Have you ever noticed that circuit designer like to put a small 0.1uF (or 100nF) capacitor adjacent close to the supply pin of an IC such as microcontroller, microprocessor, or an FPGA? Did you guys know what is the purpose of that capacitor? That capacitor is called a decoupling capacitor. The name decoupling, because of its role which for decoupling the power supply.
Before we continue, let’s talk about noise first. In electronics, noise is an unwanted random signal that present in our system. Source of noise can come from many proccess, e.g. random thermal motion of charge carrier, electromagnetic interference, mechanical part (push button, switch, motor, gear), static discharge, solar radiation, and still many other. Noise can also be considered as an AC signal component, it has frequency ranges from low to very high.
Going back to the main topic, so why does power supply need decoupling? Because in reality, power supply input is in fact can be contaminated with noise. The noise renders the supply so it contain glitches, distortion, ripples, voltage spikes, and carry an additional AC voltage components.
Another source of noise is the circuit itself. Consider a circuit that contains several ICs. When the IC performing its function, the current it drawn from the supply will be switching on and off rapidly. Remember the current flow on a PCB through a copper trace which has a resistance albeit its small ohm (Ohm Law: V = I x R). So a change in current also impose a change in voltage. The change in voltage in one IC causes change in another IC, eventually it added up and affecting the entire circuit. The generated effects can be so bad, as it can stop the circuit from functioning properly.
Noise is unavoidable, its exist in our system like a plague. The only thing we can do is to reduce the effect. Luckily the solution is simple, put a decoupling capacitor!
Remember this formula from the old school physics class? or at the college Fundamental of Electric Circuit course?
The characteristic of a capacitor is to oppose changes in voltage, therefore it can smooth the noise. Capacitor can also be though as a small local power supply, because it stores charge. If the main power supply is drop suddenly, the IC can still use the charge energy stored in the capacitor close by. If voltage spike occurred, the capacitor absord the excess voltage and reduce the impact because the spike reach the capacitor first before the IC. In another term, it improve or smooth the transient response.
If we take a closer look at the formula, we’ll realize that the bigger the capacitance and the frequency, the smaller the reactance will be, hence it goes toward short circuit. So that means in order to annihilate high frequency noise, what we need to do is just pick a capacitor with high capacitance value? That is not always true! Why? Because as the frequency goes higher, another value will come to play a role.
In addition to capacitive reactance, a capacitor also has inductive reactance (XL). This is due to the fact that everything that has a resistance also has an inductance. A capacitor is no exception, the lead, the metal foil, the insulator can act like a coil, hence it has an inductance. The simple formula for inductive reactance is:
The capacitor response to frequency is the sum of both XC and XL. Over the frequency range in the upward direction, XC will goes down, while XL goes up. The plot of reactance versus frequency is shown below:
The above picture is a commonly used scheme, see that the supply fed the capacitor first before being passed on to the IC. For a small application circuit, we probably can get away without pay too much care to decoupling, but for complex circuit, professional do the decoupling on every IC supply pin.
The sum of those two effect result in a capacitor can only filter noise up to a certain frequency ( called self resonant frequency ), and above this frequency capacitor act like an inductor and wouldn’t remove noise effectively at all. The graph below just for illustrative purpose, it shows the capacitive and inductive reactance of several capacitor value.
From the graph we can say that one capacitor value only good to cover some range of frequency. And of course circuit designer want to cover as wide as frequency as possible for their circuit. To achieve this goal, a circuit designer usually pick two value of capacitor: small one to filter high frequency noise, and large one for low frequency noise ( but its not limited to just two, more than that depend on requirement and availability ). Typical values to be used are 0.1uF, 0.47uF, 4.7uF, 10uF, 22uF, 47uF and 10mF (bulk). For smaller capacitor, a ceramic type is preferred. As the lead from the capacitor body contribute to the inductance, its important this parameter is as short as possible. A SMD ceramic capacitor is a good choice. For large/bulk capacitor, usually electrolytic/elco is choosen or tantalum, and often only one bulk capacitor present on the board is enough. Large/bulk elco is usually used for application that need to drive large current such as driving a motor.
When choosing the value for SMD ceramic or MLCC (Multi Level Ceramic Capacitor), we also need to pay attention to their package size. The smaller the SMD size the better its performance against high frequency, and the bigger the SMD size the better it is for low frequency. For example consider a circuit that uses 0.47uF SMD0402 for high frequency and 4.7uF SMD0805 for low frequency. The 0.47uF is chosen for high frequency because of its size is 0402, and the 4.7uF is chosen for low frequency because of its size is 0805. Another example is a 0.01uF SMD0402 capacitor and 4.7uF SMD0402 can have an almost identical high frequency characteristic due to the same 0402 package size. But, the 4.7uF provides better low frequency response because its capacitance value is bigger than the 0,01uF. In other words, a single 4.7uF SMD0402 sometimes is just enough and will do everything we would expect from a 4.7uF SMD0805 and 0.47uF SMD0402. Therefore the best per-pin decoupling capacitor is the highest value capacitor in the smallest size.
There’re often a case when we want to isolate some part of the circuit with particularly sensitive application for example oscillator, ADC, or in-circuit programming. Circuit designer usually would put a series resistor to provide isolation from the rest of circuit. The values often used are 10 Ohm, 330 Ohm, or even 470 Ohm.
Even better is to use an inductor or ferrite bead for ultimate isolation. The value is not too critical but often as big as we can manage ( i.e. a few ten of milli Henrys ).
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