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LMH6611 Disable Feature

IDK
Prodigy 230 points
Other Parts Discussed in Thread: LMH6611

Hi,

I'm thinking about using the LMH6611 and was curious about its operation when disabled. Its datasheet suggests that, when disabled, the equivalent circuit would be 4 diodes placed as such with a resistor in series: 

What are the exact specifications of this when the amplifier is disabled? What voltage between the inputs would cause this equivalent circuit to conduct? Are there any capacitances at the input in the actual circuit when disabled? Can the actual circuit between the inputs be properly represented by diodes and, if so, what are the specifications of these diodes?

Thanks,

Matthew

  • 0
    TI__Mastermind 22825 points

    Hi Mathew,

    To answer your questions:

    What are the exact specifications of this when the amplifier is disabled?

    Answer: When disabled, because of the amplifier essentially going "open loop", there might be enough input differential voltage across this network which could cause conduction. In normal operation, the input differential voltage is not high enough (2Vbe) to see any current flow through these diodes and hence they have no effect on operation.

    What voltage between the inputs would cause this equivalent circuit to conduct?

    Answer: 2Vbe just like any discrete silicon diode.

    Are there any capacitances at the input in the actual circuit when disabled?

    Answer: These diodes are always in place. So, the input capacitance includes the effect of these diodes (which are in the off-state).

    Can the actual circuit between the inputs be properly represented by diodes and, if so, what are the specifications of these diodes?

    Answer: The LMH6611 datasheet does not explicitly state this. But, most OPA inputs Absolute Maximum Ratings are +/-10mA input current, and input voltage range from V- to V+. Again, normal operation would not exhibit conditions which the presence of this input network affects anything.

    Regards,

    Hooman

  • 0 in reply to Hooman Hashemi
    Prodigy 230 points
    Hey Hooman,

    Alright from what you've described, it seems those 4 diodes are more or less the same diodes used for ESD protection, correct? So if the input differential voltages are kept within the normal operating limits of the op amp while disabled, the diodes shouldn't conduct any current, right?

    Thanks,
    Matthew
  • 0 in reply to IDK
    TI__Mastermind 22825 points
    Hi Mathew,
    Yes you are correct that the diodes won't be "visible" in normal operation and could only affects things in disable mode.
    These diodes are intended to limit the input differential voltage because BJT type input devices like the LMH6611 can degrade if exposed to "large" input differential voltage.

    Regards,
    Hooman
  • 0 in reply to Hooman Hashemi
    Prodigy 230 points
    Hey Hooman,

    Alright, from your reply, it sounds if I am running this in the disabled mode, I shouldn't really expect any voltage at the output as long as the input voltage range is appropriate, right?

    There's a note that says, "If the input voltage range is exceeded by more than a diode drop beyond either rail, the internal ESD protection diodes will start to conduct." So, if I plan on powering this with +/-5V supplies, I shouldn't expect any output when disabled if my input voltage range is between +/-5.5V, right?

    I know my questions are getting very specific and almost redundant, but I need to be absolutely sure.

    Thanks,
    Matthew
  • 0 in reply to IDK
    TI__Mastermind 22825 points

    Hi Matthew,
    If you are in the disabled mode, if any signal is applied to the input(s) that approaches 2Vbe or higher, causes the internal differential diodes to conduct from one input to another. With the feedback resistor in place, some of that signal will look to be present at the device output pin through this path. The LMH6611 wants to declare the presence of these diode (which are not for ESD) to allow you to explain the presence of a waveform at the output.

    With your +/-5V supply conditions, you can see an output if you were to probe using a high impedance probe assuming a large input input (explained) above is present. However, this is NOT an output "driven" by the output pin, but rather remnants of what flows through the input diodes, through the feedback path and then out to the output pin (which itself is in Hi-Z state).

    Regards,
    Hooman

  • 0 in reply to Hooman Hashemi
    Prodigy 230 points
    Hi Hooman,

    And 2Vbe will be about +/-5V with +/-5V supplies, right?

    Thanks,
    Matthew
  • 0 in reply to IDK
    TI__Mastermind 22825 points
    Hi Matthew,
    Sorry I did not explain better.
    By "2Vbe" I mean 2 diode drops (or about 2x0.6= 1.2V). That's what forward biases the diodes. Any less than that, and the diodes would not be able to conduct / forward-bias. This is the "differential" input voltage (from inverting input to the non-inverting input, or vice versa - this is not voltage relative to the system ground or other reference)

    Regards,
    Hooman
  • 0 in reply to Hooman Hashemi
    Prodigy 230 points
    Hi Hooman,

    Oh okay, thanks for clearing this up.

    Thanks,
    Matthew