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,

  • jajajib,

      People use op amps as comparators, but there are several reasons why you don't want to do it.

    If you can tell us what supply voltages, signal levels in, logic levels out, response time, etc.

    we can suggest a better solution.  Check out the ADCMPnnn parts.

    Harry

  • Hi,

    ADA4000 is used as a shmitt trigger (non inverting) Transistor is a 2N4416 and there is a serie resistor (value 100kOhm) between op-amp/comparator and transistor.

    Could you provide me with reasons people don't use op-amp as comparators ?

    thx

  • 0
    •  Analog Employees 
    on Feb 20, 2013 6:52 PM over 8 years ago

    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.

  • Hi,

    I would like to extend my issue, not only on using ADA4000 as a comparator but also as an op-amp.

    I am not sure that my electrical circuit is not the root cause of my problem (I cannot provide with the system circuit) but this ADA4000 op-amp seems to have some troubles (KO) and I would like to check if this op-amp has no internal sensitivity.

    What would be the behaviour (or failure mode) of the ADA4000 op-amp with :

    - a disymetric power supply or no synchronous power supply when power on

    - ESD

    - bad or far decoupling capacitor on power supply

    - latch up phenomenae

    - any cause which could lead to a failure on the op-amp

    thanks in advance

  • Jajajib,

      Please attach a schematic of what you are trying to do.

    Harry