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ADS8684: ADS8684 About

SWENGWOO LEE
Intellectual 270 points
Part Number: ADS8684
Other Parts Discussed in Thread: ADS8688

HI.

I have a question on the ADS8684.

When a surge is applied during EMC test, the AVDD voltage used in ADS8688 drops from 5V to 3V.

So the register is not written.

Even after reset using the reset pin, AVDD is still 3V.

If both DVDD and AVDD power of ADS8684 are turned off and on again, it will return to normal.

In addition, as a result of checking the power supply, It was confirmed that 100mA was used more when the surge was applied.

*I think the AVDD voltage goes down by using more AVDD current in the ADS8684 IC.

Have you ever had something like this happen?

Also, please suggest a solution

thank you

*LT1461 (Micropower Precision Low Dropout Series Voltage Reference Family) IC is used to supply AVDD voltage.

  • +1
    Intellectual 270 points 
    It works only when AVDD and DVDD are both turned off and on.

    If only one of the two power is turned off and then on, it will not work.
  • +1 in reply to SWENGWOO LEE
    TI__Mastermind 35570 points

    Hi SWENGWOO,

    Is there any TVS or zener diode on your supply of AVDD and DVDD to the ground? this kind of diode can provide a path for transient signal during EMC testing.

    Regards,

    Dale

  • +1 in reply to Dale Li
    Intellectual 270 points

    Thank you for reply.

    I will remove the TVS and test it.

    I am using a Voltage Reference IC called LT1461 for AVDD voltage.

    Could the Voltage Reference IC be the problem??
     

  • +1 in reply to SWENGWOO LEE
    TI__Mastermind 35570 points

    Hi SWENGWOO,

    The TVS diode is helpful against transient signal, so it's recommended. However, the parameters of TVS diode should be properly selected, and the stand-off voltage usually should match the supply voltage.

    When the ESD structure on the device is triggered, it will draw large current from the power supply. Your AVDD is supplied by a voltage reference device which usually can not provide a high current output (limit to 100mA on LT1461). I think this should be the reason to cause AVDD drop. I would suggest you to use a LDO or external power supply which can support large output current for test purpose.

    Regards,

    Dale

  • +1 in reply to Dale Li
    Intellectual 270 points

    HI

    If so, when drawing a large current from the power supply,

    Is there any problem with the operation of ADS8684?

  • +1 in reply to SWENGWOO LEE
    Intellectual 270 points

    When the ESD structure on the device is triggered, 

    Is there a way to prevent it from drawing large currents?

    Or is there a way to resume normal operation when a large current is being drawn?
  • +1 in reply to SWENGWOO LEE
    TI__Mastermind 35570 points

    Hi SWENGWOO,

    When the structure is triggered, a latch-up happens. The device stops working and normally it can only recovers when cycling the power.

    A  TVS diode can lead the transient current to the ground if the diode is properly selected in terms of power rating and standoff voltage. However, it also depends how high the energy of transient signal is.

    An external lab power supply will be helpful to verify if the device is drawing a large current.

    Regards,

    Dale

  • +1 in reply to Dale Li
    Intellectual 270 points

    HI

    If so, when drawing a large current from the power supply,

    Is there any problem with the operation of ADS8684?

  • +1 in reply to SWENGWOO LEE
    TI__Mastermind 35570 points

    Hi SWENGWOO,

    As I said, the ADC will stop working/conversion and you will see a constant data. Usually it can be recovered after cycling the power supply.

    Regards,

    Dale

  • 0 in reply to Dale Li
    Intellectual 270 points

    HI

    Just a few more questions.

    1. When AVDD is supplied to LDO with large output current and surge test is performed,
       It draws a large current (check the current with an external supply), but I've seen the adc data convert.
       According to the answer, the ADC should stop working and certain data should be checked.
       We ask for clarifications about what is different from what we tested.

    2.     I want to know about ESD structure.
       Could you please elaborate on the ESD structure you described, triggers and latches?
       If there is any related data or block diagram, I would like to receive it.

    3.     Is there any IC that is compatible with ADS8684 and does not have the above problem?
    4. Is there any TVS you recommend?
    5. If there is a reference circuit diagram, I would like to get it.
  • +1 in reply to SWENGWOO LEE
    TI__Mastermind 35570 points

    Hi SWENGWOO,

    I'm sorry for missing your post. 

    1. Based on your previous description, I suspected the ESD structure on your device was triggered because the phenomenon you saw is very similar as the behavior (drawing large current and had to recover from cycling the power supply) the ESD structure was triggered, so I suggested you to check with external power supply instead of voltage reference as a power supply. Did the external power supply work well and no drop during EMC testing?

    2. Please see SCR based ESD structure and latch-up in the TI Precision Labs ADC - Electrical Overstress on Data Converter video.

    3. There is no ADC which is completely compatible with ADS8688. This is not a problem on the ADC, see some suggestions in the item 4 and 5 below.

    4. SMBJ5.0A or SMCJ5.0A which can handle more power energy from transient signal.

    5. It's important to design the circuit and layout to improve the immunity for transient signal during EMC testing. Using protection components/circuit is one of the solutions to improve, a good pcb layout is also important. Please see TI Precision Labs ADC - PCB design for good EMC video series.

    Best regards,

    Dale