• State TI Thinks Resolved
  • Locked Locked
  • Replies 5 replies
  • Answers 2 answers
  • Subscribers 30 subscribers
  • Views 335 views
  • Users 0 members are here
Support feedback
Options
Options
Related

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

LMP91200: Issues with Inconsistent pH Sensor Data Using LMP91200 AFE

divakar ruttala
Prodigy 10 points
Part Number: LMP91200

Tool/software:

I am using the LMP91200 AFE for 2-electrode pH sensor communication in my project, and I am interfacing it with a PIC controller. While I am receiving some data from the sensor, the issue is that the data is inconsistent. Every time, it gives different values without any change in the pH solution.

  • 0
    TI__Genius 11765 points

    Divakar,

    I may be helpful to provide us a schematic and layout of your PCB that uses LMP91200. My suspicion is that there is leakage interfering with the pH measurement. Are you using a guard ring with pins GUARD1 and GUARD2?

    Thanks

    -Alex Thompson

  • 0 in reply to Alex Thompson
    Prodigy 10 points

    Hi Alex,

    Thanks for your suggestion. Yes, I have connected GUARD1 and GUARD2 to the BNC connector as part of the guard ring implementation. Please find the schematic and layout I have attached below for your review.

    Regarding the Vref (pin 9), I’ve also connected a potentiometer to allow for adjustment of the reference voltage to ensure accurate pH measurements.

    I’m still getting inconsistent values, and I think it might be due to leakage or an issue with grounding or signal conditioning. I’d appreciate any advice or suggestions after looking at the schematic.

  • 0 in reply to divakar ruttala
    TI__Genius 11765 points

    Divakar,

    You may want to try using a triax connection, and following this layout:

    There is a second guard ring around the GUARD1 and GUARD2 pins. 

    Regarding you layout, is there solder mask over the pH input signal? That will introduce leakage. Do not cover the guard ring with solder mask either.

    Cleanliness is a big source of leakage as well. Is there corrosion on the traces, are they gold plated (gold plated wont corrode)? Was the PCB washed thoroughly?

    Regards

    -Alex Thompson

  • 0 in reply to Alex Thompson
    Prodigy 10 points

    Thankyou Alex,

    Could you please tell me the stabilization time for each sample from the sensor when using the LMP91200 AFE?

    Additionally, my pH sensor cable is a coaxial cable with a coaxial BNC connector. Can I connect a coaxial male BNC to a triaxial female BNC connector, or is there another option you would suggest?

    Also, I am getting some data from the LMP91200 AFE without connecting the sensor. How can I rectify this issue?

  • 0 in reply to divakar ruttala
    TI__Genius 11765 points

    Divakar,

    Based on our experience, a stabilization time of 1 minute is typical. Ultimately, how long to wait will depend on the pH electrode chosen, and how long the pH sensor has been disconnected and powered off. These parts supply very small currents, so it can take several minutes for the pH sensor to be properly biased and ready to measure.

    Most likely that unwanted data is coming from leakage on your PCB or design. I would try using the pH sensor in a pH neutral environment, and you should be getting 0mV on the output. If you aren't, then there are leakages that must be fixed. 

    I think that BNC configuration will work, having the triax at the LMP91200 will help reduce leakages.

    Thanks

    -Alex Thompson