Several skills of operational amplifier application design
Operational amplifiers play an important role in circuits, and their applications have been extended to various fields such as automotive electronics, communications, and consumption, and will play an important role in supporting future technologies. In the practical application of operational amplifiers, design engineers often encounter problems such as selection, Power Supply circuit design, bias circuit design, PCB design, etc. In the special discussion of "Operation Amplifier Application Design" held on the Electronic Engineering Album website, Mr. Zhang Shilong, President of Shengbang Microelectronics Co., Ltd., was invited to answer and interact with engineers. Based on this topic discussion, we also summarized several techniques for operational amplifier application design to readers.
1. How to achieve weak signal amplification?
The sensor + operational amplifier + ADC + processor is a typical application circuit of an operational amplifier. In this application, a typical problem is that the current provided by the sensor is very low. In this case, how to complete the signal amplification? Zhang Shilong pointed out that for the amplification of weak signals, it is difficult to achieve good results with only a single amplifier. Some special methods and sensor excitation methods must be used, and the use of synchronous detection circuit structure can obtain very good measurement results. This synchronous detection circuit is similar to the structure of a lock-in amplifier, including the sensor's square wave excitation, current-to-voltage amplifier, and synchronous demodulation. He said that it should be noted that the current-to-voltage amplifier requires an op amp with extremely low input bias current. In addition, synchronous demodulation needs to select dual SPDT analog switches.
Another engineer friend suggested that in the selection and layout of op amps, capacitors and resistors, special attention should be paid to the selection of high impedance, low noise operation and low noise resistance. Some netizens have also supplemented the resolution of such problems. For example, netizen "1sword" suggested:
1) Pay attention to the balance in the circuit design and try to balance it. It is effective for suppressing interference. These can be found in the design manuals of the op amps of National Semiconductor, BB (acquired by TI), and ADI.
2) It is recommended to add a metal shield to cover the weak signal part (open a small mold). The metal body is connected to the circuit ground, which can greatly improve the anti-interference ability of the circuit.
3) For the nA level of the sensor output, select the op amp with input current pA level. If there is not much demand for speed, the op amp is not expensive. Instrumentation amplifiers are of course the best, because they cost more.
4) If non-instrumental op amps are used, the feedback resistance should not be too large, M-class is better. Otherwise, the resistance is relatively high. The second stage is amplified by two stages, and a simple high-pass circuit is added in the middle to suppress 50Hz interference.
Second, the offset setting of the operational amplifier
When the dual power supply is connected to a single power supply circuit, the engineer friend will encounter some dilemma in the setting of the bias voltage, such as whether the DC voltage used as the bias is divided by a resistor or connected to a reference voltage source? Some netizens recommend using a reference voltage source because of the high accuracy, and also provide a low AC bypass. Some netizens recommend using a resistor because the cost is low and convenient. Zhang Shilong did not specifically indicate which method to use for this. It just emphasizes that when the dual-power operational amplifier is changed to a single-power circuit, if the reference voltage is used, the effect is best. This reference voltage allows the system design to obtain the smallest noise and the highest PSRR. However, if the resistance voltage division method is used, the influence of the power supply ripple on the system must be considered. This usage noise is relatively high and the PSRR is relatively low.
3. How to solve the problem of zero drift of operational amplifiers?
Some netizens pointed out that the general piezoelectric acceleration sensor will be connected to a primary charge amplifier to realize the charge-voltage conversion, but when the sensor is working dynamically, the output voltage of the charge amplifier will not return to zero. How to solve this problem?
In this regard, the netizen “Frank†analyzed that there are several possibilities that will lead to zero drift: 1) The feedback capacitor ESR characteristic is not good, which changes with the change of charge; 2) The feedback capacitor is not paralleled with a resistor, in order to amplifier The work is stable, and zero drift is reduced. The resistance is connected at both ends of the feedback capacitor to form a DC negative feedback to stabilize the DC operating point of the amplifier; 3) The input impedance of the op amp that may be selected is not high enough, causing charge leakage and leading to zero drift.
Netizens "camel" and "windman" also analyzed the causes of zero drift from a mathematical analysis point of view, and believed that in addition to making the interference source drift small, the resistance of the sensor and cable must be large, and the open-loop input of the op amp The impedance should be high, and the feedback resistance of the op amp should be small, that is, the role of the feedback resistance is to prevent drift and stabilize the DC operating point. But if the feedback resistance is too small, it will also affect the lower frequency limit of the amplifier. Therefore, it must be considered comprehensively!
The guest Zhang Shilong suggested that the phenomenon that the output voltage of the charge amplifier does not return to zero is generally solved by the following methods:
1) Using the technique of switched capacitor circuits, the offset voltage can be effectively eliminated by using the CDS sampling method; 2) The offset voltage can be effectively eliminated by using the synchronous detection circuit structure.
Author: Richard Zhang
Editor-in-Chief of Electronic Engineering Album Website
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