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ADS1148: Inpulse noise test between ADC input(AINX) and AC/DC power's GND with 1.5kV

Hirotaka Matsumoto
Guru 27585 points
Part Number: ADS1148
Other Parts Discussed in Thread: TVS1401

Hi all

Would you mind if we ask ADS1148?

Our customer tested ADS1148 with follow condtions; 

Test condition : Inpulse noise test between ADC input and AC/DC power's GND with 1.5kV

As the result, the device was latched up.
Until the device powered again, the device keeps latched up condtion.

Our customer tried to insert some passive parts(diode, capacitance, common mode coil), however there was no effect.
So, if you have some knowleagde for this solution, could you share us?


Kind regard,

Hirotaka Matsumoto

  • 0
    TI__Guru** 113765 points
    Hi Matsumoto-san,

    Could I get further information regarding how the test is conducted? Is this related to a particular IEC test? Is this a contact or air discharge type test? Is the impulse applied directly to the part pins or at the some other point like a screw terminal block? Is there a particular point or input that is most sensitive?

    What is meant by 'latched up'? Does this mean the ADS1148 gets very hot? Or does it mean the ADS1148 stops communicating?

    It is important to know the state of the ADS1148 when this condition occurs. I would be good to check the DRDY pin to see if ADC is converting. Sometimes only communication is lost. Usually the RESET pin can reset the device to a known state by pulsing this pin. If possible it would be good to try pulsing the RESET pin to see if the device will return to normal operation.

    It would be helpful to know specifically what has been tried. Is it possible to see a schematic and where the diodes, passives, etc. were placed? What can be helpful is the use of low leakage TVS diodes at inputs to the system and to the supplies. Another possible option is to place low leakage stacked diode arrays near the ADC inputs such as the BAV199, or BAV199W. The goal is to prevent the transient to enter the ADS1148.

    Best regards,
    Bob B
  • 0 in reply to Bob Benjamin
    Guru 27585 points

    Bob san

    Thank you so much for your cooperation!
    We will feedback you later.

    Kind regards,

    Hirotaka Matsumoto

  • 0 in reply to Bob Benjamin
    Guru 27585 points

    Bob san

    Is this a contact or air discharge type test?
    ->AC 1.5kV impulse noise inserted to ADC inputs and AC/DC's GND(power supply's GND) both side. And, it is contact type.

    Is the impulse applied directly to the part pins or at the some other point like a screw terminal block? 
    ->A screw terminal blocks.

    What is meant by 'latched up'? Does this mean the ADS1148 gets very hot? Or does it mean the ADS1148 stops communicating?
    ->ADS doesn't get hot. ADS stops communicating. And then, the customer tried to toggle RESET pin, however ADS1148 didn't restart.
       According to turn on again, the device goes back to initial condition. So it seems that the device condition is latch up.

    It would be helpful to know specifically what has been tried. Is it possible to see a schematic and where the diodes, passives, etc. were placed? 
    ->They tried to set the diodes(low lead TVS), passives, cap etc,,, however they couldn't clear their goal(AC1.5kV impulse noise)

    The goal is to prevent the transient to enter the ADS1148.  
    ->Of course, our customer recognizes these. So, If you know pins which can not put in transient noise absolutely, could you share us?

    And then, they changed AVDD and DVDD from 3.3V to 5.0V in order to improve SN ratio, however they couldn't clear their goal(AC1.5kV impulse noise).
    At present, they could clear their goal(AC1.0kV impulse noise), so passive parts are effect for these, however it is not enough. 

    Just in case, our customer would like to use AVDD=DVDD=5.0V.
    There is no limitation(except for ABS maximum ratings) between AVDD and DVDD, right?

    We appreciate yoiur help always.

    Kind regards,

    Hirotaka Matsumoto

  • 0 in reply to Hirotaka Matsumoto
    TI__Guru** 113765 points
    Hi Matsumoto-san,

    Thanks for the additional information. If the system survives at 1kV but not 1.5kV, then most likely the issue is occurring on the pins that have the smallest series resistances. For example, if your system has some series resistance that is 10k Ohm and some that are 3.3k Ohm, the sensitivity will be greatest on those pins of the ADS1148 that are connected to 3.3k Ohms as the potential current will be much greater.

    Current through the pins with a 10k series resistor will have momentary current of about 100mA at 1kV and 150mA at 1.5kV. The current through the pins with a 3.3k series resistor will have momentary current of about 300mA at 1kV, but 450mA at 1.5kV. Here you can see that the impulse current is substantially greater with the smaller series resistor.

    The goal is to shunt this impulse current away from the ADS1148. The best place to do this is close to the terminal block, but this requires a device rated to handle large transient currents. I prefer to use a low leakage bidirectional TVS diode. TI makes a TVS flat clamp diode with one example as the TVS1401. There are many other TVS diodes from other manufacturers as well, but you want to find one that is low leakage (nA) as opposed to high leakage (uA).

    One advantage of TVS diodes is the current capacity. Generally TVS diodes can handle many amps of current for a short duration. Regular ESD diodes can switch quickly, but are limited in current capacity. Also, TVS diodes will shunt current to ground whereas the ESD diodes will put the current into the supply. The supply must be able to sink this additional current and maintain regulation. If ESD diodes are used in a signal path, then I would suggest dividing the series resistance to the ADC pins and placing the ESD protection diode (such as a BAV199W) at the junction of the two resistances. This method will allow the ESD diodes to have a current limit through the ESD protection diode as well as providing a conduction path to the supply sooner than the ADS1148 ESD diode at the device pin.

    There is also one other thing to consider, and that is the PCB layout. It may be possible that there could be an adjacent path that might can couple with the transient. This is another very good reason to use TVS diodes at the terminal block inputs. In addition, also making sure that the return path for the transient current is prevented (or isolated) as much as possible from the ground plane of the rest of the system.

    Best regards,
    Bob B
  • 0 in reply to Bob Benjamin
    Guru 27585 points

    Bob san

    Thank you so much for your cooperation always!

    Could you give us the reply for these follow questions?

    <Additional question1>
    If you know pins which can not put in transient noise absolutely, could you share us?

    <Additional question2>
    Just in case, our customer would like to use AVDD=DVDD=5.0V.(unipolar)
    There is no limitation(except for ABS maximum ratings) between AVDD and DVDD, right?
    (We assume that there is no problem.)

    And then, our customer tried to insert low leak cap, changing resistance value,,, however there is no enough effect.
    About the layout, the customer can't have enough board layout. So, it is difficult to correspond for it.
    If you have follows, could you let us know?
    -As your experience
    -As your effective method

    Kind regards,

    Hirotaka Matsumoto

  • 0 in reply to Hirotaka Matsumoto
    TI__Guru** 113765 points

    Hi Matsumoto-san,

    The device pins for the ADS1148 are tested and pass the ESD ratings as shown in section 7.2 of the ADS1148 datasheet shown on page 7.

    These ESD ratings are not the same specifications as used in the IEC impulse noise tests.  It is difficult to say if there is one device pin that may be more sensitive than another.  The best approach would be to treat each pin the same way.  If there is not enough board space you would need to treat the most sensitive inputs based on the information I provided in my previous post.

    Regarding the absolute maximum ratings, the most important consideration is AVSS as it relates to DGND.  This is often overlooked.  This connection should be very low impedance.  If the impedance is too large (even by trace inductance) where the voltage exceeds more than 300mV, then the absolute maximum is exceeded and can damage or possibly cause the device to stop working.  An impulse in the ground can be an issue if the AGND and DGND are split or connected by a trace.  We recommend that the grounds be common and a single plane if possible.

    Best regards,

    Bob B 

  • 0 in reply to Bob Benjamin
    Guru 27585 points

    Bob san

    Thank you so much for your reply!

    It is difficult to say if there is one device pin that may be more sensitive than another. 
    ->Of course, we recognize that your saying is correct.
       However, if the sensitive condition is the same other pins, we assume that there is more sensitive pin compared with another.
       As yor experience, could you give us the advice?

    We appreciate your help always!.
     

    Kind regards,

    Hirotaka Matsumoto

  • 0 in reply to Hirotaka Matsumoto
    TI__Guru** 113765 points
    Hi Matsumoto-san,

    I'm sorry for the delayed response. Maybe the word 'sensitivity' is not the best word to use when describing what is taking place at the device pins. All of the device pins are rated for voltage and current according to the absolute maximum ratings table. The voltage ratings are specified to either the analog supply or the digital supply.

    If a large voltage pulse is transferred to the PCB at the system input, then you must determine where the system allows the most voltage and current to appear at the ADS1148 device pins. Depending on the capacitors, inductance and resistance in the pathway from the system input where the pulse is issued , the actual voltage/current levels may differ at the ADS1148 pins. The series resistance will limit the current, but inductance and capacitance will store energy. When the energy is released a number of possible events can occur, such as large overshoots/undershoots and oscillations.

    TVS diodes at the system inputs can limit the effects of this large pulse by clamping the voltage to a specific level. But there are still considerations to be made with respect to the additional current created when the TVS diode clamps at a voltage higher than the supply voltages. Also, the system ground will have the additional current and noise from the pulse that can cause issues within the ground.

    PCB layout is very critical with respect to having solid ground planes. Also devices connected to ground such as TVS diodes, capacitors and any device ground connections require low inductance vias to the ground plane. This may require multiple vias at the grounding point to lower the overall inductance from the device to the ground.

    Best regards,
    Bob B