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ADS8689: AIN limit question

Brian Angiel
Expert 4370 points

Part Number: ADS8689

I'm using ADS8689 to measure the input voltage from a battery. This voltage is nominally 32V, and I use a voltage divider (70k/10k) to reduce that voltage to 4V going into my ADS8689. I have a FET between the 10k resistor and ground. This FET was added to allow me to disconnect the load when the ADC is not in use.

However, when the FET is off, the 10k is left floating, and the ADS8689 sees 32V. There is a 70k resistor between the 32V and the ADC input, and the ADC is inactive when it sees this 32V.

Do you foresee this being a problem? I understand it violated the Abs Max table, so I'm trying to confirm if this needs to be redesigned or not.

Thanks,

Brian

  • 0
    TI__Mastermind 49675 points

    Hi Brian,

    The ADC will not be damaged as long as the input current can be limited to be less than +/-10mA which is a common absolute maximum input current rating for ADCs. The absolute maximum input voltage rating on ADS8689 is +/-20V, when your over-driven signal (32V) shows up, the voltage drop on 70kohm resistors is 32V-20V=12V, so the current flowing through the resistors is 12V/70kohm=0.17mA which is much less than +/-10mA.

    However, the input protection on this ADS8689 ADC is a SCR (silicon controlled rectifier) based structure, once it is triggered, it will go to latch-up state and will remain in this state until the power supply is cycled, so it is important to prevent triggering this kind of structure. The external protection circuit is a good way to prevent the triggering of the internal SCR structure, usually a TVS diode can be used (Shottky diodes can be used as well if high voltage power supplies are available in your system), below circuit block diagram with external TVS protection is recommended for your application (a capacitor is used as a filter with the resistors). A 12V or higher standoff voltage bidirectional TVS diode can be used to protect ADS8689 ADC, for example, TI's TVS1401 from flat-clamp TVS family has lower capacitance and leakage current and is a good option. Precision Labs ADC - Electrical Overstress on Data Converters section has much more detailed information including videos and materials as below:

    https://training.ti.com/eos-and-esd-adc?cu=1128375

    http://www.ti.com/lit/ds/symlink/tvs1401.pdf

    Please let me know if you have any further questions, thanks.

    Best regards,

    Dale