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TLV170: Can I input voltage Vi over VCM（common-mode voltage）in Inverting Amplifier Circuit

yuqi liao
Prodigy 30 points
Part Number: TLV170
Other Parts Discussed in Thread: OPA627, OPA171

Hello everyone，

      I was wondering recently, when I adjusted the resistance to set the gain in the figure below to G=-0.5, could I set the input voltage to Vi = -18V.In theory, it is beyond the scope of VCM, but in practice and simulation, there is no problem.

      Thanks for answering my questions！

    

  • +1
    TI__Guru*** 145076 points

    Hello Yuqi,

    VCM is the voltage on the op amp input pins. It does not refer to the input voltage on the far side of the resistor. 

  • 0 in reply to Ron Michallick
    Prodigy 30 points

    Hello Ron

        Thank you for your answer，I want to know the theory. Before the operational amplifier is stabilized, I feel that this Vi will enter into the op amp through the input pin, so I have questions about Vi and VCM in Low gain Inverting Amplifier Circuit When Vi is greater than VCM.

        Kind Regards！

  • +1 in reply to yuqi liao
    Guru 140200 points

    Hi Yugi,

    it's always critical when the input voltage exceeds the supply voltages of an OPAmp. Not only phase inversion can happen, but also latch-up, lock-up and even damage.

    In any case, I would add a diode clamp across the inputs of OPAmp as shown in figure 33b of datasheet of OPA627.

    Kai

  • +1 in reply to yuqi liao
    TI__Guru*** 145076 points

    Yuqi,

    In cases where input exist before power up or input changes so fast that op amp can't send enough correction to keep input valid, there can be issues as Kai stated. Depending on the op amp, I might have given this information initially.

    However, OPA171 data sheet mentions that there is no phase reversal and ABS MAX table shows an input current.

    Adding a diode may not bypass all of the input current; the diode is capacitive. If diode is Schottky, it will leak badly at high temperature.