ADA4000 comparator

Hi AD,

is the ADA4000-1 can be used as a comparator instead of amplifier and is it a dedicated op-amp to work for this kind of use (e.g comparator)

It seems that ADA4000 is highly sensitive working as a comparator and it doesn't like this non linear mode.

Moreover with a transistor on the output which could create high DV/DT for example.

thanks for your help,

Parents
  • 0
    •  Analog Employees 
    on Feb 20, 2013 6:52 PM

    Hello Jajajib,

    OpAmps and comparators are designed for very different modes of operation at a fundamental level. An operational amplifier is intended to be used in systems which are linear and negative feedback is employed, keeping the operating point of all the internal parts in well behaved operation. Comparators, on the other hand, are designed to be tolerant of running open loop, or with some positive feedback, such as your schmitt trigger design.

    In the design of a comparator, the devices internal to the part are usually designed to be predictable when they become saturated, similar to some families of logic. You can force the input stage of an OpAmp into saturation by subjecting the inputs to a very large difference. In the book, "Troubleshooting Analog Circuits," by the late Bob Pease, he mentions that an old OpAmp with a slew rate of 0.5V/us with its inputs overdriven by just 5mV will break the output's linear behavior and then slew at a rate of 0.01V/us. The device was simply not designed for that function. The complexities which make up this change in performance would lead to a discussion about device behaviors (such as transit time in the active BJT, versus transit time in saturation of the BJT which dictates the response time of the device), which is outside of the scope of this forum.

    The result is that an OpAmp may be used as a comparator, but it will not be as fast or as predictable as an actual comparator. As such, by applying an OpAmp instead of a dedicated comparator, you will find that obtaining a very high slew rate (dv/dt) will be a very difficult task.

Reply
  • 0
    •  Analog Employees 
    on Feb 20, 2013 6:52 PM

    Hello Jajajib,

    OpAmps and comparators are designed for very different modes of operation at a fundamental level. An operational amplifier is intended to be used in systems which are linear and negative feedback is employed, keeping the operating point of all the internal parts in well behaved operation. Comparators, on the other hand, are designed to be tolerant of running open loop, or with some positive feedback, such as your schmitt trigger design.

    In the design of a comparator, the devices internal to the part are usually designed to be predictable when they become saturated, similar to some families of logic. You can force the input stage of an OpAmp into saturation by subjecting the inputs to a very large difference. In the book, "Troubleshooting Analog Circuits," by the late Bob Pease, he mentions that an old OpAmp with a slew rate of 0.5V/us with its inputs overdriven by just 5mV will break the output's linear behavior and then slew at a rate of 0.01V/us. The device was simply not designed for that function. The complexities which make up this change in performance would lead to a discussion about device behaviors (such as transit time in the active BJT, versus transit time in saturation of the BJT which dictates the response time of the device), which is outside of the scope of this forum.

    The result is that an OpAmp may be used as a comparator, but it will not be as fast or as predictable as an actual comparator. As such, by applying an OpAmp instead of a dedicated comparator, you will find that obtaining a very high slew rate (dv/dt) will be a very difficult task.

Children
No Data