Op amps are important analog devices. When choosing a good op amp, you need to understand the design requirements. You also need to know the manufacturing process of the op amp and some specific parameters. This article will introduce the precautions for op amp selection.
Suppose there is a perfect amplifier for any circuit design. This perfect op amp has an infinite open-loop gain and bandwidth with zero bias voltage, input bias current, input noise and supply current, and can operate at any supply voltage. Since it is truly perfect, it should be free. But this perfect op amp doesn't actually exist at all, and it can't exist. The vendor then offers a wide variety of operational amplifiers, each with its own different performance, features and price. Knowing the most important parameters of the amplifier, you can find the most suitable op amp.
Bias voltage and input bias currentBias voltage is a key factor in precision circuit design. For those parameters that are often overlooked, such as bias voltage drift and voltage noise that vary with temperature, they must also be measured. Accurate amplifiers require a bias voltage drift of less than 200μV and an input voltage noise of less than 6nV/√Hz. The bias voltage drift with temperature is required to be less than 1μV/°C.
The low bias voltage specification is important in high gain circuit design because the bias voltage is amplified to cause a large voltage output and will occupy a large portion of the output swing. Temperature sensing and tension measurement circuits are examples of applications that utilize precision amplifiers.
Low input bias currents are sometimes required. The amplifier in the optical receiver must have a low bias voltage and a low input bias current.
Among all amplifiers, the chopper amplifier provides the lowest bias voltage and the lowest temperature-dependent bias voltage drift. Many weight-measuring devices have high gain requirements and require high-quality precision amplifiers. Chopper amplifiers are a good choice.
Pay attention to the influence of power supplyAmplifiers in portable systems require operation at very low supply voltages and the supply current should be small to maximize battery life. These amplifiers typically also have good output drive capability and high open loop gain.
Although many amplifiers claim to consume very little current, care should be taken when choosing. Be sure to read the parameter list carefully to keep an eye on performance issues that may arise from working at low voltages. Some low-power op amps have a wide range of supply currents when the output voltage changes. At low supply voltages, the output current drive capability can also drop significantly. The parameter table can be consulted to determine the output current drive capability that can be achieved at a particular supply voltage.
Another option is to use an amplifier with a "off" feature. Although this amplifier has a high supply current, it can be turned off when not in operation to enter an ultra-low current state. The higher supply current allows the amplifier to have faster speeds and a large output drive capability.
Take care to avoid some common mistakesThe op amp parameter table contains a lot of information, but sometimes it can be difficult to compare which two parameter tables to determine which op amp performs better. The input common mode voltage range indicator is an example. This parameter is often misused.
To ensure proper operation, pay attention to the common mode rejection ratio (CMRR) test conditions. The test conditions given represent the common mode input voltage range. The common-mode input voltage range of the rail-to-rail input amplifier ranges from a negative supply (V-) to a positive supply (V+).
Unlike the input voltage range, the output voltage swing of the op amp is not clearly defined. Most single-supply amplifier parameter tables give voltage specifications for both high and low output swings. It represents the ability of the amplifier's output swing to be close to the positive supply and ground when the amplifier draws in and pumps current. Unfortunately, these values ​​cannot be directly compared against different vendors' parameter tables because different vendors define the output load in different ways. The key depends on whether the load is a resistor or a current source. If the load is a current source, a similar load current can be measured, which makes it easy to compare the output voltage swing between different amplifiers. If the load is a resistor, it is determined whether the resistor is connected to the power supply voltage Vcc, or to the reference voltage Vcc/2, or to ground. Connecting the load to Vcc/2 will allow the amplifier's output stage to pump and sink current, but the amplifier's output current is equivalent to half of the load ground or positive supply. This difference in output current allows the op amp to swing close to the value of the positive and negative supplies. This can be misleading to some extent because in most single-supply DC application circuit designs, the load is directly grounded and the amplifier output swings below the positive supply.
Capacitance drive capability is a parameter that is often defined in the parameter table. All amplifiers have different degrees of sensitivity to capacitive loads. Some low power amplifiers may become unstable with respect to capacitive loads of only a few hundred picofarads. Therefore, the parameter tables of these amplifiers may hide this fact.
To determine the sensitivity of the amplifier to the output capacitor, it can be determined by an overshoot plot relative to the capacitive load. Another good diagram is a small signal response map that can be used to observe the extent of overshoot and the fall time of a particular capacitive load. Some parameter tables also provide a gain-bandwidth map of the relative capacitive load.
One way to reduce overshoot and damped oscillations is to connect a series RC network in parallel with the output load. The optimum value of this network (also called the damping circuit) can be determined experimentally. Other methods of reducing overshoot and damped oscillations can also be found in the application description of the device.
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