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Irregularity in the working of LMP91000...

Atul Goel
Prodigy 10 points
Other Parts Discussed in Thread: LMP91000

Hello,

           I am using lmp91000 with different type of sensors having same characteristics i.e. the output voltage should increase as the increase in that particular gas, but i am getting opposite responses with some gases it is increasing and with some gases it is decreasing with the same settings of the lmp91000, Please help what could be the issue and suggest me the solution.

  • 0
    TI__Genius 9655 points

    Hi Atul,

    have you left enough time to the sensor to reach a steady state conditon ?

    Are they biased ? If so have you stored them wiht bias connected ?

    If they are not biased, have you stored them wiht RE and WE shorted ?

     

    regards,

    Domenico.

  • 0 in reply to domenyko
    Prodigy 50 points

    There is great Irregularity with temperature of lmp91000. For instance I would like to measure 16 ppm of CO, in normal condition of 25C everything is good, if lmp91000 cooled to -25C, baseline goes to -16 ppm or other lmp91000 to +16ppm without gas. There was cooled ONLY lmp91000.

  • 0
    TI__Genius 9655 points

    Hi Atul,

    can you provide me wiht the part numbers of your gas sensors ?  I want to double check the sign of the current.

    regards,

    Domenico.

  • 0 in reply to Dmitriy Silaev
    TI__Genius 9655 points

    Hi Dimitriy,

    can you provide me wiht the register's configuration of the LMP91000  and the part number of the CO sensor ?

    regards,

    Domenico

  • 0 in reply to domenyko
    Prodigy 50 points

    Hi Domenico!

    gas sensor is 7E/F, (sensitivity is 0.1 microA) of CiticeL, I tried Alphasens also, there was the same result.

    registers:

    TIACN - address 0x10      data 0x14,   35k,  10 Om

    REFCN - address 0x11    data 0x00,   BIAS 0,    SIGN 0,  INT_Z =20%, REF_SOURCE internal = 5V

    MODECN -address 0x12   data 0x87     Temperature measurement (TIA ON)   Shorting Fet enabled

    Everything works good untill temperature....

                                                                                                        

  • 0 in reply to Dmitriy Silaev
    TI__Genius 9655 points

    Hi Dimitriy,

    if I well understood you are cooling just the LMP91000 (the gas sensor is not at -25°C) and you are measuring the baseline (programmed internal zero, in your case 20% of VDD). If so why have you enabled the shorting FET feature ? Moreover if you are in Temperature measurement TIA ON, are you measuring the baseline at C2 pin ?

    I would try the measurement without the FET feature enabled.

    A shift in the baseline is expected (TcVos of the TIA) and it is about 4uV/°C.

    regards,

    Domenico.

  • 0 in reply to domenyko
    Prodigy 50 points

     Hello Domenico! You understood well.The gas sensor is not at -25°C. And I am measuring the baseline at C2 pin. Usually FET connect WE and RE electrodes when device is switched off, and I thought that FET in LMP91000 is working when device is in idle mode. Can you explain function of FET in LMP, and in what period of time it should be selected? It would be great if shift in the baseline is  4uV/°C. Tomorrow I will try without FET feature.

  • 0 in reply to domenyko
    Prodigy 50 points

    Great Domenico! You are right! The problem was with FET.

  • 0 in reply to Dmitriy Silaev
    TI__Genius 9655 points

    Hi Dimitriy,

    The FET feature needs to be enabled when you put the LMP91000 in the following mode of operaitons:

    Standby

    Deep Sleep

    Temp measurement TIA OFF

    In few words, the FET needs to be enabled when the TIA is OFF, it ensures that WE and RE stays at same potential and ensure a faster startup when the TIA is turned on.

    Since the FET bit and the Mode bits are in the same register (MODECN), you can select both in one command.

    regards,

    Domenico