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LMP91200MT pH Sensor

Other Parts Discussed in Thread: LMP91200

Hi hello community!

I'm preparing a design for pH meter.

I've received the samples of LMP91200 pH sensor provided by Ti / National.

We came across a simple question during the reading of the datasheet and application notes.

We are going to use Cortex-M3 LPC1769 as the main processor and we would like to know how the pH measure comes out the sensor.

To clarify that, we need to know how to read the measure value in mv. to be converted in the pH measure at the Cortex processor. Is it enough to connect the Vout PIN to the microprocessor?

Which pH probe do you recommend to use?

Please, if you need any further details on my question, let me know.

This is the most critical part of the design.

Regards,

  • Hi Mariano,

    the LMP91200 can be placed between the pH probe and the ADC of your microcontroller. When the LMP91200 is configured in pH measurement, at the Vout pin you have a voltage which is proportional to the pH of the liquid under test.

    The output voltage of the LMP91200 can be configured in order to be ground referred or Vocm referred.

    pH unit = 7+Vout_LMP91200/alpha,

    where

    alpha=-59.16mV/pH @ 25°C

    Vout_LMP91200 = VOUT (gnd referred) - VCM in case of LMP91200 set with single ended output

    Vout_LMP91200 = (VOUT-VOCM)  if the ADC is connected between VOUT and VOCM pin.

    Regarding the pH probe, I cannot suggest one, it is strictly dependent on the application. Since the principle of operation of a pH electrode is based on a well known equation (Nernst equation) any combined pH probe can be connected to the LMP91200.

    Regards,

    Domenico Granozio

     

  • Domenico,

                      Your answer was extreamly helpful. Thank you so much!!

    Best Regards,

      

  • Hi All,

    I need your suggestion, I am making pH meter by using LMP91200 IC. I did not able to find out ph by numeric calculation. I am finding by this way

    pH(X) = pH(S) + [ E(S) - E(X) / Z ]  

    Where

    pH(S) = known pH value inside sensor is 7

    E(S) = reference electrode potential known Ag/Agcl  is o.22 V

    E(X) = measured electrode potentials (output range of LMP91200 is -0.3 to 3.9 V )

    Z = Nernest factor equal to Z= 2.3 RT/F

    by solving above equation i am getting  pH(X) = -54.90076  using E(X) = 3.9 V & temperature is 300 K

    Please give me suggestion for finding pH value. 

     

  • Hi Rahul,

    I suggest the follwing approach:

    Ph = 7 + Vout_LMP91200/alpha(T)
    Vout_LMP91200 is measured in mV
    alpha(T) = -(1/298)*T*59.16    (mV/Ph init)
    T is measured in degKelvin

     When you read the output of the LMP91200, remeber to remove the VOCM contribution.

    regards,

    Domenico

  • Thanks Domenico,

    According to ph = 7+ Vout_LMP91200/alpha(T)  I  got accurate pH value. But another problem is with Vout_LMP91200. My sensor output is +-386.2 mV then I connected at INP and VCM pin of LMP91200. 

    According to LMP91200 data sheet I found output is  -0.3V to VDD +0.3V then output becomes  - 0.3V to 3.9V for VDD = 3.3V.

    My question is for pH how much I will get differential output voltage at Vout pin of LMP91200 for -386.1 mV to +386.1 mV input. Please clarify my problem. 

     

  • Hi Rahul,

    when you connect the pH probe between INP and VCM the anolog ground of your probe is forced at VCM potential.

    It means that if you have a VREF=VDD=3.3V and the internal analog ground is set 1/2VREF, your sensor will have an output which will be in the range between (1.65V-386.2mV) to (1.65V+386.2mV).

    It is true only if your liquid under test is not grounded. In case the liquid is grounded the LMP91200 cannot be used.

    regards,

    Domenico

  • Hi Domenico,

    I have ADWAelectrodes AD1230 probe. It have only BNC connector. There is not guard pins. I can not receive any reasonable analog values. When I close my hand to BNC connector analog Readout of LMP91200 changes. But if I use lemon or water for Sensor there is not any changes. What do you think about this situation?

    I'm looking for your reply,

    Best Regards,

    Mehmet Emin