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AM335x ADC Driver's Guide Vref Value

Doug Kehn
Intellectual 405 points

Hi All,

In the 'Formula Used for Calculation' is Vref set to VDDA_ADC or VREFP?  For example, if VDDA_ADC is 1.8v and VREFP is 1.25v, should Vref be 1.8 or 1.25?

I have a custom AM3352-based board with VDDA_ADC = 1.8v, VREFP = 1.25v, VREFN = 0v (ground).  I applied 1v to an ADC input and read 2275.  This equates to a Vref of 1.8.  I kind'a expected the ADC to read 3276 (Vref = 1.25).

If Vref is supposed to be VREFP, what configuration is needed to get the ADC to use VREFP instead of VDDA_ADC?

Thanks and Regards,
...doug

  • 0
    TI__Guru 63145 points

    If you are using VREFP and VREFN as the ADC reference voltages with VREFP set to 1.25V, VREFN should be set to 0.55V.  VREFP and VREFN should always be symmetrical with respect to (0.5 x VDDA_ADC). If VREFP is 0.5 volts greater than (0.5 x VDDA_ADC), the VREFN should be 0.5 volts less than (0.5 x VDDA_ADC). See the third parameter in the TSC_ADC Electrical Parameters table in the AM335x Data Sheet.

    You would need to select the VREFP input as the positive ADC reference by configuring the SEL_RFP_SWC_2_0 bits in the respective STEPCONFIG register, and the VREFN input as the negative ADC reference by configuring the SEL_RFM_SWC_1_0 bits in the respective STEPCONFIG register.  See the AFE Functional Block Diagram and the STEPCONFIG register definition in the AM335x TRM to better understand details of selecting the expected ADC references.

    Regards,
    Paul