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OPA128: Input protection.

Samuel Kerem
Prodigy 70 points
Part Number: OPA128


Page 7 of the data sheet mentions "Input FET gate-to-substrate ... diodes".... that can be forward biased.

Does that mean that OPA128 will survive if the signal source > Vcc, but the input current is greatly limited e.g.<10 microA?

  For example: OpAmp power: Vdd=0V, Vcc=+15V.     In- is at gnd. V_signal can reach +25V but is connected to In+ through 1 MegaOhm. There will be current flowing (25V-15V)/1Meg=10uA. The OpAmp output rails, but is this situation harmless?

Thank you

Samuel

  • 0
    TI__Guru 68231 points

    Samuel,

    Vbe of the forward biased diode implies certain current: 

    Thus Vbe=~Vt*ln(Id/Is) where Is is a saturatioj current of few fA and Vt=KT/q=~26mV at 25C

    If you limit the current to 10mA, Vbe will be limited to ~0.6V and almost all of the voltage will be dropped across the resistor thus preventing any damage to IC (op amp does NOT 'see' any voltage greater than 15.6V).