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LMP91000: Modeling 3-Lead Amperometric Cell in Potentiostat Configuration

Victor Aprea
Intellectual 940 points
Part Number: LMP91000

I'm using the LMP91000 in a product, and I'm trying to model the LMP91000 for simulation purposes. I'm struggling with the statement in datasheet section 7.3.1.6., which I find pretty difficult to parse in the context of the block diagram.

> ... in this configuration the Control Amplifier (A1) is ON and provides the internal zero voltage and bias in case of biased gas sensor.

If I model the sensor as two resistors, one connecting the Counter Electrode to the Reference Electrode, and one connecting the Reference Electrode to the Working Electrode, and put A1 and TIA amplifiers into the my simulation wired per the block diagram, providing the configured Bias Voltage to the positive terminal of A1 and the configured Internal Zero Voltage to the positive terminal of TIA, and simulate the outcome, I don't get the Bias Voltage reproduced across the Reference and Working electrodes.

My simulation only comes out right if the Bias Voltage is expressed relative to the Internal Zero Voltage? That is to say if my Internal Zero is set to 1.5 Volts, and I set the Bias Voltage to -0.2V, then for the purposes of simulation I have to set the positive terminal of A1 to 1.5 - 0.2 = 1.3V in order to measure 0.2V between the Reference and Working Electrodes.

Have I understood this behavior correctly?

  • 0
    TI__Mastermind 43785 points
    Hi Victor,

    I apologize for the delay, but our gas sensor expert will get back to you soon.

    Thanks,
    Ren
  • 0
    TI__Expert 4875 points
    Hello Victor,

    The datasheet does not specify what voltage you should be seeing at the cell inputs, but rather the final output of the LMP91000. Have you measured VOUT? Why do you expect the bias voltage to be present between RE and WE?

    The variable Bias Voltage you program is referenced to ground, and so is the Internal Zero voltage. The voltage from WE to RE will not be the same. From my understanding, the Bias Voltage is chosen based on the electrochemical cell that you are using.

    For design guidance to determine the right bias voltage and settings for the LMP91000, take a look at our simulation tool in WEBENCH to model your application. This tool allows you to choose from a variety of different electrochemical sensors and will help to choose the proper configurations for the LMP91000. It is located on the right-hand side of the product folder page.

    Thank you,
    Audrey
  • 0 in reply to Audrey Dearien7
    Intellectual 940 points
    One of the major functions of the LMP91000 is to produce a configurable bias voltage between the working and reference electrodes. ..
  • 0 in reply to Victor Aprea
    TI__Expert 4875 points
    Hello Victor,

    What is the reason for generating 0.2V from WE to RE?

    The bias voltage block is used to configure the bias, but is not equal to the voltage you will measure from WE to RE. If your sensor requires a bias voltage of 0.2V, then you will configure the Variable Bias Block to be 0.2V. The bias is set based on the expected bias of your electrochemical cell at RE, and the error between RE and V_Bias is compared at A1. That error is amplified to compensate the electrochemical cell such that the voltage potential between RE and WE remains constant.

    Regards,
    Audrey
  • 0 in reply to Audrey Dearien7
    Intellectual 940 points
    Sure, I'm just trying to simulate an electrochemical cell as two resistors, though, and by doing what I suggest, I'm able to get the bias voltage to show up where I expected it. Thanks though, I'll mark this as resolved.