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TLV8541: Practical TIA limits

Stephen Fenwick
Intellectual 260 points
Part Number: TLV8541
Other Parts Discussed in Thread: LMP91002

I have an application with a gas sensor that has an output of about 5nA/ppm, and I want to be able to accurately detect very low concentrations (e.g. 1ppm) as well as high concentrations (e.g. 500ppm). Cost and power (battery application) are critical; we really can't afford more than about 10uA continuous draw, and cost is important so using a more integrated part like the LMP91002 is problematic.

I'd like to use this part with a very large feedback resistor (maybe 39Mohm) for the low ppm case and a smaller resistor (maybe 680k) in parallel with a FET to disable it for the wider range. Then the MCU that reads the output of the TIA could decide when to switch between them.

My question is, is it practical to use a 39M feedback resistor with this part, and expect accurate readings?

  • 0
    TI__Mastermind 25990 points

    Stephen

    At room temperature, your 39M feedback resistor should not cause any problem in detecting 1ppm of gas concentration since the error of even 1pA of input bias current is less than 0.1mV at the output.  However, you need to consider your highest operating temperature to determine your largest error that is contributed by bias current.  This part does not have a warranted bias current over temperature specification but the general rule of thumb is a 2x increase in bias current for every 10C increase in temperature.

    This amplifier was essentially designed with gas sensing in mind, so I think you should be ok going forward this option.  Most gas sensors don't see temperatures over 50C, so if that is similar to your application, you should be ok.

    Chuck

  • 0 in reply to Chuck Sins
    Intellectual 260 points

    Thanks for the quick reply.

    Ib(typ) is given as 100fA, so assuming 2x per 10C means we should be under 1pA even at 55C. 1V/39Mohm = 25pA, so it sounds like we're in good shape. And, we can have a temp sensor of some sort to help null out the temperature effects, correct?

    Thanks!

  • 0 in reply to Stephen Fenwick
    TI__Mastermind 25990 points

    You should be ok but I don’t understand your 1v comment. 1v across 39M would be 25nA, not 25pA. So pA bias current error is an even smaller percentage error than possibly what you initially thought.

    Best of luck on your application.

    Chuck

  • 0 in reply to Chuck Sins
    TI__Mastermind 25990 points
    Stephen
    There has not been a new entry in a while so I am closing this post out. Feel free to re-open the post by hitting reply if you need further assistance.
    Chuck