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LMP91000: Test points for LMP91000

Jason Kahana
Prodigy 140 points
Part Number: LMP91000

Dear Community,

I am trying to implement the LMP91000 for a dissolved oxygen probe (2 wire) that requires ~670 mV bias.  I am using a Cypress PSoC MCU with their internal VDAC at 3360 mV driving into VREF of the LMP91000.  I intend to use the PGA at 20% to get me to 672 mV final.  I am currently trying to test my setup and I'm getting some weird results.  Firstly the VDAC out is reading 3358 mV when unconnected to anything, but once I connect it to the VREF on the LMP91000, i get 1353 mV at the pin on the PSoC.  Vout on the LMP is giving me the same value.  

My settings are as follows:

LMP91000_TIA_GAIN_EXT | LMP91000_RLOAD_10OHM,
LMP91000_REF_SOURCE_EXT | LMP91000_INT_Z_50PCT | LMP91000_BIAS_SIGN_POS | LMP91000_BIAS_20PCT,
LMP91000_FET_SHORT_DISABLED | LMP91000_OP_MODE_AMPEROMETRIC

Can someone help me locate my 672 mV?

  • 0
    TI__Expert 5145 points

    Hi Jason,

    you are using a 2-wire sensor, so you should connect RE and CE together as in the following picture:

    without any sensor connected to the LMP91000 you should see the internal ref = 3.360V / 2 = 1.680V between WE and Ground.

    On the RE or CE pin (that you should have connected together), you should see the internal zero + vbias = 1.680V + 0.672V = 2.352V referred to Ground.

    The 672mV should be then present CE (or RE) and WE.

    Please let me know if this replies to your question.

    Best regards,

    Carmine

  • 0 in reply to Carmine Iascone
    Prodigy 140 points

    On WE, I am indeed getting 1.680V.  That's great!

    On the RE/CE pins, I am NOT gettng 2.352V-  I am getting 1.008V, which is 1.680 - 0.672.  

    That said, I'm getting the 672 mV from WE referred to CE.  This is exactly what I want.

    Is this how it should work.  I have the bias sign as positive.  

    Thanks so much!!!!