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
  • 0
    •  Analog Employees 
    on Apr 23, 2019 6:10 PM over 1 year ago in reply to jgirgis

    Hi Jgirgis,

    Yes, that's the idea with the safety capacitor. If your system is in a sealed box, then it's unlikely that you need isolation.

    Only other question that comes to mind is if the sensors (conductivity or otherwise) are in direct contact with seawater, and if so, is the circuit ground also tied to seawater potential somehow? (Conductive stainless steel case, for example.)

    -Mark

Children
  • Hi Mark,

    Thanks for the advice. Just to be logistically clear, the safety capacitor should be between the grounds if it is a Y cap? or should it be between iso-power and ground or power and iso ground? I found an unknown 250V x1/y2 capacitor lying around so I am unsure if that could be used versus just buying a Y cap.

    For the sensor, the J1 5 pin screw header is what is interfacing to some stainless steel electrodes and a pt100 sensor. The last pin for ground is unused. Additionally the case that holds the application is non conductive. 

  • 0
    •  Analog Employees 
    on Apr 23, 2019 7:27 PM over 1 year ago in reply to jgirgis

    Definitely between grounds. Since your circuit is in a box and not tied to the AC mains, you don't necessarily need to use a y-cap, so go ahead and try the one you found.

    -Mark

  • 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?