MT-084 TUTORIAL

MT-084 TUTORIAL

Using Op Amps As Comparators
Even though op amps and comparators may seem interchangeable at first glance there are some important differences. Comparators are designed to work open-loop, they are designed to drive logic from their outputs, and they are designed to work at high speed with minimal instability. Op amps are not designed for use as comparators, they may saturate if over-driven which may cause it to recover comparatively slowly. Many have input stages which behave in unexpected ways when used with large differential voltages, in fact, in many cases, the differential input voltage range of the op amp is limited. And op amp outputs are rarely compatible with logic.
Yet many people still try to use op amps as comparators. While this may work at low speeds and low resolutions, many times the results are not satisfactory. Not all of the issues involved with using an op amp as a comparator can be resolved by reference to the op amp data sheet, since op amps are not intended for use as comparators.
The most common issues are speed (as we have already mentioned), the effects of input structures (protection diodes, phase inversion in FET amplifiers, and many others), output structures which are not intended to drive logic, hysteresis and stability, and common-mode effects.