ADA4000 comparator

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.

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.