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ADS1298: damage ADS1298

Juergen Schramek
Prodigy 150 points
Part Number: ADS1298
Other Parts Discussed in Thread: TPS60403

we are using the ADS1298 in an ecg-application.

 DVCC: 3.3V

AVCC: +2,5V

AVSS: -2,5V

 

We have the situation that during test we lose some ic because of damage.

The effect after this happened is:

 

  • Input-Impedance of Din is only 500R
  • in some cases the supply-current (normal 75mA) rise to 150…250mA

 

First thing we found was a peak on DVDD off 2ms to 4,2V during Power-Up. We put an 3.3V LDO in line to eliminate this peak.

 A other thing that I can see with the oscilloscope is a short peak at AVSS to 0,8V ( it is coming out of the ADS1298).

So we put an Shottky-Diode in and we could limit it to 400mV.

 

Do you have an idea?

  • 0
    TI__Mastermind 38745 points
    Hello Juergen,

    Have you verified that at no time do the voltages or currents ever exceed the limits given in section 7.1 in the ADS1298 datasheet? Or do you think there could be a case where any of those limits are exceeded?

    Brian
  • 0 in reply to Brian Pisani
    Prodigy 150 points
    Hi Brian,

    during Powerup i see a peak on the AVSS of about +0.8V for 2ms. But this peak is comming out of the ADS1298. What can lead to DIN become only 500R input-resistant?

    Juergen
  • 0 in reply to Juergen Schramek
    TI__Mastermind 38745 points
    Hello Juegan,

    What do you mean that the voltage is "coming out" of ADS1298? If AVDD - AVSS was lower than normal, i would not expect that to cause damage. This is especially true since DIN there should be no circuit connection between DIN and AVSS inside the chip.

    Are AVSS and DGND connected in your circuit? Have you probed DIN with respect to DGND to see if there is an overvoltage event on that pin? If you reset or power cycle the device after this event, does the device return to normal or is it permanently damaged?

    This behavior could be caused by on overvoltage event on the pin. This could be triggering protection circuitry that does not shut off or it could be damaging the device to where it could not be used.

    Brian
  • 0 in reply to Brian Pisani
    Prodigy 150 points
    Hi Brian

    We use for AVDD+2,5V and for AVSS -2,5V.
    We use the combination of TPS60403 and TPS72325 for AVSS.

    At Power up I see an overschoot at AVSS of +750mV related to DGND.
    In section “Absolute maximum ratings” in the Datasheet AVSS to DGND is limited from -3V…+0,2V.
    With a resistor connected between AVSS and DGND and -2,5V disconnected I also see the peak.

    If I measure the 500R at DIN with a DVM the damage is permanent.

    I do not see any violation to the max. ratings except that at powerup at AVSS.

    Juergen
  • 0 in reply to Juergen Schramek
    TI__Mastermind 38745 points
    Hello Juergen,

    The violation of the absolute maximum between DGND and AVSS could very well be damaging the device. You may need to clamp the voltage with a diode circuit.

    Brian
  • 0 in reply to Brian Pisani
    Prodigy 150 points

    Hi Brian,

    could you please take my problem serious. We lost two days for answering questins about informations that I gave you at the first request.

    I did add a shottky diode. But that limits the peak only to 400mV. The limit in Datasheet is 200mV. The current, that is the source of that peak is coming out of the ADS1298.

    What can be the reason for that current coming out of the ADS1298?

    an an attachmentI send you oszilloscope-pic of the poweron with 3.3V(yellow) and -2,5V (green)

    Thats the way we connectet the ADS1298 power-Pins

    Thank you for your help

    Jürgen

  • 0 in reply to Juergen Schramek
    TI__Mastermind 38745 points
    Hello Jurgen,

    I apologize if I do not sound like I am taking this seriously. TI takes all perceived claims of damage or possible defects very seriously. I promise I am doing my very best to understand this issue. At this point I honestly don't know enough about the problem to diagnose it.

    What is the state of the DIN pin during power-up? There is a protection circuit within the device on the DIN pin that could be turning on during power up if DIN is larger than DVDD.

    Have you tried depopulating and shorting out the ferrite beads L1, L4, and L5? We typically recommend a low inductance from the supply to the supply pins, but I have never heard of this causing damage before.

    If you do not think I am able to help you sufficiently, please read TI's page about failure analysis: www.ti.com/.../failure_analysis.page

    Regards,
    Brian Pisani
  • 0 in reply to Brian Pisani
    Prodigy 150 points
    Hello Brian,

    sorry for the rough talk.

    DIn is direct connctet to MCU-GPIO (MOSI). Both devices are supplied with the same 3.3V.
    The ferrit beads do have 0,6 RDC, and 2200R at 100MHz. Between AVSS, AVDD and the ADS1298 we use 3,3µH inductance in addition.

    Juergen
  • 0 in reply to Juergen Schramek
    TI__Mastermind 38745 points
    Juergan,

    No worries. I understand this is frustrating.

    Perhaps try to probe DIN during power on to see if it ever rises above DVDD during that time. Could you also send me a more complete schematic so I can check for anything else that may be causing issues?

    Brian
  • 0 in reply to Brian Pisani
    Prodigy 150 points

    Hi Brian,

    datasheet of TPS72325 tells us that "enable threshold Negativ" is -1.5V. Until that level the ldo is diabled.

    Attached a pic. Yellow -> 3.3V, green chargepump TPS60403 ( input for TPS72325 ). Blue -> output of LDO -2,5V.

    Juergen

  • 0 in reply to Juergen Schramek
    TI__Mastermind 38745 points
    Jürgen,

    Are you saying that the LDO that regulates AVSS is not enabled when this occurs? If so, there is nothing keeping AVSS below ground. Its voltage is not deterministic in that scenario. Do you see the same behavior when the LDO is enabled from the beginning?

    Brian
  • 0 in reply to Brian Pisani
    Prodigy 150 points

    Hi Brian,

    we have still the problem with damaged digital inputs at the ads1298.

    If a input is damaged we can mease 500 Ohm to Gnd and 3,5kOhm to VDD.

    Do you have an idea what could cause such a damage.

    Jürgen