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LMP91000 AFE

Other Parts Discussed in Thread: LMP91000

Hi. I purchased an LMP91000SDEVAL board to be evaluated for 3 electrode sensor. Our plan is to put 0.5V between WE and RE. The current I am expecting to read is from 10nA to 200nA. Will LMP91000 works for this current range? I would like to know the suggested settings on Vref, Vdd, TIA. The Vout will be going to an ADC with Vref of 2.5V. Thanks.

  • Hi Fishead,

    below the setting of the LMP91000 and external supply/Reference.

    Vdd=3.3V

    Vref=2.5V

    Bias: 20% VREF

    RTIA = 350Kom

    Internal Zero = 67% VREF (I'm assuming the current is entering in to the WE pin of the LMP91000),

    regards,

    Domenico

  • Dear Domenico,

    What's the importance regarding whether current is entering or leaving WE pin of LMP91000 respecto to the Internal Zero ?

    My registers are:

    Vdd=5V

    Vref=2.5V

    Bias: 8% VREF

    Bias Sign: positive

    RTIA = 350Kom

    Internal Zero = 20% VREF

    For this configuration what is the current range LMP91000 can measure ?

    Thank you.

    Best regards,

    Alejandro

  • Hi Alejandro,

    if the current flows in the WE pin, the output of the TIA will start from the Internal zero level (0 ppm) and decreases (reaches 0V) as soon as the gas concentration increases.

    If the WE flows out from the WE pin, it works in the opposite way, at 0ppm the VOUT of the TIA is equal to internal zero and as soon as the gas concentration increases VOUT appraches the positive rail.

    In your case, I do not know the sign of the current, but I can say the following:

    if the current flows in the WE pin, the output dynamic is jsut 500mV, so the input current will be at max 500mV/350Kohm,

    if the current flows out form the WE pin, the output dynamic is 4.5V, so the input current will be at max 4.5V/350Kohm.

    So you can see that setting the Internal zero in the right way (according the sign of the current provided by the sensor), it is possible to have a better output  dynamic.

    regards,

    Domenico

  • Dear Domenico,

    Since there is the electronic current and the ionic current, I get a little puzzled with this sign convention.

    Using the tester, I can measure the following voltages:

    Vwe - Vre = -200 mV

    Vwe - Vce = -888 mV

    Vwe - GND = 500 mV

    Vre - GND = 701 mV

    Vce - GND = 1,391 V

    Also the current measured using AFE software is negative. For a dummy cell of 1 Mohm, I get -200nA.

    What can you suggest ?

    Thank you.

    Best regards,

    Alejandro