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TIC12400: design confirm

Amelie Zheng
Genius 10865 points
Part Number: TIC12400

Hi Team,

My customer now use TIC12400. They would like to use ADC to measure Analog signal(INx) without interrupt. The accuracy is ±2-3%.

They would like to set wetting current as 0mA. INx input analog signal(such as 2.5V/1.5V), read ADC value periodically。

Is it ok?

  • 0
    TI__Mastermind 33030 points

    Hi Amelie,

    A sampling accuracy of ± 2-3% is possible and there is a total of 1024 samples for a 0-6V sampling range. But there are several factors that may impact the accuracy. 

    • There is an internal voltage divider that is used to scale larger voltages to within the ADC's 6V range.  As a result, there is up to ± 110uA leakage current that will need to be accounted for.  This is specified as the IIN_LEAK_0mA parameter in the Electrical Characteristics table of the datasheet.  This resistor voltage divider is also depicted in the Functional Block Diagrams.
    • The Analog MUX used to connect the Input Pins to the ADC/Comparator requires that the supply voltage Vs is at least 1.5V larger than the voltage trying to be measured.  If the overhead voltage is less than 1.5V, the Input Voltage will be attenuated as it passes through the MUX before it is measured by the ADC.  This overhead attenuation is a similar to a variable input resistance being placed in parallel with the external switch.  The datasheet has some figures called "ADC Code vs. Equivalent Resistance at INx" that shows the ADC Max and Min codes for a supply voltage between 6.5V and 35V.  These figures show that the ADC code "minimum" value will be associated with a Vs = 6.5V and can be less than the actual voltage level when the input voltage is less than the required 1.5V due to the MUX overhead restriction.

    If the input voltage is greater than 7.5V then any voltage within the 0-6V ADC range should be sampled correctly.  The ± 110uA leakage current will be drawn through the Input Pin and impact the voltage slightly, but once this is understood, it should not impact accuracy of the measurement.

    Regards,

    Jonathan

  • 0
    Prodigy 21 points

    pls. see the the  "8.2 Functional Block Diagram" section of the datasheet

    1.  I would like to know the value of the R1 and R2.

    2.  I would like to know whether the MUX has a impedance isolation component, so that the ratio of  external resistance divider is not affected by the R1/2 divider.

  • 0 in reply to jincheng peng
    TI__Mastermind 33030 points

    Hi Jincheng,

    The total input resistance for an INx pin to GND that will be in parallel with the external load is 240 kΩ, with each resistor being approximately 120 kΩ.

    No, there is no impedance isolation and the internal impedance from this internal resistor divider will effectively be in parallel with the external load.

    Regards,

    Jonathan