Several skills of operational amplifier application design
Operational amplifiers play an important role in the circuit, its application has been extended to all areas of automotive electronics, communications, consumer, etc., and will play an important role in terms of technical support in the future. 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. We are also based on this symposium, summed up a few tips application design op amp 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 noted, for amplifying weak signals, only a single amplifier is difficult to achieve good results, must use some more specific methods and sensor excitation means, and synchronous detecting circuit structure can be obtained very good measurement results. This synchronization detecting circuit is similar to lock-in amplifier structure, comprising a sensor excitation square wave, the current-to-voltage amplifier, and synchronous demodulation of three parts. He said should be noted that the current-to-voltage amplifier should be chosen very low input bias current of the op amp. 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 Internet users to solve such problems have also been supplemented, as friends "1sword" recommendations:
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), ADI and other companies.
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 sensor output of nA, pA stage select input current to the op amp. If there is not much need for speed, the op amp is not expensive. Of course the best instrumentation amplifier, it is the high cost of some.
4) if the choice of a non-instrument amplifier, feedback resistor is not too great, better M-European level. Otherwise, the resistance is relatively high. Then subsequent stage amplifying stage 2, the intermediate addition of a simple high-pass circuit, 50Hz interference suppression.
Second, the operational amplifier bias setting
When the dual power op amp connected to a single power source circuit, engineers encounter some friends dilemma in terms of the bias voltage is provided, such as a DC bias voltage dividing resistor is good or good to the reference voltage source? Some users suggested by the reference voltage source, on the grounds that high accuracy, also can provide a low AC bypass, some users recommended resistance, low cost and convenience reasons, this, Zhang Shilong not specifically what way except when stressed dual power op amps into a single power supply circuit, if the reference voltage, the result is the best. This reference voltage allows the system design to obtain the smallest noise and the highest PSRR. However, if the resistor divider embodiment, power supply ripple must be considered on the system, this usage is relatively high noise, low PSRR.
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 stable, zero drift reduced, at both ends of the feedback capacitor and the resistor, the DC negative feedback can form stable DC operating point of the amplifier; 3) may be selected by the input impedance of the operational amplifier is not high enough, resulting in charge leakage, resulting in zero drift.
Sign "camel" and "windman" also causes zero drift from the perspective of a detailed analysis of mathematical analysis, except that the source of interference that drift of the sensor must also be small, the cable resistance is large, open loop operational amplifier input 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 switched capacitor circuit technique, using CDS sampling mode can effectively eliminate offset voltage; 2) synchronous detection circuit structure can effectively eliminate the offset voltage.
Author: Richard Zhang
Editor-in-Chief of Electronic Engineering Album Website
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