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LT1881 vs LT1636 dynamic load

Using the LT1881 as the difference amplifier for a dynamic load.

The output of the Opamp is slammed against the positive rail regardless of the value of Vctrl.

If I simulate the same circuit using an LT1636, like I saw in someone else's design, and the design works just fine. I am at a loss as to why one does and one does not work.

Can someone help me understand?

  • Hi,

    Difference in response for LT1636 and LT1881 is due to difference in Input Voltage Range (IVR).

    Using single ended supply of 12V, GND, LTC1636 accepts 0V input while the LTC1881 requires (V- + 1V).
    Thus, 25mV input is out of Input Voltage Range.
    For your supply and using LT1881, the minimum input voltage would be 1V.

    LT1881 IVR:

    I have tried LT1881 only that I used -1V for V- instead of GND, the 25mV was accepted.

    Here is the response of simulation for LT1636 Single Ended Supply and LT1881 with -1V at V-.

    Hope this helps.

    Thanks and regards.

  • Absolutely perfect!

    In all of the years that I have been an EE, I *never* had to pay attention to that part of an op amp's specification and so I overlooked it entirely.

    Thank you for patiently teaching me!

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