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OPA698: How to limit the output voltage

Takuya Hosokai
Prodigy 10 points
Part Number: OPA698

I'm using the limiter function of OPA698. It works always quite well. Input pulse signal is reduced in an output signal below one what I want. However, the current situation is not convenient for me because I don't know why the limiter function works in the amplifier. I need to explain to people how the limiter function of OPA698 works. I'm able to find some information about the limiter from Wikipedia, where several ways of the limiting function are described.

Can anybody tell me about the working mechanism of the limiter function of OPA698?

Sincerely,

  • 0
    Guru 47485 points

    Morning Takuya-san

    Essentially on each side of the output there is a separate amplifier that is comparing the high impedance node voltage to the limiter input voltage, once that high impedance node (essentially the comp node that is usually simply buffered to the output) exceeds the set point, that separate buffer takes over and drives the output stage with the limiter input voltage, clamping the output at that level. It releases its control as the comp node voltage drops below the set point and the amplifier operates normally. This is very simplified, but essentially what is going on. 

  • 0 in reply to Michael Steffes
    Prodigy 10 points

    Dear Michael,

    thank you very much for your quick reply. I could probably understand the basic concept of OPA698. Can you tell me the detail about the "clamping"? It seems like the limiter system is like one used in digital oscilloscope, where overdrive output signal is also clamped in a protection circuit. But in the oscilloscope, the excess signal sometimes appears as dummy (delayed) signal, since the excess one is stored in a protection circuit and has to be flowed at a certain time. Using OPA698, I do not see such a dummy signal and recovery time when output signal goes below limit voltage is very fast. This is quite nice and I like it.

    Again, cay you tell me more about the system of the clamping?

    sincerely, 

  • 0 in reply to Takuya Hosokai
    Guru 47485 points

    The earlier versions of this type of device did use a diode clamp that had recovery issues (CLC502, CLC501) - this is a much more active circuit type clamp with a the output voltage transitioning from the op amp type output to just a buffered version of the clamp input DC voltage. - It is unlikely the HSP group is going to give you the internal schematic, but perhaps under NDA

    Incidentally, for historical perspective, the need for this originally came from

    1. 2stage subranging 12Bit ADC residue amp

    2. military radar channels where a jamming signal might overdrive the channel where this allowed quick recovery in the signal chain.