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ADS4449: input protection

Nav
Prodigy 150 points

Part Number: ADS4449
Other Parts Discussed in Thread: LMH6554,

Greetings!

We have designed a board based on the ADS4449 ADC with DC coupled configuration based on the ADC driver LMH6554. The driver circuit we are using is based on Fig 15 from following document: www.ti.com.cn/.../tidu172.pdf i.e. DC coupled with the ADC driver powered from +3.65/-1.65V supply.

During our testing we found that two of the ADC channels have been damaged. So we are wondering if it could be due to the ADC driver being driven to saturation which could have exceeded the absolute max limit for ADC channel inputs (1.9V)? If so, we wanted to understand what is the safe current limit for the built-in ESD diodes? We can add a series resistor to limit the currents to safe levels of ESD diodes. We understand that adding series resistors could limit bandwidth but we only need ~100MHz -3db analog bandwidth. 

Notice that the above mentioned circuit also doesn't use any explicit input protection. 

  • 0
    TI__Guru* 91947 points

    Navin,

    Do you have any idea how long this input could have been exceeding the max input? Was it continuously high during this time or could it have been pulsed?

    Regards,

    Jim

  • 0 in reply to jim s
    Prodigy 150 points

    Jim,

    We believe the LM6554's output could have saturated for long durations to either rail. We have the 25+12.4 ohm resistors as per the circuit mentioned above which should be limiting the input ESD diode current on ADS4449. If we consider the LMH6554 single ended output swings to be 1V below rail voltages, this would result in a worst case continuous single ended current of (3.65-1 - 1.9)/(25+12.4) = 20mA on P channel of ADC. Shouldn't the ESD diodes be able to withstand this low current levels even continuously?

  • 0 in reply to Nav
    TI__Mastermind 45670 points

    Hi Navin,

    We will need to verify this with design to understand the current handling capability.

    We will get back with you on this requirement.

    Regards,

    Rob

  • 0 in reply to Rob Reeder
    TI__Guru* 91947 points

    Navin,

    Design has confirmed you need to limit the current to 25ma.

    Regards,

    Jim

  • 0 in reply to Rob Reeder
    TI__Mastermind 45670 points

    Hi Navin,

    I spoke to design on this, the current limit on the analog inputs is 25mA per pin.

    Regards,

    Rob

  • 0 in reply to Rob Reeder
    Prodigy 150 points

    Thank you Rob/Jim. Based on the ADC driver LMH6554 datasheet "Output swing (single-ended)"  figure it is stated that the typ. swing is +/-1.42V with supplies of +/-2.5V. So assuming it maintains about 1V headroom from rails, with a 3.65V supply, the max. output of LMH6554 is about 2.65V. The ADS4449 AVDD is 1.9V. So the input current is limited to (2.65-1.9)/(25+12.4) = 20mA. So it is well below the 25mA. Am I missing something here?

  • 0 in reply to Nav
    TI__Guru* 91947 points

    Navin,

    I would check the voltage at the ADC input pins and set the meter to hold peak value. If there are two resistors in series, I would replace the first with the a part that equals the two, and insert a current meter in place of the other and set this to record peak current. This way you will know for sure what the device is seeing.

    How did you verify the ADC channel was damaged?

    Regards,

    Jim