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LMP91000: how is VOS_RW measured?

Vincenzo Pizzolante
Mastermind 25426 points
Part Number: LMP91000

hi,

In LMP91000 datasheet is not very clear to  us how VOS_RW is measured.

Our believe is that:

  • VOS_RW = -550uV;
  • Bias = 0V;
  • Internal Zero=50% VCC (3.3V/2 = 1.65V);
  • RL = 10 Ohm;
  • TIA_GAIN = 35k;
  • 3-electrode sensor is connected with zero output current;
  • LMP91000 in 3-electrode Amperometric mode;

Offset voltage between RE and WE causes current to flow into TIA through 10 Ohm resistor (-550uV/10 Ohm = -55uA), then TIA gain current for 35k (55uA*35k = 1.925V), Vout achieves GND rail and cutted-off, cause we use Vcc=+3.3V.

Currently with our samples of LMP91000, VOS_RW is very small, but during manufacturing, is probability that we have samples with -550uV offset, which is a no-go for our apps.

Any advice is more than welcome

KR

Vincenzo

  • 0
    TI__Genius 9675 points

    Hi Vicenzo,

    What values are you seeing for your Vos_RW measurement? 

    I would say based on your configuration and datasheet spec, it could be possible for you to see at most a 550uV offset. Unfortunately, we have limited resources for understanding some of our datasheet specs but I will reach out to a colleague and try to get clarification on how this spec was measured.

    Thanks,

    Jalen

  • 0 in reply to Jalen Tate89
    TI__Mastermind 25426 points

    hi Jalen,

    currently we have -1.14uV at root Temp.

    For what we understood, Vos_RW is +/-550uV in the -40+80*C range, and if we perform temperature compensation, +/-550uV-18.6% = +/-447uV.

    Correct? If so, there should be problem, just decreasing TIA gain to 14k and increase ADC resolution.

    But Vos_RW voltage forces us to do calibration of offset error (Vos_RW) and gain error (TcVos_RW) for each sample.

    Even removing offset caused by Vos_RW, gain error caused by TcVos_RW of +-4uV/*C in range of +-25*C gives +/-100uV; +/-100uV/RL = +/-10uA, for our sensor with sensitivity of 40nA/ppm, that will be 10/0.04=+/-250ppm.

    This is an absolute error which will be added to gain error of own sensor. Sensor have measurement range 0 - 300ppm, thus is there no way without calibration with two points (offset&gain). Correct?

    We just want be sure that we correctly understand how to use the spead data. Vos_RW - it is a sum of OpAmp's input offset voltages of A1 and TIA. Correct?

    We plan to do calibration during manufacturing by shorting CE & RE pins in Amperometric mode, A1 will be with gain = 1, and at CE will be it's input offset voltage. Connecting to that point WE pin, allow current to flow in/out from TIA, Vout value we can measure (enshuring TIA gain), and by multiplying measured current (which at input of TIA) with RL, gives us Vos_RW value.

    Sounds reasonable or are we mistaking / misunderstanding something?

    thanks a lot in advance

    KR

    Vincenzo

  • 0 in reply to Vincenzo Pizzolante
    TI__Genius 9675 points

    Vincenzo,

    I am not quite sure where the number of 18.6% for temperature compensation comes from.

    But to answer your questions, you will need to calibrate for both the offset and gain. It is important to note that the +/-4uV/C is spec'd at the extremes of the -40 to 85C range so most likely not at 25C.

    Overall, looking at your approach for doing calibrating seems to be a reasonable approach.

    Jalen