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UCD3138A: The AD sampling values are higher than the theoretical values

Yanli Zheng
Prodigy 10 points
Part Number: UCD3138A
Other Parts Discussed in Thread: UCD3138OL64EVM-031

Hi,

When AD0 is used for the address pin sampling, the corresponding AD sampling value is higher than the theoretical value. After checked the waveform that the AD0 pin voltage value is higher during sampling process. At first , we suspected that it was because of sampling value of the previous channel. Later we found that only samplling this channel would have such a problem. It is worth noting that the voltage corresponding to our channel is 150mV. Before AD sampling,  the voltage is 0.15V (10uA*15k), but the voltage rises to 0.17V when the sampling is enaled.May I ask, what causes the high AD value? Any good suggestions? Thanks.

  • 0
    TI__Intellectual 1370 points

    Hi Yanli,

    The 10uA current source from UCD internal AD or external? Any external source connects to this AD port?

    Can you share the details steps that can reproduce this issue?

    Thank you!

    Jacob

  • 0 in reply to Jacob Wang(SZ)
    Prodigy 10 points

    Hi Jacob,

    We use the internal 10uA current source.

    1. Enable the 10uA current source. PMBusRegs.PMBCTRL3.bit.IBIAS_A_EN = 1;

    2. enabled AD and start conversion.(only sample this channel) We used software trigger.

    3. Cyclic sampling more than 100 cycles to ensure that the current source charging is complete.

    4. Disable the 10uA current source. PMBusRegs.PMBCTRL3.bit.IBIAS_A_EN = 0;

     

    Thank you! 

    YL

  • 0 in reply to Yanli Zheng
    TI__Intellectual 1370 points

    Hi Yanli,

    On port AD00, do you connect other source to this pin beside UCD internal source? I have tested the internal current source, it is very accurate.

    I suggest you disconnect other source to this pin.

    Thank you!

  • 0 in reply to Jacob Wang(SZ)
    Prodigy 10 points

    Hi Jacob,

    There are one 15K resistor and one 4700p capacitor. We do not connect any other souce.

    Thank you.

  • 0 in reply to Yanli Zheng
    TI__Intellectual 1370 points

    Hi Yanli,

    I have tried to reproduced your issue on my EVM UCD3138OL64EVM-031.

    Below is the circuit schematic. I replaced the 44.2k resistor with a 15k resistor, the capacitor is 1000pf.

    My test code as below.

    void init_pmbus_address_adc12(void)
    {
    AdcRegs.ADCCTRL.bit.SAMPLING_SEL = 6;
    AdcRegs.ADCCTRL.bit.MAX_CONV = 0; //only one channel to sample
    AdcRegs.ADCCTRL.bit.SINGLE_SWEEP = 1; //Use single sweep mode

    AdcRegs.ADCSEQSEL0.bit.SEQ0 = 0; //address 0
    //AdcRegs.ADCSEQSEL0.bit.SEQ0 = 1; //address 1

    AdcRegs.ADCAVGCTRL.bit.AVG0_EN = 0; //Disable averaging
    //AdcRegs.ADCAVGCTRL.bit.AVG1_EN = 0; //Disable averaging

    AdcRegs.ADCCTRL.bit.ADC_EN = 1; //Enable ADC

    AdcRegs.ADCCTRL.bit.SW_START = 1; //Send start bit high
    AdcRegs.ADCCTRL.bit.SW_START = 0; //Send start bit low
    }

    And my test result as below:

    Resistor Resistor(AD00-AGND) voltage adc value adc voltage
    15 15.01 0.149 243 0.14831543

    I failed on reproducing your issue.

    I have some suggestions:

    1. Measure the resistor value between ADC00 to AGND.

    2. Change chip and board to find out the difference.

    3. Decrease the capacitor value.

    Please let me know if you have any further progress.

    Thank you!

    Jacob