OPA991: Common Mode above 20V

Part Number: OPA991

Tool/software:

Hello,

Just wanted to confirm that the common mode voltage offset is going to remain flat from 20V and above?  

We're going to be operating at 36Vin.

Thank you, Keith

  • Keith,

    The answer to your question is a little complicated:

    1. The jump you see in offset is due to a switching from P-channel MOSFET to N-channel MOSFET.  This switch is how the op amp can have a common mode range from the negative supply rail to the positive supply rail.  The offset will be relatively flat above and below the transition.  By "relatively flat" I mean there is a very small slope on the offset signal (even in what you see in the graphs).
    2. The transition point happens 2V from the power supply rail.  It does not always happen at 20V.  In the graph shown the power supply voltage is ±20V, so the transition happens at +18V.  If you have a single 40V supply, the transition will happen at 38V.
    3. Op Amp Input and Output Swing Limitations covers this topic in detail.

    I hope that helps.  Best regards, Art

  • Thanks Art, makes sense.

    Could you please remind me on the different colors in the curves?  

  • Keith,

    The different colors are just to help differentiate the curves.  Each curve represents a single device (one sample).  If you look at multiple curves you will notice that some curves have larger transitions, and different levels of offset.  The graphs in this data sheet look like they represent about 10 or 15 samples.  Generally many more devices are included in the characterization but it would be hard to read the curve it hundreds of devices were shown.

    Best regards, Art