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AM6442: ADC reading lower than expected (12-bit ADC – 1.8 V input reads ~1.7 V)

Anjana Rao
Prodigy 60 points
Part Number: AM6442


Tool/software:

Hello,
We have dseigned a custom board based on AM6442 where we are using 8 channel ADC's. Here we have observed few things as below

  • When there is no input voltage supplied and the pin is left floating we see a reading 0.3-0.4V
  • When the voltage is supplied above 1.7V to 1.8V we see the reading going only upto 1.7V only.

Please confirm whether this is expected behaviour from AM6442 or not?

Below is the formula used to convert the raw value to analog voltage value.

  • ADC_VMIN: 0.1 V

  • ADC_VMAX: 1.8 V

  • Resolution: 12-bit (4096 counts)

  • Scaling formula used:

    scalingFactor = (ADC_VMAX - ADC_VMIN) / RESOLUTION;

    vin = (adcValue * scalingFactor);

Note:

However, for other voltages in the range 0 V to 1.7 V, the ADC readings are quite accurate and closely match the expected values.

  • 0
    TI__Intellectual 790 points

    Hello Anjana,

    Regarding your observations:

    1. ADC Pins left floating: This is expected behavior. When a pin is left floating with no input voltage or not tied to VSS, it is expected that there will be some noise/leakage.

    2. Voltage reading only up to ~1.7V: This is not expected behavior if your setup is configured correctly. The ADC measures the input by comparing it to its reference span, from VREFN (low) and VREFP (full-scale). The ADC should be able to measure the full-scale up to VREFP if the reference pins and input are supplied correctly. To help diagnose this, could you please provide:

    • A schematic showing your ADC input circuitry and signal source
    • The actual voltages present on VREFP and VREFN Pins
    • What voltage you're measuring relative to VREFP when the 1.7V limiting occurs

    Regards,
    Karam

  • 0 in reply to Karam Haddad
    Prodigy 60 points

    Hello,

    Please find attached the screenshots addressing the points you mentioned:






    Regarding the actual voltages present on the VREFP and VREFN pins, they could not be measured directly.

    As the input approaches 1.8 V, the ADC reading saturates around 1.66 V to 1.7 V.

    Note: 

    Please note that the 1.8 V power supply is sourced from the PMIC, part number TPS6522053RHBR.


    Thanks.

  • 0 in reply to Anjana Rao
    TI__Intellectual 790 points

    Hi Anjana, 

    Thank you for your response. The schematic seems to mainly cover power, could you also please provide the schematic for the input circuitry/signal source?

    Regards,
    Karam