Do you know the role of decoupling capacitors? Decoupling capacitors are used in many circuit design applications, but designers often have trouble omitting the use of decoupling capacitors. Do not underestimate the use of decoupling capacitors, its role is unquestionable.
When do you need a decoupling capacitor? What is its role? Below we will examine what happens when no decoupling device is used.
Figure 1: Buffer circuit with decoupling and no decoupling (measurement results)
Figure 1 shows the snubber circuit used to drive the RC load with decoupling capacitors and without decoupling capacitors (C1 and C2). We note that the output signal of the circuit contains high frequency (3.8MHz) oscillation without the use of decoupling capacitors. For amplifiers without decoupling capacitors, low stability, poor transient response, startup failure, and many other anomalies often occur.
Figure 2: Current with and without decoupling
Figure 2 illustrates why decoupling is important. It should be noted that the inductance of the power trace will limit the transient current. The decoupling capacitor is very close to the device, so the inductance of the current path is small. In the transient process, the capacitor can provide an extremely large amount of current to the device in a very short time. Devices that do not use decoupling capacitors cannot provide transient currents, so the internal nodes of the amplifier will sag (often called interference). Devices with no decoupling capacitors can cause device discontinuities due to internal power supply disturbances because the internal nodes are not properly biased.
In addition to using decoupling capacitors, you also need to take a short, low-impedance connection between the decoupling capacitor, the power supply, and the ground.
Figure 3: Comparison of good and bad PCB layout
Figure 3 compares the good decoupling board layout to the poor layout. You should always try to keep the decoupling connections at a short distance while avoiding the presence of vias in the decoupling path because the vias increase inductance. Most product specifications give recommendations for decoupling capacitors. If not given, you can use 0.1uF.
Properly connecting decoupling capacitors will save you a lot of trouble. Even if you don't use decoupling circuits on the test bench, it works well, but if you go into mass production, you may have problems with your product or other practical factors. Therefore, do not ignore the use of decoupling capacitors.
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