Is the decoupling capacitor really what you need?

Before working as an application engineer, I was an IC test development engineer. One of my projects is to characterize the I2C temperature sensor. After writing some software, I manually soldered a prototype board. Due to the rush of time, I saved the troublesome decoupling capacitors. Who needs it, right? I have been collecting data for about a week, but no results are matched to the expected results. So I made a lot of changes, trying to improve performance, but it didn't work. Finally, I decided to add a decoupling capacitor. As expected, the problem was solved. This makes me wonder if I always need to use decoupling capacitors?

What is its role?

To answer this question, you need to test what happens when you don't use a decoupling device. Figure 1 shows the snubber circuit used to drive the R-C 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.

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 supply 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 internal discontinuities in the device 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. 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 the recommended values 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 a decoupling circuit on the test bench, it works well, but if you go into mass production, you may have problems with this or another due to process changes and other practical factors. Learn from my lesson, don't fall into the trap of not using decoupling! From the perspective of practical experience, the op amp will teach you the circuit debugging.