Need clarification on CN0359 Configuration

Hi Rafs 

The excitation voltage of 0.7V corresponds the Vpp. With regards on the intermittent readings, may I suggest you try to use a resistor (have the same resistance with your probe) to simulate the conductivity sensor to check if the wrong reading happen. Let me know if you still get the same values.

Regards,
rey

Hi Rolmedo, thanks for your reply. I used a resistor to so simulate the conductivity and got stable values. However, with the resistor I used the 2-electrode configuration. Per the user guide for the CN0359, I shorted pin1 to pin2 and pin3 to pin4 of the conductivity terminal. The conductivity sensor I am using uses the 4-electrode configuration, have there been any issues with the board and 4 wire conductivity sensors? 

Additionally, I ran into another post on this forum where someone explained that the voltage configuration on the board is for the Vp, not the Vpp. They also said that a Vpp of 0.7V is too low for the board. Do you know anything about this? I've linked the conversation below. 

(+) EXC voltage - CN0359 - Q&A - Reference Designs - EngineerZone

Hello Rafs ,

Sorry for the earlier confusion. To clarify, the excitation voltage setting refers to the peak voltage (Vp).
Since your sensor specifies a maximum excitation of 0.7 Vpp, you may want to try setting the excitation voltage to 0.35 V, which corresponds to 0.7 Vpp across the cell.

regards,

rey

Hi rolmedo , 

I have tried a different sensor, also a 4-electrode sensor, and again I am not getting the right readings. I was able to get good readings by using a resistor. However, with the resistor I used the 2-electrode configuration. Per the user guide for the CN0359, I shorted pin1 to pin2 and pin3 to pin4 of the conductivity terminal. Any ideas why I may be getting bad readings when connecting a 4-electrode sensor to the board?

It’s possible that this is related to the device configuration. Have you tried the configuration I suggested in my previous reply?

I set the excitation voltage to 0.3v, so as not to exceed the 0.7Vpp and I am still not getting the correct readings. I keep getting values that are in the range of 3mS/cm, instead of the 950uS/cm. I tried different excitation frequencies and setup and hold times, but I continue to get incorrect values. 

rolmedo Can you confirm that it is the Rev C board that has the capacitive coupling added to the excitation and sensor output signals? The sensor the customer is using is of the type that caused us to make the change - we found that tiny, planar sensors like this were being degraded by the residual DC voltage on the excitation, and possibly the input bias current. It also might be worthwhile to try a higher excitation frequency.

-Mark

Thanks for the input,  MarkThoren   
Yes, it's the Rev C, where we added capacitive coupling to the excitation and sensor outputs. 

Rafs , may I know which board revision you are using?

I am using the Rev C version of the board.

Hi Rafs . I haven’t seen any documentation reporting read errors for the 4‑electrode configuration on Rev C. Can you share your hardware setup and firmware versions (ADC config, wiring, and any jumpers)? I’m replicating your setup now and will report what I find.

-rey