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ADS1278: ADC draws a lot of current when connecting IOVDD pins

Raman Sridahran
Prodigy 130 points
Part Number: ADS1278

Hi,

This is my first foray into hardware myself so please call out any idiotic things I may be doing.

Im connecting an ADS1278 up with a QFP64 socket adapter similar to this one: https://www.waveshare.com/gp-qfp64-0.5.htm

Im not even communicating with the chip yet - just setting up power rails. 

I connected all Analog and Digital GNDs to the same ground my regulators are connected to.

I set up a 5V regulator, connect it to all the 5V rails on the chip per the datasheet (all Analog power supply pins).  Power it on, nothing explodes, all seems OK.

But when I set up a 3.3V regulator, as soon as I connect it to the IOVDD pins, the regulator gets really hot.  I dont have an accurate ammeter to measure the current, but the regulator gets really hot very fast.  I suspect I burned out a regulator by leaving it on too long; as I had to replace the 3.3V regulator once.

Could this much current draw from the IOVDD pins be correct?  Could I have burned/damaged my ADC?  Or is this actually normal?

  • 0
    Prodigy 130 points

    I changed out the QFP64 socket adapter with another one, and it still has the issue.  I also ordered a current sensor which I will get tomorrow.

    Looking at the datasheet, IOVDD current should be no more than 1mA (12V supply to 3.3V regulator).  So 1mA * 8.7V = 0.0087W heat...which should be less than a degree increase in the temperature of the regulator.  But, I can feel its temperature skyrocket within seconds.  

    So IOVDD is definitely drawing waaay too much current, but I have no idea why this is is acting this way.  Any ideas are welcome.

  • 0 in reply to Raman Sridahran
    TI__Mastermind 44670 points

    Hello Raman,

    The IOVDD current should never be higher than 1mA, and typically will never be higher than 0.25mA.  The part is likely damaged and will need replaced.

    The most likely cause is the absolute maximum supply voltage has been exceeded (3.6V ABS MAX).  Can you confirm the proper voltage on your regulator before connecting it to the ADS1278?  You can also exceed the maximum voltage if you connect the supply voltage when it is already ON.  You should connect all supply wiring with the power supplies OFF, and then power up the device after all connections have been made.

    One other possibility is that IOVDD pin is shorted to ground in your socket.  These pins can easily bend when inserting into a socket, and can short between adjacent socket pins.  Both IOVDD pins are located directly next to ground pins.  I suggest visually inspecting the pins to make sure they have correct alignment.  You can also use a handheld ohmmeter and measure the impedance between IOVDD and ground when the device is inserted into the socket.

    Regards,
    Keith Nicholas
    Precision ADC Applications

  • 0 in reply to Keith Nicholas
    Prodigy 130 points

    Hi Keith,

    Thanks for the reply.

    Yes, I verified my regulators output was exactly 3.3 before connecting.  But,  I wouldn't be surprised if I accidentally applied more than 3.6V at some point in the past.  I ordered a new chip and am awaiting it in the mail.

    I checked the pins and they all look good; and visually there appears to be no shorting when the chip is seated in the QFP64 socket. 

    I measured resistance between GND and the two IOVDD pins.  One fluctuates a lot; and one is stable at around 9Ohm.  I guess I dont know what to make of this; or if these resistances changes when powered on; but I suspect 9Ohm is not correct.

    When you say that all terminals should be connected before powering up - are you referring to ALL input voltages and GNDs (Analog Supply, Digital Core Supply, Digital IO Supply, Positive reference input, Negative reference input)? 

    Or is it safe to connect all GNDs along with All IOVDDs; and test that.  Then disconnect the IOVDDs and connect all Analog Supplies; and test that...etc?

  • +1 in reply to Raman Sridahran
    TI__Mastermind 44670 points

    Hello Raman,

    The resistance measurement between the supply pins and ground should be several kilo-Ohms.  I would say that the device is damaged and you will need to get new parts.

    Do not connect a power supply that is already energized directly to the ADC power pins.  The inductance in the power supply cables can cause the voltages to spike and damage the device.

    I would recommend connecting the device and powering up all of the power supplies at the same time.  If the power supply sequence is out of order, the device will not be damaged.  However, if you do not follow the power-up sequence in Figure 84 of the datasheet, you will need to assert the /SYNC pin after the supplies have stabilized to ensure a proper reset of the device.

    Regards,
    Keith

  • 0 in reply to Keith Nicholas
    Prodigy 130 points

    Thanks. Ill give this a try when the new chip gets in this Friday.

  • 0 in reply to Raman Sridahran
    TI__Mastermind 44670 points

    Thanks for the update.

  • 0 in reply to Keith Nicholas
    Prodigy 130 points

    Thanks - the new chip does not overheat the power supply!  It appears I did damage my old chip.

  • 0 in reply to Raman Sridahran
    TI__Mastermind 44670 points

    That is great news.  Thanks for the update.  Keith