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Analog front end design for electrochemical gas sensors

user5175262
Prodigy 10 points
Other Parts Discussed in Thread: LMP91000, ADS1226, LMP90100, ADS1120, ADS1220, ADS114S06, ADS124S06

Hello,

I was working for some months with electrochemical gas sensors typically from Alpha Sense company which require an analog front end to be designed. Until now I did some prototyping and I have created some AFE using operational amplifiers and other passive components. Lately I ran on the LMP91000 chip from TI and seems very nice especially for 3 pin electrochemical gas sensors. Until now I used the A4 sensors from alphasense and I did not considered this chip also from the fact that those sensors have 4 pins and an external operational amplifier was required in addition to the LMP. 

In this moment I need to use sensors from spec-sensors.com which are 3 pin and LMP91000 can be used, but those sensors also have a smaller output current typically 4.5-20 nA/ppm. I need to be able to measure tens of ppbs therefore I suppose I need a big amplification. Moreover I need to have 4 of them on a single board. Alongside with the 4  LMP91000s I want to use a 24 bit ADC to convert the output of the AFE into numerical form. I have found the following ADC from TI: ADS1226 which has 2 differential channels therefore for 4 sensors I will need to use 2 of them and LMP90100 which has 4 differential channels and also another PGA inside which, I think, will help with the amplification.

Last note that I want to mention is that I am not an electrical/electronic engineer, but a computer science one and therefore I have limited knowledge of analog design. 

My questions are: is the LMP91000 suitable for those sensors(from spec)? Is the LMP90100 ADC the better choice to go along with LMP91000 AFE or should I seek another one? Any other guidance when using such sensors is welcome.

Thank you!

  • 0
    TI__Expert 5695 points
    Hello user5175262,
    As an starting point to determine if the LMP91000 is a good fit for your 3 pins sensors I recommend to use a design tool called Webench(r). This tool can be launched going to the webpage with link listed below and is located in the middle-right portion of the page. The Webench(r) interface for LMP91000 will allow you to select on different sensors or load the key parameters (e.g. sensitivity nA/ppm, full scale range ppm, etc.) of yours if is not there. There is a nice tutorial before using the tool.
    www.ti.com/.../toolssoftware

    The ADS1226 looks a great ADC option due to an excellent dynamic range and programmable gain. We found that for majority of applications the ADCs inside most common micro controllers (typically 12-16 bits) are good enough. Please refer to the applications section (page 24 on the LMP91000data sheet).
    www.ti.com/.../lmp91000.pdf

    Hope this helps and let us know if you need another question.

    Regards,
    Jose
  • 0 in reply to Jose Arteaga4
    TI__Mastermind 20885 points
    Hello user5175262,

    I just wanted to comment on the potential ADC options. The LMP91000 provides a single-ended output. In that case you need an ADC that can measure 4 single-ended inputs.
    I would look at ADS1220 (24 bit) or ADS1120 (16 bit) or if better performance is required then ADS124S06 (24 bit) and ADS114S06 (16 bit).

    Regards,