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LMP91000: External gain for LMP91000

Laxmikant Savadekar
Prodigy 20 points
Part Number: LMP91000

We are using LMP 91000 with Ozone gas sensor manufactured by Spec sensors, USA. We have obtained necessary settings of LMP from the sensor supplier
Internal Zero - 20%
Vref = 2.5V
Rload = 10ohm
Gain : 350Kohm
Bias = -2%

We tried calibrating the sensor using above settings.
Voltage difference observed at point C2 in 0 and 5 ppm gas is just about 20 mV. Supplier informs that at least 80mV is expected due to change in gas concentration. Gases are from calibrated from Laboratory. Sensor supplier confirms the correctness of the schematic and also LMP settings. We are exploring the option of using higher gain. However LMP settings do not allow reducing R load less than 10ohm or increasing the internal gain more than 350KOhm.
We tried using external gain of higher value ( 450KOhms) but could not find much improvement.

Request your support to address this issue.

  • 0
    TI__Genius 9380 points

    Laxmikant,

      The sensor manufactures have standard settings for their sensors. However standard settings don't always apply to every design or IC that can be used to measure the sensor. The LMP91000 is a transimpedance amplifier that is very sensitive and low current.

    Most designers, begin with the incorrect settings for the LMP91000 board. Use the GUI Software that was created for the EVM to find a sensor that is similar or the same as you are using and change the default settings to these values to begin. Only if you are using a standard solution with the sensor can you know exactly what value you should be getting. Adjust the settings until you get the correct voltage feedback. 

    Let me know if you have any problems.

  • 0 in reply to Gordon Varney
    Prodigy 20 points

    Hello Gordan,

    Thanks for your reply. 

    Tried using the GUI program but it is asking for EVM board for sensor settings.

    Is it possible to use the GUI program without EVM board. We do not have EVM board as of now.

    Can we connect the evaluation kit received from sensor supplier. 

    Regards,

    Laxmikant

  • 0 in reply to Laxmikant Savadekar
    TI__Genius 9380 points

    Laxmikant,

      Yes, really what are are looking for is the list of sensors and then go into the register settings. The GUI asks for the EVM, but you can open and look around the GUI by itself. Find a similar or compatible sensor and see what the suggested settings are. Use these settings and see if it works for you. Let me know if you have any problems.