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ADS131M04: Values of 2 channels differ from each other when inputs are left open

Franz Schellhase
Expert 3605 points
Part Number: ADS131M04


Hi team,

Can you have a look at the following customer inquiry please?

With both, sample and EVM, we now have the problem that with "open" inputs, i.e. without externally connected strain gauge force sensors, the measured values differ between the channels by up to a factor of 2.
Excerpt from the circuit diagram for this:


We haven't found an explanation here yet, but also have to look for all application notes.

Do you have suggestions what the issue might be here?

Thank you,

Franz

  • 0
    TI__Mastermind 18900 points

    Hello Franz,

    Can they clarify the use case a little bit more? What do they mean by measured inputs vs. open circuit? Can they quantify their gain settings along with their measured input and expected output code value?

    Specifically, open implies the X7 header is not connected with the sensor as described. According to the schematic, the R59 and R60 lightly pull the inputs to GND (which isn't the same as an open circuit, floating input with no pull downs). So, VIN = VAINP – VAINN would would be the equivalent of 0V, which would correspond to 0x0 in codes for the ideal use case. Hopefully you can see the confusion.

    Best,

    -Cole

  • 0
    TI__Mastermind 18900 points

    Hello Franz,

    Talking with the team some more, the questions are still important to figuring out their problem.

    I will say, usually strain gauges are in just one leg in a bridge circuit and I'm surprised not to see the other resistors used for biasing the bridge so if they haven't taken in resistor tolerances within their error, I'm sure that'll contribute.

    I thought I'd mention that each channel will have its own bias current, offset, and other errors associated with it. So, if we're talking about the differences between 1uV and 2uV, this is expected. I would just like to reaffirm that each error will line up with the specs of the datasheet and they can review the ADC TIPL section on errors linked here: https://training.ti.com/ti-precision-labs-adcs-statistics-behind-error-analysis?context=1139747-1140267-1128375-1139104-1128656  

    Best,

    -Cole