In accordance with the parameters of integrated operational amplifiers, integrated operational amplifiers can be divided into the following categories. Precision operational amplifiers generally refer to the offset voltage is less than 1mV operational amplifier and at the same time emphasize the offset voltage with the temperature change drift value should be less than 100 V. For DC input signals, the VOS and its temperature drift is small enough on the line, but for AC input signals, we must also take into account the input voltage noise and input current noise of the operational amplifier, the input voltage noise and the input current noise in many applications appear to be more important. In many applications, input voltage noise and input current noise are more important. At the same time, many application designs require the use of programmable high-precision operational amplifiers (PVGAs) to dynamically adjust the amplification in the signal chain.
There are challenges in selecting the best precision op amp for implementing the input processing designs of many high-end sensors.
The selection of operational amplifiers becomes particularly difficult when the type of sensor and/or the environment in which it will be used brings with it a number of special requirements, such as ultra-low power consumption, low noise, zero drift, rail-to-rail inputs and outputs, reliable thermal stability, and reproducibility for thousands of readings and/or consistent performance under harsh operating conditions.
In complex sensor-based applications, designers need to make multiple considerations in order to obtain a precision operational amplifier with the best combination of specifications and performance, while also considering cost. Specifically, chopper-stabilized operational amplifiers (zero-drift amplifiers) are ideally suited for applications requiring ultra-low offset voltages as well as zero drift. Chopper op amps achieve high DC accuracy by continuously running a calibration mechanism implemented on the chip.
The difference between precision op-amp circuits and ordinary op-amp circuits:
Ordinary op-amp circuits are generally similar in composition, precision amplifier circuits will be more power supply decoupling, filtering and other specially designed circuits. The main difference lies in the operational amplifier, precision operational amplifier performance is much better than the general operational amplifier, such as open-loop amplification is greater, CMRR is greater, slower, GBW, SR is generally smaller. Out-of-regulation voltage or out-of-regulation current is relatively small, temperature drift is small, low noise and so on. The performance of a good precision op-amp is far from the general operational amplifier can be compared, the general op-amp offset is often a few mV, while the precision op-amp can be as small as 1uV level. To amplify small signals, you must use precision op-amps, with the general op-amps, it will bring itself into a large interference. To improve through the peripheral circuitry, small or fine-tuning can be, but can not be drastically or completely changed.
In the future, with the introduction of a variety of new sensors, people are increasingly demanding performance of electronic equipment, a large number of automation equipment into use, low-distortion, low noise, high-precision amplifiers will be in the field of medical electronics, measuring instruments, automotive electronics, industrial automation equipment and other fields to show their skills. High-precision operational amplifier performance indicators will be with the times, towards a lower voltage current noise, lower distortion voltage, lower distortion voltage temperature drift, greater bandwidth, smaller power consumption, higher voltage direction of continuous innovation, products continue to introduce new, to meet the customer's ever-increasing design needs.
The most commonly used precision operational amplifier is the OP07, and its family, OP27, OP37, OP177, OPA2333. there are many others, such as the products of the U.S. AD company, many of which are led by OPA. Integrated operational amplifier is one of the most widely used devices in analog integrated circuits. In a variety of systems composed of operational amplifiers, due to different application requirements, the performance requirements of operational amplifiers are also different.
In the absence of special requirements of the occasion, try to use general-purpose integrated op-amps, which can reduce costs, but also easy to guarantee the source. When a system to use more than one op-amp, as far as possible, the use of multi-op-amp integrated circuits.
Evaluating the performance of integrated op-amps, should look at its comprehensive performance. Generally use the good value coefficient K to measure the good degree of integrated op-amps, which is defined as follows: where, SR for the conversion rate, the unit of V / ms, the larger the value, indicating that the AC characteristics of the op-amp the better; Iib for the input bias current op-amps, the unit is nA; VOS for the input voltage out of phase, the unit is mV. Iib and VOS value the smaller, indicating that the op-amp dc characteristics of the better. Therefore, for the amplification of audio, video and other AC signals circuit, choose SR (conversion rate) is more appropriate; for the processing of weak DC signals circuit, the choice of higher precision op-amps is more appropriate (both out-of-phase current, out-of-phase voltage and temperature fluctuations are relatively small).
The actual choice of integrated op-amps, in addition to the coefficient of merit to be considered, should also take into account other factors. For example, the nature of the signal source, is a voltage source or current source; the nature of the load, integrated operational amplifier output voltage and current to meet the requirements; environmental conditions, integrated operational amplifier allowable operating range, operating voltage range, power consumption and volume and other factors to meet the requirements. 1. Power supply mode of integrated op-amps
Integrated op-amps have two power supply terminals, +VCC and -VEE, but there are different power supply modes. For different power supply methods, the input signal requirements are different.
(1) Symmetrical dual power supply
Operational amplifiers are mostly powered in this way. Positive power (+E) and negative power (-E) are connected to the +VCC and -VEE pins of the operational amplifier, respectively, relative to the public **** terminal (ground). In this way, the signal source can be directly connected to the input pin of the op-amp, and the amplitude of the output voltage can be up to positive and negative symmetrical supply voltage.
(2) Single-supply mode
Single-supply mode is to connect the -VEE pin of the op amp to the ground. At this time, in order to ensure that the internal unit circuit of the op amp has a suitable static operating point, the input of the op amp must be added to a DC level, at this time the output of the op amp is based on a DC level with the input signal changes. For the AC amplifier, static, the output voltage of the operational amplifier approximate VCC / 2, in order to isolate the output of the DC component access capacitor C3.
2. Zeroing of the integrated operational amplifier
Because of the integrated operational amplifier of the input offset voltage and the input offset current effect, when the operational amplifier composed of a linear circuit input signal is zero, the output is often not equal to zero. In order to improve the circuit's arithmetic accuracy, the requirements of the out-of-phase voltage and out-of-phase current caused by the error compensation, which is the operational amplifier zeroing. Commonly used zeroing methods are internal zeroing and external zeroing, and for the integrated operational amplifier without internal zeroing terminal, to use external zeroing method.
3. integrated op-amp self-excited oscillation
op-amplifier is a high-amplification multi-stage amplifier, connected to the depth of negative feedback conditions, it is easy to generate self-excited oscillation. In order to make the amplifier can work stably, it is necessary to add a certain frequency compensation network to eliminate self-excited oscillation.
In addition, to prevent low-frequency oscillation or high-frequency oscillation caused by the power supply internal resistance measures in the integrated operational amplifier positive and negative power supply input to the ground must be added to an electrolytic capacitor (10mF) and a high-frequency filtering capacitor (0.01mF ~ 0.1mF).
4. Protection of integrated op-amps
The safety protection of integrated op-amps has three aspects: power protection, input protection and output protection.
(1) Power supply protection. The common failures of the power supply are power polarity reversal and voltage jump. Power reverse protection and power supply voltage surge protection circuit, for the poor performance of the power supply, in the power supply on and off moment, there is often a voltage overshoot. The FET current source and voltage regulator clamp protection, voltage regulator regulator value is greater than the normal operating voltage of the integrated operational amplifier and less than the maximum allowable operating voltage of the integrated operational amplifier. the current of the FET tube should be greater than the normal operating current of the integrated operational amplifier.
(2) Input protection. If the input differential mode voltage of the integrated op amp is too high or the input *** mode voltage is too high (beyond the limit parameter range of the integrated op amp), the integrated op amp will also be damaged.
(3) Output protection. When the integrated op amp is overloaded or the output is short-circuited, if there is no protection circuit, the op amp will be damaged. But some integrated op amp internal current limit protection or short circuit protection, the use of these devices do not need to add output protection. There is no internal current limiting or short circuit protection for integrated op amps.