5000 straddling between the two ground planes. I'd try it without isolation and see what you get - in fact, you could start by just shorting grounds together and leave the ADUM5000 in place; this would be a useful debugging exercise anyway.

    -Mark

  • Hm, this is good information.

    Would I also short both the iso3.3V and  3.3V rails together?