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LMP91000 VREF question

Gaston_Melo_Arg
Mastermind 7675 points
Other Parts Discussed in Thread: MSP430F2274, LMP91000, LM4120

Hi to all,

I'm using the lmp91000sdeval connected to the msp430f2274 board. I'm able to configure the lmp91000 through the I2C lines. As it says in the lmp91000_board.pdf  pag 3

the vref must be between 1.5V to Vdd (if not connected PVOUT is 0). I'm planning to use the lmp91000 in a low power applcation and the lm4120 that is used as vref in the board consumes 160uA ( not soldered in the board).

What are the drawbacks If I connect the Vdd line to the Vref pin?

Regards

Gastón

  • TI__Prodigy 725 points

    Hi Gaston,

    it depends. Are you planning to use zero biased sensors or biased sensors?

    For zero biased, if you connect Vref (you can do this internallly and leave the pin open) to VDD, you internal ground, the zero gas voltage at Vout will depend on VDD. If your µC measures VDD, this should not be a problem, as in this case VDD is known.

    For biased sensors, also the bias voltage would be derived from VDD, which might cause some problems to the sensor, because the bias voltage might get out of spec. Again not too much of an problem, if you know VDD and can reprogramm Vbias, if Vdd goes away too much. However I would recommend using an external reference for biased snesors.

    Hope this helps

    Andreas

  • in reply to Andreas Kraemer
    Mastermind 7675 points

    Hi Andreas, 

                      I'm using the evalation module as standalone with the msp430f2274 board and I don't have the Vref pin connected.

    My configurations for the REFCN register is 0x20 (REF_SOURCE:INTERNAL and INT_Z:50%) . Measuring the PVOUT I have less than 1.5V (~1.25V) . The strange thing happens when i start debug my program,  the PVOUT falls to 0V. I'm wondering if this is correct, because with this config the zero would be 1.5V, so PVOUT will increase from this point.

    I'm using an unbiased sensor but planning to use biased sensor also.

    Regards

    Gastón