Please forward this error screen to 216. The differential voltages must stay within the limits analog circuit design tutorial pdf by the manufacturer.
LM139 family, allow the input potential to drop 0. This parallels the characteristics of comparators and can be substituted in applications with low-performance requirements. Comparators are often used, for example, to check whether an input has reached some predetermined value. In most cases a comparator is implemented using a dedicated comparator IC, but op-amps may be used as an alternative. Comparator diagrams and op-amp diagrams use the same symbols. Figure 4 shows a comparator circuit.
Note first that the circuit does not use feedback. The circuit amplifies the voltage difference between Vin and VREF, and outputs the result at Vout. If 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 Fig. 5, for example, will provide stable operation even when the Vin signal is somewhat noisy.
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. 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. A dedicated voltage comparator will generally be faster than a general-purpose operational amplifier pressed into service as a comparator. If there is a fixed voltage source from, for example, a DC adjustable device in the signal path, a comparator is just the equivalent of a cascade of amplifiers. When the voltages are nearly equal, the output voltage will not fall into one of the logic levels, thus analog signals will enter the digital domain with unpredictable results. To make this range as small as possible, the amplifier cascade is high gain. Slew rate has no meaning for these devices.
When the inverting input is at a higher voltage than the non inverting input, the output of the comparator connects to the negative power supply. While in general comparators are “fast,” their circuits are not immune to the classic speed-power tradeoff. High speed comparators use transistors with larger aspect ratios and hence also consume more power. Depending on the application, select either a comparator with high speed or one that saves power. Likewise if a comparator is needed to implement a relaxation oscillator circuit to create a high speed clock signal then comparators having few nano seconds of propagation delay may be suitable. A comparator normally changes its output state when the voltage between its inputs crosses through approximately zero volts. Small voltage fluctuations due to noise, always present on the inputs, can cause undesirable rapid changes between the two output states when the input voltage difference is near zero volts.