Operational amplifier is a very important component in the measurement and control circuit, and has a very wide range of applications in the measurement and control circuit. As a commonly used device, operational amplifiers are more and more widely used in measurement and control circuits. The measurement and control circuit has more requirements for operational amplifiers than other applications, so more stringent requirements are put forward for the design of operational amplifiers. If the operational amplifier is not properly designed in the actual design process of the measurement and control circuit, it is likely to seriously affect the performance of the entire measurement and control circuit, or even make the circuit unable to work normally. This article will conduct in-depth research and analysis on several common misuses of operational amplifiers in the design of measurement and control circuits, and propose some solutions.
1.Package selection
In the design process of the measurement and control circuit, the packaging problem of the operational amplifier will inevitably be encountered. Generally speaking, there are three packaging methods for operational amplifiers: one package in one package is called single op amp, two in one package is called dual op amp, and four in one package is called quad op amp. Whether it is a single op amp, dual op amp or four op amp, the power supply is connected together. Dual op amps and quad op amps have two input terminals, non- inverting and inverting, while single-op amplifiers also have a zero-adjustment terminal on this basis. In the actual design process, if zero adjustment is very important, a single op amp must be used. There are two points that must be paid attention to in a single op amp. One is that the zero-adjusting terminal may become another input terminal, which can cause huge errors; The second is that the two input terminals of the single op amp are far apart, so the negative feedback connection is very long, which increases the area of the loop, which becomes an important problem for high-speed circuits. However, dual op amps and quad op amps do not have such problems. Therefore, in high-speed applications, dual op amps and quad op amps should be used as much as possible. In addition, the op amp circuits of dual op amps and quad op amps are relatively close. If the requirements for circuit isolation are relatively high, a single op amp is still needed.
2.Misuse it as a voltage comparator
Voltage comparators are also commonly used devices in measurement and control circuits, and their appearance is similar to operational amplifiers, and the symbols on the circuit diagram are similar. In this way, some designers may mistakenly believe that the two can be used interchangeably. The most common problem occurs in dual op amps and quad op amps. Some designers use redundant op amps as voltage comparators to save circuit space and reduce production costs. A careful comparison of the two shows that the input stage is superficially the same, but the output stage of the operational amplifier is slightly more complicated than that of the voltage comparator. The output stage of the operational amplifier is mainly in the form of complementary output, while the output stage of the voltage comparator is in the form of open-drain output or the collector. If the operational amplifier is used as a voltage comparator, it may not work properly and even burn out the circuit. Therefore, when both of them are needed at the same time, chips that package the operational amplifier and the voltage comparator together, such as TLV2302, TLV2702, TLV2704, and TLV2304, can be used.
3.Treatment of redundant operational amplifiers
Generally speaking, when using dual op amps and quad op amps, it is very likely that redundant op amps will appear. If these operational amplifiers are not properly processed, problems such as excessive noise, excessive power consumption, and overheating are likely to occur. If the operational amplifier is not terminated, it is easy to cause a large and rapid voltage change, thereby forming strong electromagnetic interference. If the two ends of the operational amplifier are grounded separately, the output stage is easily saturated, and the ground potential difference is easily affected by the outside, causing the output voltage to change between the positive and negative power supply voltages. It is recommended to connect the non-inverting input terminal of the operational amplifier to the middle of the positive and negative power supplies, or to the ground of the split power system.
4.Error introduced by zero adjustment
A single op amp generally has a zero-adjusting terminal, and its voltage is generally low, and the effect of the offset of the op amp can be compensated by applying a voltage to it. When the impedance of the zero-adjusting terminal is too low, it is easy to become other input terminals of the operational amplifier, and the operational amplifier is generally very sensitive to the signal changes at the zero-adjusting terminal, making the voltage gain of the zero-adjusting terminal larger than other input terminals. In addition, when the current of other circuits returns to the power supply, the voltage drop generated on the power supply wire becomes the noise of the operational amplifier, which may become the main noise source of the entire system. In order to solve this problem, you can choose to connect the sliding end of the zero-adjusting potentiometer to the V+ or V- pin of the operational amplifier according to the actual situation.
5.Leakage current coupling interference
High input impedance operational amplifier is one of the more commonly used operational amplifiers in measurement and control circuits. For this kind of operational amplifier, once there is a relatively high potential around it, it is likely that there will be leakage current due to poor insulation, which will cause interference. The interference voltage is generally much larger than the offset voltage of the operational amplifier, and even much higher than the input signal of the operational amplifier, because high-impedance operational amplifiers are generally used to amplify weaker signals. For high input impedance operational amplifiers, even very small leakage currents may cause serious adverse consequences. For this problem, a shielding method can be used to suppress it. Specifically, the input end of the operational amplifier is enclosed by a conductor post to form a shielding layer, and then connect the shielding layer to a position with a lower impedance.
6.Drive capacitive load
Generally speaking, the load of the measurement and control circuit is mostly capacitive. This is because the load circuit and the load capacitance are inherently capacitive. If the capacitive load is too large, it is very likely that the operational amplifier will be overloaded. In addition to high-power operational amplifiers, general operational amplifiers can only provide about 15 mA of current. For example, if a first-order RC filter is connected to the input of the operational amplifier, its corner frequency is 1/2πRC. It can be seen from the formula that proper adjustment of the values of R and C can obtain the required corner frequency. However, in the low-noise design process, since the rms voltage of thermal noise is proportional to the square root of the resistance, a combination of small R and large C is required. However, this is likely to cause overload of the op amp and cause greater noise. In addition, capacitive loads may also cause oscillation, which requires proper phase compensation.
Conclusion
This article has conducted in-depth analysis and research on several common misuses of operational amplifiers in the design of measurement and control circuits, and proposed improvements. However, there are many influencing factors that affect the application of operational amplifiers in measurement and control circuits. In addition to understanding the basic application knowledge of operational amplifiers, it is also necessary to continuously accumulate experience to avoid misuse in the design as much as possible, and strive to continuously improve the operational amplifier Application level in measurement and control circuits.
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