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LMP91000 with 4-lead fuel cells

Other Parts Discussed in Thread: LMP91000, LMP7701

Hello everyone,

I recently learned about the LMP91000 AFE and was excited to use it in my application. However, I use 4-lead fuel cells from City Technology.

I was wondering if anyone has found a workaround to interface 4-lead sensors with LMP91000. If it's not possible, to use LMP91000, does anyone in Texas know if there are plans on developing a new AFE chip to intarface 4-lead fuel cells?

Thanks in advance,

André Araujo.

  • Hi André,

    are you referring to the dual gas sensor 4COHS (H2S + CO)? If so we do not have yet a solution. Those sensors require 2 independent TIAs.

    regards,

    Domenico

  • Hello Domenico,

    Actually, my application uses these sensors: http://www.citytech.com/PDF-Datasheets/mno2.pdf and http://www.citytech.com/PDF-Datasheets/mnd2.pdf.

    Thanks for the support,

    André Araujo.

  • Hi André,

    I went through the Datasheet you sent, I've never seen that cells before. The AUX pin offers a good feature to remove the effect of the temperature.  I have some questions and maybe you can help me in order to identify a possible way to use the LMP91000 with those cells.

    1. I've read that AUX pin and WE have same behaviour at temperature changes, so have I to assume that AUX pin sources/sinks a current (baseline current) as the WE or AUX pin is a voltage output ?

    thanks,

    Domenico

  • Hi Domenico,

    Indeed the "Auxiliary" pin seems to work in the same way as the WE (named "Sensing" in City Tech's documentation). The recommendation from City Tech is that 2 current follower amplifiers should be used in the AFE and then the outputs from these two amps should be subtracted. The AUX pin is used to compensate for temperature effects as well as improve interference response from other gases. "Reference" and "Counter" pins must be connected to the potentiostatic circuit.

    The recommended circuit for NO2 cells is the following:

    The recommended circuit for NO cells is the following:

    Maybe using two LMP91000 could work (one for te WE and another for the AUX), but somehow it would be necessary to use CE and RE pins connected to both LMPs or just one... What do you think?

    Or if there's a standalone TIA available, then it wuold be possible to build an extra amplifier to deal with AUX pin.

  • Hi André,

    I've worked around the LMP91000 and I came with 2 possible solutions which should work for both biased and zero bias gas sensor:

    First solution. 2x LMP91000 (programmed with same internal zero and bias voltage), there will be a minimal offset between the 2 internal zero voltages (maybe less than 1mV), At CE=R pin of the second LMP91000 you have a buffered version of the internal zero (at least for zero bias gas sensor).

    Second solution 1x LMP91000 + 1x LMP7701 + JFET p channel + Rload + RTIA (in this case the internal zero offset of the External TIA is given by the offset of the LMP7701)

     

     

     

  • I've attached the pictures of the schematics.

  • Hi Domenico,

    I think it's worthy giving a try in one of those solutions! Thank you very much for the support.

    Best regards,

    André Araujo.

  • Hello guys

    Can I get an update on this subject? Did any of you actually design a solution for the dual sensor (4-lead).

    In my case, I'm interested in the 4COSH (CO+H2S) sensor, and I'm leaning towards the 2x LMP91000 solution but I haven't designed it yet.

    // Daniel, INFICON

  • Any progress in designing for the 4COSH?

    Fiachra