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ADS1000: ADC is railing when the board temperature rise

May
Genius 9880 points
Part Number: ADS1000

Hi,

My customer is using ADS1000A0IDBVT to send the digital value the uP when a certain device is connected or disconnected. When the temperature of the ADC rise to ~ 50 degree C, the ADC starts railing to full scale causing false detection. The ADC is rated for 125 degree C. Do you have an explanation to what could be happening with the ADC?

Thank you.

Regards,
May

  • 0
    TI__Mastermind 24940 points

    Hello May,

    I suspect that the input signal is the course of the error and not the ADC. Have you confirmed the input voltage to the ADC while it is railing? in other words the output of the signal input source?

    Another possibility is the power supplies. Have you confirmed that the rails are stable and at the level expected during this instance?

    Regards

    Cynthia 

  • 0 in reply to Cynthia
    TI__Genius 9880 points

    Hi Cynthia,

    Just received response from customer, please see details below.

    Yes, I have confirmed that the input signal is 0.0 V (shorted to ground) while observing the output of the ADC (Pin 4) is railing to full scale counts.

    I have also verified that the reference voltage rail VDD is stable at 5.0xx V when the temperature of the board rises.

    I did attach a thermocouple to the ADC, and I noticed that the output voltage railing at 0.0 V input analog voltage occurs when the ADC temperature exceeds ~ 49 degree C.

    Thank you.

    Regards,
    May

  • 0 in reply to May
    TI__Mastermind 24940 points

    This is odd, and will need more information. 

    Is this all done on a single PCB, or are there wires connecting multiple boards?

    Can you share the schematic? As well as a scope shoot of the digital communications, if normal behavior and bad behavior?

    Regards

    Cynthia

  • 0 in reply to Cynthia
    TI__Genius 9880 points

    Hi Cynthia,

    Please see response from customer.

    Yes, this all done on one PCB. I have attached the schematics for the ADC, as well as two scope shots showing the ADC at 0 counts and 2047 counts when the temperature exceeds ~50 degrees C.

    ADS1000.zip

    Thank you.

    Regards,
    May

  • 0 in reply to May
    TI__Mastermind 24940 points

    Hello May,

    There is something else going on here, note in the scope shots that the high and low levels in the digital communications is not consistent, both the SDA and SCLK have inconsistent voltage levels. though it is more obvious on the SDA line, the clock also dips. 

    Are there any other devices on this digital bus? when communicating to the other devices, does the digital transmission also display this behavior? this may be due to board layout. What  logic level is the I2C using? from the scope shot it looks as if the scale is 1V, and thus high is 3V. Is this correct? Note that the ADC's VDD also sets the I2C logic levels, which based on your schematic is 5V.  The I2C transmissions needs to adequately meet the below logic high and low levels to be interpreted correctly by the device. 

    Regards

    Cynthia