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ADC128D818: No voltage input ADC？

kevin.huang_FJXM
Expert 2925 points
Part Number: ADC128D818

At present, the circuit uses ADC128D818CIMTX/NOPB chip to collect voltage. The schematic diagram and serial port driver reading values are shown in the figure:

Problem description: During normal power supply, the correct voltage value can be collected; However, when the collected voltage is not powered, a high voltage will also be read with a probability of about 50%,

Positioning process:

The normal power supply of IN0 and IN1 PVPP is 2.5v, and when not powered, the voltage collected is 5v, and the drive return value is 2560,

The normal power supply of IN3 is 12v, and when not powered, the voltage collected is 12v, and the drive return value is 2560,

The normal power supply of IN4 is 3v3, and when not powered, the voltage collected is 6v, and the drive return value is 2560

IN5 and IN6 are not connected to any devices, and there will also be small driver return values of 267/268

1. Using an oscilloscope to detect the waveform before and after overvoltage collection, there is no actual voltage present, only minimal external interference, and the collected problem voltage time is completely out of sync, eliminating the possibility of external noise interference causing collection errors.

When there is an error in the collected voltage, the drive return values are all 2560. According to the manual, calculate the formula DOUT=[ Δ VIN/VREF] × 2 ^ 12, carried in at 2560, Δ The VIN is 1.6v,

Please help to take a look. We have the problem of voltage collection errors when there is no power supply. What is the reason for the return value error to be the same? Why does this collection error state occur? If there are any other issues or unclear descriptions, please feel free to communicate at any time,

  • 0
    TI__Expert 5590 points

    Hi Kevin,

    Thanks for your question! One possibility may be that the 1.6V you read at the input is a result of the ADC that is still running despite the inputs being powered off. The voltage is likely the same (1.6V) at all inputs as the bottom resistor (1.4Kohms) is the same in all 5 input resistor dividers. Using a buffer may help in this situation. A few questions:

    1. Are the conversion results correct when the inputs are powered on?
    2. Can you probe the ADC input pins when the input power supplies are turned off and the report the voltage readings?
    3. Does changing the 1.4Kohm bottom resistor change the 1.6V voltage that is measured at the input pins when input power is off?

    Best,

    Samiha

  • 0 in reply to Samiha Sharif
    Prodigy 20 points

    hi， ，

    1. The voltage collected when the normal voltage is input is also normal;
    2. When the collection error occurs, no real voltage exists
    3. Change the PVPP_CPU0 resistor R2416(originally 1.4k) to 3k, and the collection error is the same as before

    Are there any other directions we can look?

  • 0 in reply to shuhao yu
    TI__Expert 5590 points

    Hi Shuhao,

    Thanks for your response! That is interesting, I would expect some voltage at the input pins when the collection error occurs. I will attempt to recreate the case and see what I observe and get back to you in a few days.

    Best,

    Samiha

  • 0 in reply to shuhao yu
    TI__Expert 5590 points

    Hi Shuhao and Kevin,

    Some additional comments:

    1. Although I can't see them in the attached schematic, are there pullup resistors on the SDA and SCL lines, as recommended on page 36 of the datasheet?
    2. Could you please try and resize the input resistor dividers to follow the datasheet recommendations on page 35. The bottom resistor should be between 10kohm-100kohm.
    3. This thread (linked) mentions that a similar issue may be caused by damage to the input ESD structures. Could you measure the input leakage current at the input pins with the inputs powered off and check if it is more than the expected 5nA? 

    Best,

    Samiha

  • 0 in reply to Samiha Sharif
    Prodigy 20 points

    hi， ，Thank you very much for your support, this issue is solved

    1.The SDA and SCL had pull-up

    2.Design bottom resistor R1&R2 between 10kohm-100kohm，now，when not powered ，can read normal 0v，Finally found the root cause。

    thanks~~