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LMP91000 Sensor AFE configuration settings verification

Natarajan Chockalingam
Prodigy 65 points
Other Parts Discussed in Thread: LMP91000

Hello,

Sensor AFE : LMP91000

Parameters : Programmable TIA gain, Internal Zero selection (Reference voltage), variable BIAS voltage, FET switch

I am trying to figure out the proper settings for various toxic gas sensors using LMP91000 eval board.  I can read/write configuration registers through I2C interface and was able to verify them with the logic analyzer. I would like to verify the effects of these configuration register settings like programmable internal zero, programmable TIA gain, and variable bias on the LMP91000 eval board before placing any actual sensor.  The only parameter i was able to verify is the programmable internal zero which shifts the output scale at the Vout pin.  Could someone explain me how these parameter settings affect the 3 electrode pins and help me with simulating a small input range and playing/verifying with these programmable registers in the stand-alone mode ??  I also have the two interface boards -- SPIO4 and ADC161S626BEB for interface with the computer through usb and configured using the WEBENCH graphical user interface.  

Thanks,

Raj

  • 0
    TI__Prodigy 725 points

    Hi Raj,

    let me try to explain.

    What you can program with the LMP91000 is

    • the internal zero. You have verified this, by measuring the output, which shifts its DC value.
    • The TIA gain. One way to measure this, would be injecting a small current into the WE pin. You can also apply a small voltage to the WE pin and you will observe the respective change in voltage at the output. Really this amplifier can also be seen as a inverting voltage amplifier, with a gain of RTIA / RLOAD. This voltage off course would need to be respective to the internal zero, not board ground.
    • Finally the bias voltage. You can just measure this voltage using a DMM. If you put the board in front of you, so you can read the text on it, you need to measure between the middle and the lower pin.

    May I ask you, why you try to do these measurement?

    Hope I could help you.

    Best regards

    Andreas Kraemer

  • 0 in reply to Andreas Kraemer
    Prodigy 65 points

    Thanks for the explanation Andreas,

    With the bias voltage, i still could not see the bias voltage between the WE and CE pins.  It always remain at the same potential (basically (vdd - internal zero)).The reason i am trying to do the measurements is that, I do not want to place a toxic gas sensor with improper configuration settings.  I just wanted to understand how each parameters work on the board before testing.  Could you help me with this bias voltage measurement?  Thanks again!

    Regards,

    Raj

  • 0 in reply to Natarajan Chockalingam
    TI__Prodigy 725 points

    Hi Raj,

    that sounds strange. I am attaching a picture here, how I did the measurement. Vbias was set to 10% of Vref = 0.4096V, which is what I measure.

  • 0 in reply to Andreas Kraemer
    Prodigy 65 points

    Hello Andreas,

    My mistake was, I did not short the CE and RE in the J_CE_RE_GND jumper.  Thanks a lot for the info.  

    Also, it would be good to have a custom configuration setting that could be saved on the chip in addition to the default setting (FAB default) during power on.  In that way, it could work standalone without a need for the microcontroller to configure everytime when it is powered on. 

    Thanks again,

    Raj

  • 0 in reply to Natarajan Chockalingam
    TI__Prodigy 725 points

    Hello Raj,

    about the configuration setting. Unfortunately this is not possible with the LMP91000. It would require internal non-volatile ,e,ory, which the LMP91000 does not have. It may be possible to include this in future parts, if technology allows.

    Andreas

  • 0 in reply to Andreas Kraemer
    Mastermind 7675 points

    Hi Andreas,

    It is possible to reconfigure the lmp91000? 

    What I do is:

    1) Configure lmp91000 at start up

    2)go into lmp3

    3)measure variable

    What I want to do is between 2) and 3) reconfigure lmp91000, is this possible?

    Gastón

  • 0 in reply to Gaston_Melo_Arg
    Mastermind 7675 points

    Solve it!! Time between configurations

    Regards

    Gastón

  • 0 in reply to Gaston_Melo_Arg
    TI__Prodigy 725 points

    Hi Gaston,

    not sure if I understand correctly, you problem is solved?
    Andreas