Reg Doubt in LMP91051 gas/Chemical sensor

Dear All,

One of my customer is in initial testing phase of the NDIR gas bench with Thermopile sensor ST120Quad with your LMP91051

Analog front end IC in the output of Thermopile sensor for signal conditioning.


They observed that the amplitude of sensor (at the output of AFE) is less. Please find attached the signals captured during passing

Fresh air and when passing a Gas mixture( CO= 3.57%, CO2=14.50% and Propane = 2090ppm). Channel 1 is CO and HC, Channel 2 is CO2 and Ref.

Gain set in channel 1 is = 4000 and Channel 2  = 8000   .

The channel switching time set is 5ms.  IR LED ON/OFF time is 250ms( 250 ms ON and 250ms OFF).


Please let us know whether this is ok? waveforms are attached for reference. please advise.


9 Replies

  • Hi Aravind,

    I will double check the costumer waveforms and the device settings with our LMP91051 designer, and I will let you know our comments as soon as possible.

    Best regards,

  • In reply to Carmine Iascone:

    Hi Aravind,
    I double checked with the LMP91051 designer, and we suspect that the problem could be in the way the thermopiles are connected to the LMP91051s. Could you kindly ask to the customer if he can forward to us the schematic of these connections for a double check?
    Thanks in advance.
    Best regards,
  • In reply to Carmine Iascone:

    Dear Carmina,

    Thanks for your reply. please find attached schematics, please help to review the same and advise your valuable feedback, thanks.

    Rgds/Aravind.SCHEMATIC1 _ Signal conditioner.pdf 

  • In reply to Aravind .R:

    Hi Aravind,

    The typical NDIR Sensing Application Circuit using the LMP91051 is shown below:

    So basically, for each gas that the customer want to detect, it is necessary to use a single LMP91051, and connect the IN1 pin to the Active thermopile, and the IN2 to the reference.

    Reviewing the customer schematic, the configuration of U3 is not correct, because the IN2 pin should be connect to the REF thermopile (pin 3, of the ST120 thermopile), and for the HC detection it is necessary to use a further LMP91051.

    Please let me know if you have any comments.

    Best regards,


  • In reply to Carmine Iascone:

    Dear carmina,

    Thanks. please review the attached schematics and also help to reply for below query.

    1. The LMP91051 (AFE) IC consists of only one ground terminal. This terminal should be connected to analog ground or digital ground?
    2. Each AFE has one CMOUT pin which will output 1.15volts in the case of 3.3volts power supply. We are planning to use 3 AFE’s, so shall we connect all the AFE’s CMOUT pins directly to make one CMOUT for the thermopile sensor or you have any precise method to combine all the CMOUT’s?  pls advise suitable method of combining the CMOUT.(in the schematics we have combined the CMOUT pins using resistors)


    1. We are also wanted to update our design using LMP91050 (4 numbers each one for a gas) which is basically single channel AFE. Is there any distinct advantage over dual channel AFE when we are using Single channel? pls clarify.
    2. Pls note that the LMP91050 does not have VIO pin. So there is no confusion of analog ground and digital ground. Why this kind of variations available for LMP91051 and LMP91050 (obviously both are used for same sort of applications)

     Rgds/Aravind.7026.SCHEMATIC1 _ Signal conditioner.pdf

  • In reply to Aravind .R:

    Dear All,

    Look forward for valuable reply on previous post, thanks.


  • In reply to Aravind .R:

    Dear Aravind,

    We reviewed the schematic, and our comments are the following:

    1. In the LMP91051, the analog ground and the digital ground are internally connected;
    2. It is ok to combine the three AFE’s CMOUT pins using resistors, but please modify their value from 10 to 10k ohm, in order to limit the current requested from the output buffers, in case of little mismatches between the three CMOUTs;
    3. In the ADCs, please connect directly the AFE’s CMOUT outputs instead of the combined:
      1. CMOUT1 to U6 VIN1N (pin 2);
      2. CMOUT2 to U6 VIN2N (pin 4);
      3. CMOUT3 to U7 VIN1N (pin 2);

    Switching from LMP91051 to LMP91050, there are the following advantages/disadvantages:

    • DISADVANTAGES: increase the BOM, using 4 LMP91050s instead of 3 LMP91051s;
    • ADVANTAGES: the overall system is simpler. In fact all the 4 thermopile outputs are available at the same sampling instant, and can be elaborated at the same time. Using the LMP91051, instead, only CO, CO2 and HC outputs are available at the first sampling step, while the REF output is available only at the second step. Furthermore, using the LMP91050s it is possible to use the serial interface only for the initial configuration of the AFEs, instead using the LMP91051, the serial interface is highly used, in order to switch continuously from the channel 1 to the channel 2.

    LMP91051 provides the flexibility to separate the analog and IO voltage references, if needed, in order to avoid the use of additional level shifters. Unfortunately, this configuration is not available in the LMP91050.

    Please let me know if this reply to your questions or you needs further information.

    Best regards,


  • In reply to Carmine Iascone:

    Hello Carmine,
    Does the circuit require Lamp with ST120 for CO2 for LMP91051 for NDIR operation. Will the circuit work with only LMP91051 ans ST120,

  • In reply to sumathi Sapthagiri:

    Hi Sumathi,
    Yes, in the NDIR applications, the lamp is required.
    Best regards,