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Output inversion of the time of the current increase in the non-inverting circuit.[LMV358-N/TLV2442]

Tomoaki Yoshida
Expert 2200 points
Other Parts Discussed in Thread: TLV2442, LMV358-N, OPA2313, LMV612, TLV2462A

Hi,

I would like to know behavior of non-inverting circuit by LMV358-N or TLV2442.

In the following circuit configuration, output voltage will reach to 0V when the input current continues to increase.
After that, when input current is increasing, negative input voltage of amplifier will be high because output voltage is 0V.
As shown in the following figure, whether the output voltage which may be inverted to Vcc?
In exceeds the recommended operating voltage, whether there is a possibility that happen?
I also output absolutely as the output voltage exceeds the recommended voltage is not want to invert.
Thanks.
  • 0
    TI__Guru* 93105 points

    Yoshida-san,

    Driving the inverting input with an ever-higher input current causes the op-amp output, Vout, to move more and more negative. Vout will eventually hit the the lowest output level, Vol, that it can achieve for the current being forced through it. The output transistor saturates and the amplifier is no longer operating in linear mode.

    Since Vol cannot go any lower the voltage drop across the feedback resistor increases causing the inverting input to become higher and more positive. Eventually, the positive common-mode voltage range (Vicr) is exceeded. That upper limit is about (VDD) - 1.3 V for the TLV2442. When that happens and if driven hard enough, the output may invert. Inversion happens when the input stage FET drains are no longer out of phase with input gate signal. Instead the drains track the input gates and the output inverts.

    The parameter that you really have to watch and not exceed is the Absolute Maximum Input Current, which is 5 mA for the TLV2442. If the current exceeds that limit there is the risk of damaging the input ESD cells, and/or the amplifier input circuitry itself. The absolute maximum specifications for voltage, or current, should never be exceeded.

    Regards, Thomas

    PA - Linear Applications Engineering

  • 0
    TI__Mastermind 49896 points
    Hello Tomoaki,

    What Thomas said for the TLV2442 is also true for the LMV358-N. There is no guarantee that the output will not invert when Vcm is exceeded. The only guarantee is to use a R-R input device (LMV612, OPA2313) that will not collapse when the Vcm approaches the upper rail.

    We know the LMV358-N will invert if the inputs are taken BELOW V- by 500mV or more. This is unlikely in the scenario you describe above.

    It may be possible if there is a very large opposite current on the input. The output would hit the positive rail and the negative input would be forced negative (below ground) with increasing source current. But this would require several mA of negative current as you would have to overcome the forward-biased current of the ESD protection didoes (several mA) to get into the -500mV region. Again..very unlikely, especially if you do not have a negative voltage source.

    Regards,
  • 0 in reply to Paul Grohe
    Expert 2200 points
    Hi Paul,

    Thank you for your reply.

    I understood I can use R2R input device TLV2462A.
    The need is not, but we decided to use this device of R2R input TLV2462A.

    thank you.

    Tomoaki Yoshida