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
  • +1
    •  Analog Employees 
    on Apr 22, 2019 8:17 PM over 1 year ago

    Hi jgirgis,

    The most likely culprit here for your RF interference is the ADuM isolator parts.  There are techniques on how to handle this, and thankfully someone wrote them up in a nice little app note.  Have a read through this and try implementing these techniques and see if you're able to cut down on the noise.

    https://www.analog.com/media/en/technical-documentation/application-notes/AN-0971.pdf

    Cheers,

    Brandon

  • I assume the situation is depicted in Figure 4. Dipole Radiation Between Input and Output. Is there any way to mitigate interference without remaking the eval board?

    Are the ADuM parts necessary for the circuitry? 

  • 0
    •  Analog Employees 
    on Apr 23, 2019 5:14 AM over 1 year ago in reply to jgirgis

    Does your end application require isolation? If not, then the ADuM parts aren't necessary. If you hadn't thought about isolation before, then the answer is probably that you don't need them.

    If the end application is a handheld device, or a piece of lab equipment in a relatively benign (electrically) environment, then this would be the case.

    On the other hand, if the conductivity cell is in a situation where it's connected to a metal pipe at some potential (such as Earth ground), and the data collection equipment is at some other potential (like a laptop with any number of other instruments attached to it), then isolation can prevent unpleasant surprises.

    If you determine that isolation is necessary,  a high-voltage capacitor between grounds would be the first step. Y-capacitors are typically used for this purpose: https://www.digikey.com/en/product-highlight/v/vishay-bc-components/vy-series-ceramic-caps 

    -Mark

  • Hi Mark,

    My application requires many sensors, including the cn0349, for a MCU to interface to. The application is in a sealed case in a marine environment. As I said I have an LTE modem and GPS on the master board. I think I only have a couple sensors on the 3.3V line, such as a temp sensor, GPS, and ADC. I am not sure isolation is needed or not. As a down and dirty way of reducing EMI on the evaluation board, are your saying adding a safety capacitor between grounds can help? Reading the AN-0971 shows a 160pF safety capacitor between PWR and GND instead.

    from page 18:

  • 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

  • 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. 

Reply
  • 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. 

Children
  • 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?

  • 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