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Original question:

CCS/LMP91000: wiring of 4-electrode gas sensor with LMP91000

CCS/LMP91000: AFE circuit for 4 electrode gas sensors using LMP91000 & LM7721

Manaskant Mishra
Prodigy 20 points
Part Number: LMP91000
Other Parts Discussed in Thread: LMP7721

Tool/software: Code Composer Studio

Hello Josh,

Is it necessary that the Rgain and Rload of LM7721 be same as that of LMP91000?

  • 0
    TI__Genius 9380 points

    Manaskant,

      I don't have the part number of your sensor. However, in many cases the fourth terminal is much like a second sensor in the same package and is used for correction and verification.

      Normally the gain and load resistors would be the same as the primary op-amp gain and load resistors, unless your sensor requires gain compensation for correction, then follow the instructions for your sensor.

  • 0 in reply to Gordon Varney
    Prodigy 20 points

    Dear Gordon,

    I'm interfacing the "4-electrode A4 series gas sensors" from "Alphasense" namely CO-A4, SO2-A4, etc with LMP91000 and LM7721. The thing is, I have successfully interfaced the CO-A4 sensor by keeping the Rload and Rgain of LM7721 same as that of LMP91000 and it gives nearly the same result and sensitivity as the default AFE board provided by "Alphasense" itself. However, in case of SO2-A4 I'm not getting the desired result by keeping the configurations same for both ICs. Thus, I wanted to know if there is any hard and fast rule or formula to choose the gain and load resistors for LM7721. Furthermore, in some threads I have seen LM2251 used for the 4th electrode. So, I'd be grateful if you can tell me as to which one of these ICs (LM7721 or LM2251) be more suitable for my application.

    Sincerely,

    MANASKANT MISHRA

    Embedded Firmware Developer / R&D Engineer

    D&K Technologies

  • 0 in reply to Manaskant Mishra
    TI__Genius 9380 points

    Manaskant,

      Yes you may have to adjust the Rload and Rgain values as needed. This is not uncommon.

      As for as the LM2251, I can't find any information on this part number. I do know that the LMP7721 is a great part for sensor applications due to its ultra low input current and low noise. You will be happy with this part.