Tip&Tricks for a good layout : why the placing of the decoupling capacitor is so crucial? T

#decoupling# #capacitor# #pcb design# #JLCPCB#

Lately I have had this doubt: is the position of the decoupling capacitor really significant? Well, the quick answer is YES!

Here you will see a brief experiment and its results that I hope will make you understand how significant actually is.

First of all, what are decoupling capacitors and why are they needed?

• Well, they are needed to stabilize voltage and eliminate noise in electronic circuits by providing a local and high-speed reservoir for components (such as microcontrollers etc.). It will prevent the EMI radiation, as well as isolate the sensitive components from the power supply noise. More specifically, when a circuit needs current instantly the supply rails can't react instantly because of wiring inductance and resistance, thus the supply voltage sags. Putting a decoupling capacitor helps with this by keeping the circuits stable.

What do you need? 

• An oscilloscope, a signal generator and a power supply. The circuit built featured a NPN BJT transistor, with a resistor of 20 Ohm on its source, the gate connected to ground and drain to Vcc. Connected to the gate there was a source generator generating a square wave of 5V of amplitude at 1kHz. The source was connected to the scope/oscilloscope with DC coupling.
• After building the circuit with a breadboard, you can put a capacitor at difference distances from the voltage source and see its effect on the voltage drop.

Obtained data analysis

• the voltage drop without a capacitor is around 4.62V. This is HUGE! Since the voltage power is of 5V.
• the 1000uF electrolytic/ 1uF film capacitors are the one that reduced more significantly the voltage drop. Generally the electrolytic however are slower.
• the more the capacitor is near to the voltage source the more it attenuates its voltage drop.

Key findings from data

The experimental data reveals a critical struggle between power stability and signal speed. Without any decoupling, the 4.62V drop on a 5V rail represents a near-total power failure every time the transistor switches. In a real-world application, this wouldn't just be electrical noise; it would cause a processor to brown out or a sensor to provide false readings because the voltage hits rock bottom.

These results also expose a performance trade-off tied to internal resistance. The 1000uF electrolytic capacitor acts like a massive water tank with a slow, narrow valve. While it holds enough energy to prevent the voltage from sagging, it cannot release that energy fast enough to keep up with the 1kHz square wave. This explains why the signal rise time slowed to 153ns with the large cap, whereas the 1uF film version, acting as a smaller but much faster bucket, snapped the signal into place in just 90ns.

CONCLUSION

When designing a PCB's layout, ALWAYS put the decoupling capacitors near its voltage source, the nearest the better!

Do not put them distant!!