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LMP91000EVM: problem with the data reading

Patrizio Calabrese
Prodigy 20 points
Part Number: LMP91000EVM
Other Parts Discussed in Thread: LMP91000

Hi

I'm a student of Uniroma2 university in rome,

We are using the LMP91000EVM for a project, to capture the data we're using an arduino uno board, in particular we are using the arduino SPI functions for the communication. We are finding some issue in the data reading from the ADC of the evaluation board.

The data we read is not comparable with the voltage value in the differential input of the ADC. As in the schematic in http://www.ti.com/lit/ug/snau121a/snau121a.pdf the input on the ADC should be Vout - VREF that is correctly across the capacitor C8 (we measured with a multimeter), but the binary word we obtain from the ADC is not equal to that voltage value.

I'd like to understand if there could be an error in the SPI lecture of the data or if is not possible to read the data without the SPIO-4 board

Hoping for help

  • 0
    TI__Mastermind 35835 points
    Hi Patrizio,

    What is the data that you receive, and what do you expect?

    Can you show the data transaction on an oscilloscope?

    Thanks,
    Ren
  • 0 in reply to Ren Schackmann
    Prodigy 20 points

    Hi Ren,

    No, unfortunatelly i can't use an oscilloscope.

    As a test without connectig any electrode in this configuration

    the current in input at the TIA amplifier is null so the Vout is equal to the internal zero voltage setted via I2C and thi is constant if we generate an infinite loop that keep the LMP91000 in this condition.

    In fact if we use the 67% as internal zero Vout should be 1.675V and measuring Vout from the pin with a multimeter it is correct,

    If the input of the ADC is differential the voltage converted into digital should be 1.675V - 2.5V = -0.825V but the binary word i receive is 0x3572 that with a 16 bit converter with two's complement give 1.043V

  • 0 in reply to Patrizio Calabrese
    TI__Mastermind 35835 points
    Hi Patrizio,

    I understand that you expect LMP91000 to output 1.675V and that is also observed by multimeter. I suggest that you use an oscilloscope to verify the voltage is constant. It would make sense for the ADC to acquire a lower voltage if the output were switching to zero.

    Thanks,
    Ren