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Original question:

LMP91200: reference to build Ion-selective electrode measuring circuits

LMP91200: mV output pH & Ion-selective electrodes

Gopi Krishna Saladi
Prodigy 50 points
Part Number: LMP91200
Other Parts Discussed in Thread: LMP91000, TIDA-00561, LMP91000EVM

https://e2e.ti.com/support/sensors-group/sensors/f/sensors-forum/1135597/lmp91200-reference-to-build-ion-selective-electrode-measuring-circuits?tisearch=e2e-sitesearch&keymatch=LMP91200#

Dear ,

As a follow-up to my previous question on ion-selective electrodes measuring through LMP91200,

all our sensors - pH, NH4+ and NO3- provide mV output. Is it safe to assume that LMP91200 is a perfect peg? or do you have any other suggested parts?

For unknown reasons, the circuits we built are not providing repeatability.

I really appreciate any help you can provide.

Gopi Krishna

  • 0
    TI__Expert 8810 points

    Gopi,

     The LMP91200 is very focused on a PH probe with standard settings. If you are seeing very small currents in the probe then look at the LMP91000 as it has adjustable settings like the shunt value and the transimpedance gain. Its basically the same part but adjustable and customizable.

    Knowing and understanding the probe is a key factor. If the probe is standard then some of our parts are specific to that probe and will minimize the setup and layout. When you need something less dedicated to a specific probe then you need a part that is more versatile like the LMP91000. It will require some extra work in determining the external parts, the internal setting and then the formulas to provide the values you need. The only way you can do this is to have a standard solution for your probe so you can adjust these values to get a known result. Then you can trust the values for the unknown measurements.  

    Most of the time problems with measurements has to do with the internal settings. When a probe creates micro-amps (uA) of current the feedback will be milli-volts. To overcome the low readings you need to increase the Rload resistance settings. Rload is a shunt resistor that will produce a voltage drop based on the current through the probe. increasing the Rload to 10 Ohms or even 100 Ohms will produce a larger voltage drop. The transimpedance (TIA) amplifier will convert the probe current to a voltage. If the changes in the current probe are very small, then you can increase the gain of the TIA by adjusting the feedback resistor value.

    Let me know if you have any problems. 

  • 0 in reply to Gordon Varney
    Prodigy 50 points

    Dear Gordon -

    Thank you for the detailed explanation as always.

    1. Once we build the commercial product, we intend to use a standard online process pH sensor with a measurement range of 0-14 pH. The probe output is mV. (like Sensorex S272 for now). We have taken TIDA-00561 as a reference earlier. We shall now include the LMP91000 in our exploration. Do you have any reference circuits for LMP91000?
    2. Does it mean, LMP91200 cannot be used for ion-selective electrodes? From your previous guidance, we have noted that the working principle of the ion-selective probes is similar to that of pH. Somehow I could not find a reference schema on public search.

    Thanks again.

  • 0 in reply to Gopi Krishna Saladi
    TI__Expert 8810 points

    Gopi,

      The LMP91200 was designed to minimize the work, parts count and effort to create a PH probe measurement system. All that means is that there is not a lot of adjusting that can be done. If the probe that you have, has a lower current level for the relative gas then there will not be much room on the LMP91200 to increase the gain or sensitivity level. 

    The LMP91000 is my preferred go to sensor IC because it can be adjusted for sensitivity and gain. The zero can be adjusted and and it can be used for other applications due to its versatility. 

    Lookup the LMP91000EVM/NOPB  and the users documents for it. https://www.ti.com/lit/pdf/snau121 This is a good reference to the design and layout.

    Layout is very critical due to the sensitivity of the AFE.