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Part Number: LMP91000
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?
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.
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In reply to Carmine Iascone:
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!!!!
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