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LMP91000: AFE Output voltage

Vijay Kolaventy
Intellectual 870 points
Part Number: LMP91000
Other Parts Discussed in Thread: ADS8166, , ADS8332

Hello

I am trying to use the LMP91000 with gas sensors like Spec, Alpha, Citytech etc. I want to use 4 of them with ADS8166. My questions are

1. The Voltage reference from the ADS8166 is 4096mV. If use this as external VREF for LMP91000 with the internal zero set to default 50% how should I calculate the voltage output on the VOUT?

2. What the calculations to get matching VOUT from LMP91000 to the 16 bit ADC.

For example, the Spec CO sensor output is 4.75nA/ppp with a range of 0-1000ppm. I like to what should be my settings on the LMP91000 and how are they calculated so I can use them for other gases.

Regards

Vijay

Regards

  • 0
    TI__Guru* 90906 points

    Dear Vijay - 

    If you open the EVM GUI, you can choose from the AlphaSense and City Tech Devices settings there. 

    For example: 

    For the Spec Sensors device, as you have the table from it up there,

    in that table you can see they say the bias voltage is 0 to 5mV, and the scale is from 0 to 1000ppm with ~4.75nA/ppm output - 

    Those folks have an open source zip file you can register for and download, they have an LMP91000.c and .h file that you will hopefully find useful - 

  • 0 in reply to Josh Wyatt
    Intellectual 870 points

    Hello Josh

    I am trying to understand on how to calculate the VOUT assuming that the sensor output will be between 0-1000 ppm at ~4.75nA/ppm. Since we are not engineers and the software you have referred is for to help us to calculate settings for the LMP91000 but it does not help us in choosing the right VREF and ADC.

    Regards

    Vijay

  • 0 in reply to Vijay Kolaventy
    TI__Guru* 90906 points

    Vijay - 

    in the documents download section for the Spec Sensors, where i pointed yesterday for the .c and .h files that are part of the larger code example, there is also a PDF entitled  D-ULPSM Rev 0.3 LMP91000 Settings  - Here for part # 110-102 (their CO sensor) they list out Vref as 2.048 and the ADC they are using is inside the PIC24F on their EVM (schematic in same section) has 12 bit ADC. (MSP430 or MSP432 could be used here just as easily)


    LMP91000EVM can be connected to the AlphaSense and City devices and has 16 bit SAR ADC onboard http://www.ti.com/product/ADC161S626

  • 0 in reply to Josh Wyatt
    Intellectual 870 points

    Josh

    I already have these files with me and we have already got the sensor working with MSP4306989 launchpad. I also have the SPEC digital sensor kit. I want to learn the calculations behind these settings. I am trying to develop a 4 gas sensor with 4 LMP91000s and multichannel 16 bit ADC. I want to know what is the optimal VREF, and how calculate the settings for different gas sensors from different manufacturers. The ADS8332 and ADS8166 come with 4.096V VREF. I dont know if this is suitable. I want to understand the rationale so I will be able to decide the BOM and also the settings.

    Regards

    Vijay

  • 0 in reply to Vijay Kolaventy
    TI__Guru* 90906 points

    Vijay - 

    The LMP91000 is a transimpedance amplifier, which means it converts current to voltage. The voltage output is  basically equal to the the reference voltage at the non inverting pin of the TIA minus the gain times the current delivered by the working electrode pin. 

    Therefore, Vout = (Vref_div (internal to LMP91000)) - (RTIA (gain) x Iwe)

    Page 5 here has a worked example using CO-AF sensor from Alphasense ==>