“The voltage comparator, as the name implies, is one of the two input terminals as a reference, the other one is compared with the reference, and the output has only two states of high level and low level. An analog signal can be converted into a digital signal by means of a voltage comparator.
1. The working principle of the voltage comparator
The voltage comparator, as the name implies, is one of the two input terminals as a reference, the other one is compared with the reference, and the output has only two states of high level and low level. An analog signal can be converted into a digital signal by means of a voltage comparator.
+The potential of the input pin>-When the potential of the input pin is, output a high level;
-When the potential of the input pin>+the potential of the input pin, the output is low;
Comparator input and output diagram
2. Differences between voltage comparators and op amps
The pin structure and circuit symbols of the voltage comparator are similar to those of the operational amplifier, and sometimes the operational amplifier is directly used instead of the comparator. In essence, the comparator is an open-loop application of the operational amplifier, but in fact the two are two complete different devices. The main differences are as follows:
①Internal composition is different
One difference between op amps and comparators comes from the lack of internal phase compensation capacitors. Since the operational amplifier is used as a negative feedback circuit, an anti-vibration phase compensation capacitor needs to be set inside the IC. The comparator does not constitute a negative feedback circuit, so there is no built-in phase compensation capacitor. Comparators without phase compensation capacitors have better responsiveness than op amps because the phase compensation capacitors limit the input-to-output response time. On the other hand, depending on the presence or absence of the phase compensation capacitor, when an operational amplifier is used as a comparator, its responsiveness is much lower than that of a comparator due to the limitation of the phase compensation capacitor. This needs to be taken into account when the op amp is used as a comparator.
②Different output characteristics
As can be seen from the above figure, the operational amplifier adopts a dual transistor push-pull output, while the comparator is an open-collector output, so when using it, the comparator needs an external pull-up resistor from the positive power supply terminal to the output terminal. Operational amplifiers can be used in linear amplification circuits (negative feedback) as well as non-linear signal voltage comparisons (open loop or positive feedback). Voltage comparators can only be used for signal voltage comparison, not for linear amplifier circuits (comparators have no frequency compensation). But comparators have faster slew rates and shorter delays than op amps.
③Accuracy is different
The design of the comparator is for voltage threshold comparison, the required comparison threshold is accurate, the rising or falling time of the output edge after the comparison should be short, and the output conforms to the TTL/CMOS level/or OC, etc., and the accuracy of the intermediate section is not required. , and the driving capacity is also different. General situation: using op amps as comparators, most of them cannot reach full-scale output, or the edge time after comparison is too long, so it is better to use less op amps as comparators in the design. The flip speed of the comparator is fast, about the order of ns, while the flip speed of the op amp is generally of the order of us (except for special high-speed op amps). The op amp can be connected to the negative feedback circuit, but the comparator cannot use the negative feedback. Although the comparator also has two input terminals, the non-inverting and the inverting, but because there is no phase compensation circuit inside, if the negative feedback is connected, the circuit cannot be used. Stable work. There is no internal phase compensation circuit, which is the main reason why the comparator is much faster than the op amp.
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