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TMS320F28335: Voltage at ADC input before power supplied to uP

stevenh
Guru 19935 points
Other Parts Discussed in Thread: ADS8528

What happens when there is a voltage (>0.7V and <2V) at the uP ADC input before power is applied to the uP?  The uP is being powered by a linear voltage regulator.

  • 0
    TI__Genius 11951 points
    Steven,
    Since it is outside the datasheet requirements it is hard to say exactly what will happen, but it can cause internal non-destructive latch-up events which in turn make the device the behave in numerous unexpected ways (digital or analog side) that go away when the device is powered correctly or the device can be permanently damaged. I recommend you add clamping diodes between the analog pins and VDDAIO if you cannot handle this through power sequencing.
    Regards,
    Joe
  • 0 in reply to Joe Prushing
    Guru 19935 points

    Thanks for the reply.

    Can you explain how clamping diodes between analog pins and VDDAIO mitigates the issue? What is VDDAIO internally tied to?  

    What if I only use a series resistor? will that be sufficient?  Or do I need both a VDDAIO and series resistor. A several hundred ohm series resistor would add that much delay to the acquisition time.  I acquiring data from 11 channels (2us/channel acquistion time for each channel) every 10ms. 

    Also, I am using both the uC ADC and an external ADS8528.  Do you know if the ADS8528 has the same issue?

  • 0 in reply to Joe Prushing
    Guru 19935 points

    Before power is supplied to the uP or, in general, to any of the circuitry, there won't be any voltage on VDDAIO.   However, the clamping diode is good for the situation where the board is powered.

    What is the max input current to the ADC input pin?

  • 0 in reply to stevenh
    TI__Genius 11951 points

    Using a clamping diode between the ADCIN and VDDAIO should keep the voltage at the ADC input to within a diode drop of VDDAIO.  So even if the board is not powered but an external voltage voltage is driven on the ADCIN the uP should be protected.  As the device powers, the voltage will increase on the ADCIN, but should never violate the DS restriction.

    The max input current to an ADC input is 20mA, but there is also a 20mA max for the device, so if there exists multiple ADC inputs with a voltage you have to consider the accumulated current as well.

    I cannot comment whether ADS8528 has the same restriction, but I believe they have E2E forums as well in which you may be able to find your answer: http://e2e.ti.com/support/data_converters/

  • 0 in reply to Joe Prushing
    Guru 19935 points
    Hello Joe,

    Is it ok to have a voltage less than -0.3V (e.g. -0.5V) at the ADC input if the current is limited to be less than -20mA.

    Thanks,
    Stephen
  • 0 in reply to stevenh
    TI__Genius 11951 points
    Stephen,
    I'm afraid we have to advise that both the voltage and current requirements be met. -0.3V is an absolute rating in the DS and exceeding this can cause unpredictable results or even damage the device.
    Regards,
    Joe