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?
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.
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?
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.)
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.
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.
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?
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.
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?
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.
Hm, this is good information.
Would I also short both the iso3.3V and 3.3V rails together?
If the ADUM5000 is still in place, then don't tie them together. If this test determines that isolation is not necessary, that is, no degradation to your conductivity measurement as a result of shorting grounds, then the next step would be to remove the ADUM5000, shorting the supplies (such that the formerly isolated side of the board is now powered directly.)
Should the same be done with the ADUM1250?