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LMP91000: issue when enter to standby mode with Alpha sense O2A3 sensor interfaced

FELIX GEORGE
Prodigy 10 points
Part Number: LMP91000

Tool/software:

I am developing a portable gas detector using LMP91000 and alpha sense O2A3 oxygen sensor. Its a battery operated device and it required sleep mode implemented. The O2 sensor interfaced with LMP91000 and i could able to read oxygen readings too. 

I am facing an issue while turn the LMP91000 in standby mode. I am measuring  the Oxygen concentration once in every second. 

When the LMP91000 turn to standby mode after the read, the VOUT is increasing with maximum out. There is no issue if i continue the LMP91000 in wakeup mode.

What is the reason for this issue? Could you please share the AFE configuration to put the LMP91000 to standby mode when O2A3 sensor interfaced.

There is no problem if i use the LMP91000 in 3 lead electrochemical mode. I could able to put standby and wakeup without any issue during the measurement time.

  • 0
    TI__Expert 5670 points

    Hi Felix,

    in standby mode, the TIA should be off. Do you have anything connected to C1 and/or C2 that could be pushing the Vout high?

    Thanks

    -Alex Thompson

  • 0 in reply to Alex Thompson
    Prodigy 10 points

    Hi Alex,

    Currently the C1 and C2 are kept open. No components are connected. I have attached my circuit diagram here for reference. 

    Regards,

    Felix George

  • 0 in reply to FELIX GEORGE
    TI__Expert 5670 points

    Felix,

    Are all the leads on the cell still on? The output can drift up in that case.

    Regards

    -Alex Thompson

  • 0 in reply to Alex Thompson
    Prodigy 10 points

    Hello Alex,

    I was investigating about the issue; The issue is happening when the LMP91000 is switching from one mode to another (standby to 3 electrode mode and 3 electrode mode to standby). Now I have connected a current meter in series to 2 electrode O2 sensor and the current measured is 70uA after the stabilization period.

    Currently the LMP91000 is at standby mode. when the LMP91000 switch from standby to 3 electrode mode there is a immediate raise in current from 70uA to 110uA and slowly back down to previous state(70uA). After that when the LMP91000 switch from 3 electrode mode to sleep, observing immediate down from 70uA to 40uA and slowly back to previous state(70uA). What is the reason to trigger this issue?

    Can you suggest LMP91000 configurations to switch from 3 electrode mode to standby and vise versa. My standby to 3 electrode switching period is once in every second and the AFE kept active only from a period of 10ms.

  • 0 in reply to FELIX GEORGE
    TI__Expert 5670 points

    Hi Felix,

    I would try setting up the LMP91000 like this:

    This schematic is from the LMP91000 EVM, and should work. I do see some minor differences between your schematic and this one, so try setting it up like this schematic and see if the issue disappears. This schematic also has an ADC, you do not need to copy that. Just focus on the LMP91000 configuration and how the 3-lead cell in connected.

    Regards

    -Alex Thompson

  • 0 in reply to Alex Thompson
    Prodigy 10 points

    Hello Alex,

    I am trying on hardware based on the above suggestion, Following are the AFE configuration during 3-electrode mode and standby mode,

    TIA_GAIN - 14k

    R_LOAD - 100E

    REF_SOURCE - External 

    INT_Z - 20%

    BIAS_SIGN - Positive 

    BIAS_VOLT - 0%

    FET_SHORT - Disabled 

    Can I use the same configuration in both 3-electrode and standby mode? Do you have any suggestion in configuration for 3-electorde and standby mode?

  • 0 in reply to FELIX GEORGE
    TI__Expert 5670 points

    Felix, you should not need to use a different configuration in standby mode, that should just be something that you can switch in the Mode register. For your external reference voltage, is it stable? Could it turning off A1 while in standby mode?

    Thanks

    -Alex Thompson

  • 0 in reply to Alex Thompson
    Prodigy 10 points

    Hi Alex, External reference voltage i am not maintaining continuous. We are using external voltage booster to maintain 3.3V from a 3V source. I just turning ON the booster before walkup the LMP91000. My reading sequence as follows,

    1. Turn ON voltage booster.

    2. Walkup LMP91000 from standby

    3. Starting ADC conversion and wait to complete

    4. Read converted ADC result

    5. Enter LMP91000 to standby

    6. Turn OFF voltage booster.

    When voltage booster at OFF condition, the Reference voltage switching to VDD as VREF. The above operation is continuing in every second(the entire read process takes around 10ms to complete 1 read).  Is it above sequence is fine to measure the concentration? Do you have any recommendation in this?

  • 0 in reply to FELIX GEORGE
    Prodigy 10 points

    Hi Alex,

    One more doubt related to sensor circuit. Is there any need of connecting external load resistance between CE/RE and WE for Alpha sense O2A3 sensor. 

    Regards,

    Felix George

  • +1 in reply to FELIX GEORGE
    TI__Expert 5670 points

    Felix,

    I would try not using the voltage booster and try your sequence with a constant/steady reference voltage. If 3.3V is needed, try it with the voltage booster always running. I don't think it is a good idea to keep changing the reference voltage between standby and active mode.

    There should be no need for an external load resistor, use the internal variable load.

    Regards

    -Alex Thompson

  • +1 in reply to Alex Thompson
    Prodigy 10 points

    Hi Alex,

    I have done the following changes and the vout is sturdy now by doing the following changes with few additional power consumptions.

    • AFE kept active during measurement
    • constant reference voltage

    Now the AFE working as expected with O2 sensor interfaced with the circuit.

  • 0 in reply to FELIX GEORGE
    TI__Expert 5670 points

    Felix,

    I'm happy to hear the VOUT is working properly now. Please reach out to us with any additional questions!

    -Alex Thompson

  • 0 in reply to Alex Thompson
    Prodigy 10 points

    Hi Alex,

    Sure. Thanks for the support.

    Regards,

    Felix