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LMP91000: Expected behaviour of bias blocks in 3 lead amperometric cell mode when INT_Z is set to bypass?

Robert Alfonsi
Prodigy 80 points
Part Number: LMP91000

I am planning to use the LMP91000 in an evaluation project with a 3 lead amperometric cell: an NO2 cell, requiring a negative bias.

I wanted to be able to have access to the precise voltage used for the internal zero at the trans-impedance amplifier and one way to do this seems to be to use bypass mode for INT_Z where I am providing the INT_Z from an external reference source.

If I then use the LMP91K in 3 lead mode, will the A1 amplifier and variable bias blocks still be in functional mode i.e. driving the external Vref plus/minus (0-24%) as bias to the CE terminal?

  • 0
    TI__Expert 4875 points
    Hello,

    Yes, the internal zero that you use to drive CE is based upon your choice of reference, either external or from the supply voltage. The external reference voltage you provide will be used for both the TIA and A1. When you selected the External Reference option of REF_SOURCE, all of the following selections, INT_Z, BIAS_SIGN, and BIAS are all based upon that External Reference voltage.

    When you program INT_Z to bypass the internal zero circuitry, you directly connect VREF to the TIA and you still have control over VREF Divider/Variable Bias for the A1 circuit.

    Regards,
    Audrey
  • 0 in reply to Audrey Dearien7
    Prodigy 80 points
    Great thanks Audrey for the speedy reply!
  • 0 in reply to Robert Alfonsi
    Prodigy 80 points
    Hi Audrey, I wonder if you can confirm you have tested out this behaviour you describe on silicon?

    I have had a board laid out and was monitoring the bias output. It seems to work as expected for internal zero fields set to 00,01,10 (20, 50,67%), but when set to bypass 0x11, the bias output slowly drops to about 33% of Vref, suggesting it is high impedance; The bias block does not look active and does not change its output much (only about 2%) when toggling the bias from positive 10% to negative 10% of Vref.
    regards
    Robert
  • 0 in reply to Robert Alfonsi
    TI__Expert 4875 points

    Hi Robert,

    I believe I misunderstood before. You are correct that A1 is off when the bias circuitry is bypassed. In order to operate in 3-lead cell mode, you must use the internal zero voltage based on the options in the register address INT_Z (67%, 50% or 20%).

    Regards,

    Audrey