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ADS127L11: ADC Not reading negative differential input values.

Rachel Johnson
Prodigy 10 points
Part Number: ADS127L11


Tool/software:

Hello, 

I am working with the ADS127L11 on a custom PCB.  I am able to read the ADC values from 0-2.5V right now.  My VREFP = 2.5V, VREFN = 0V, AVDD1 = AVDD2 = 2.5V, AVSS = -2.5V.  I can read 0-2.5V on AINP-AINN but cannot read negatives. I had VCM floating before but tied it board GND which changes the range.  Is there any reason why AINN would be cutoff on the negative end? I was able to verify my AFE works as expected. 

Thanks

  • 0
    TI__Mastermind 47770 points

    Hello Rachel,

    Welcome to the TI E2E community.

    Since the ADS127L11 is a differential input, can you measure the AINP and AINN pins with respect to DGND?  The inputs relative to DGND can be in the range of +/-2.4V when using the internal input buffers, and +/-2.5V when not using the internal buffers.

    An example of a -1V input differential voltage would be AINP=0V=GND, AINN=+1V.

    What is the output code (conversion result) when trying to read a -1V differential input voltage using the above conditions?

    Also, it may be a good idea to measure the supply voltages with respect to DGND to confirm AVDD1=AVDD2=+2.5V and AVSS=-2.5V.

    VCM is an output voltage at mid-scale (with AVDD1=2.5V, AVSS=-2.5V, this pin will be near 0V), used to drive the input common mode pin of a fully differential amplifier.  You can leave this pin floating if not connected to an input amplifier.

    Regards,
    Keith Nicholas
    Precision ADC Applications

  • 0 in reply to Keith Nicholas
    Prodigy 10 points

    Hi there, 

    I confirmed AVDD1 = AVDD2 = 2.5V according to board ground. AVSS = -2.5V as well. VREF = 2.5V. 

    I injected 0.5V at the input, and AINP = 0.25V and AINN = -0.25V. the output code is 0x61332F 

    Injecting -0.5V at the input and AINP = -0.25V, AINN = 0.25V, and the output code is 0x610000

    The register configs are as follows: Config1 = 0x03, Config2 = 0x00, Config3 = 0x04, Config4 =0x80.  GAIN and OFFSET registers are set to their power on reset values. 

  • +1 in reply to Rachel Johnson
    Prodigy 10 points

    It looks like the status register was included in the code. I am updating with the following correct information: 

    with 1V injected at the input, AINP = 0.5V, AINN = -0.5V, and output code is 0x332F00

    with -1V injected at the input, AINP = -0.5V, AINN = 0.5V, and output code is 0xCCD200

  • 0 in reply to Rachel Johnson
    TI__Mastermind 47770 points

    Hi Rachel,

    Looks like you are now getting the correct codes for both +1V and -1V differential inputs.

    Note that you need to sign extend the negative code if using a signed 32b integer; 0xCCD200 becomes 0xFFCCD200 for 32b signed, or decimal equivalent of -3354112d.

    Please let me know if you have any additional questions.

    Regards,
    Keith