Vin is lower than VREF, then Vout will fall to its negative saturation level, equal to the voltage at the negative side. In practice, this circuit can be improved by incorporating a hysteresis voltage range to reduce its sensitivity to noise. The circuit shown in Figure 1, for example, will provide stable operation even when the Vin signal is somewhat noisy. This is because of the difference in characteristics of an operational amplifier and comparator, using an operational amplifier as a comparator presents several disadvantages as compared to using a dedicated comparator. Op-amps are designed to operate in the linear mode with negative feedback. Hence, an op-amp typically has a lengthy recovery time from saturation. Almost all op-amps have an internal compensation capacitor which imposes slew rate limitations for high frequency signals. Consequently, an op-amp makes a sloppy comparator with propagation delays that can be as long as tens of microseconds. Since op-amps do not have any internal hysteresis, an external hysteresis network is always necessary for slow moving input signals. The quiescent current specification of an op-amp is valid only when the feedback is active. Some op-amps show an increased quiescent current when the inputs are not equal. A comparator is designed to produce well-limited output voltages that easily interface with digital logic. Compatibility with digital logic must be verified while using an op-amp as a comparator. Some multiple-section op-amps may exhibit extreme channel-channel interaction when used as comparators. Many op-amps have back to back diodes between their inputs. Op-amp inputs usually follow each other so this is fine. But comparator inputs are not usually the same. The diodes can cause unexpected current through inputs. Design A comparator consists of a high gain differential amplifier whose output is compatible with the logic gates used in the digital circuit. The gain is high enough that a very small difference between the input voltages will saturate the output, the output voltage will be in either the low logic voltage band or the high logic voltage band of the gate input. Analogue op amps have been used as comparators, however a dedicated comparator chip will generally be faster than a general-purpose operational amplifier used as a comparator, and may also contain additional features such as an accurate, internal reference voltage, adjustable hysteresis, and a clock gated input. A dedicated voltage comparator chip such as LM339 is designed to interface with a digital