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OPA355: ESD diode to positive rail

Mircea Dabacan
Prodigy 60 points
Part Number: OPA355

Hello,

I use OPA355 in an high impedance non-inverting input stage: at the inverting input, I have the feedback resistor Rf=200ohm, and a resistor to GND Rin=1k.

At the non-inverting input, I have a resistive divider of 637k to Vin and 363k to GND.  the OPA355 is supplied +3V and -2.5V.

The stage is supposed to withstand an input voltage (Vin) range of +/-60V, even if saturated for high voltages. When V+ tries to pass 3.5V or -3.1V, I expect the ESD diodes to open and clamp the voltage. The ESD diode current should stay below 60V/600k=0.1mA.

This works as expected for negative voltages, but it does not work for positive voltages. The circuit behaves as the diode from the non-inverting input to the positive rail would be missing.

Furthermore, I repeated the experiment with the unpowered OPA355. The rail voltages  stay relatively low (few hundreds milivolts), due to other loads on these rails.For negative input voltage, the non-inverting input voltage clamps at 0.5V below the negative rail, as expected. 

However, for positive input voltage (up to 60V), the non-inverting input voltage does not clamp and goes up to +8V. This permanently damages the OPA355, by increasing the offset voltage. The offset voltage grows with the stress amplitude and duration.

Is this expected behavior?

Is this something I do not understand in the data sheet?

Thank you,

Best regards,

Mircea

  • 0
    TI__Expert 7970 points
    Hi Mircea,

    The ESD diodes in these devices are not designed to sustain non-ESD voltage or current events. They are specifically created only to handle high voltage short duration ESD strikes, and will very likely fail in the scenario you are using them for. In this case you will need to use external protection diodes to keep the device safe.

    Regards,
  • 0 in reply to Jacob Freet
    Prodigy 60 points

    Hi Jacob,

    How should I read the absolute maximum ratings and note 2 from the data sheet TI post at:

    www.ti.com/.../opa355.pdf

    Signal Input Terminals, Voltage(2) .................... (V–) – 0.5V to (V+) + 0.5V

    Current(2) ..................................................... 10mA

    (2) Input terminals are diode-clamped to the power-supply rails. Input signals that can swing more than 0.5V beyond the supply rails should be current limited to 10mA or less.

    10mA means the current during the "short duration ESD strikes"?

    There is no mention in the data sheet about the length of the 10mA pulse... With the specifications above, I understand the 10mA can last indefinitely. Is there a note in the data sheet I don't see?

    Thank you.

    Best regards,

    Mircea

  • 0 in reply to Mircea Dabacan
    TI__Expert 7970 points
    Hi Mircea,

    My apologies, with these devices you are correct that the inputs can be driven beyond the limits if the current is limited below 10 mA.

    Regards,