[SOLVED] requirement of decoupling capacitor.......

[student52]

Hey all,

I have seen quite a few micro controllers recommend a capacitor between vdd and vss. There are other things which do not require this coupling capacitor as well. What purpose does this capacitor serve??? Can circuit work without that capacitor???

Thanks in advance,
Regards.

 

[ckshivaram]

Decoupling capacitors are there to ensure that no paracitic DC components will be propagated to the rest of the circuit or inside the IC. It is clearly for protection reasons for both sides.
It is important to have the decoupling caps right next to the parts, because otherwise the impedance of the trace will reduce their effectiveness...

Decoupling capacitor - Wikipedia, the free encyclopedia

---------- Post added at 11:52 ---------- Previous post was at 11:48 ----------

A decoupling capacitor is used to reduce the change in power supply voltage.

You attach capacitors across VCC/GND to try and keep the voltage more constant. Under a DC circuit, a capacitor acts like an open circuit so there is no problem with shorting there. As your device is powered up (VCC=5V), the capacitor is charged to capacity and waits until there is a change in the voltage between VCC and GND (VCC=4.5V). At this point, the capacitor will discharge to try and bring the voltage back to the level of charge inside the capacitor (5V). This is called "smoothing" (or at least that is what I call it) because the change in voltage will be less pronounced.

Ultimately, the voltage will not ever return to 5V through a capacitor, rather the capacitor will discharge until the charge inside it is equal to the supply voltage (to an equilibrium). A similar mechanism is responsible for smoothing if VCC increases too far beyond its average (VCC=5.5V perhaps).

 

[Raza]

Hi,
If you are aware of the Analogue and Digital electronics then it will be very easy to understand, anyways:
In analogue circuits take an example of an Audio amplifier, when you are listening music and in between get a noise ripple it will simply disturb the music quality for a short time and will continue, simple.
But in Digital circuits if a noise is superimposed on DC supply and it enters the Microcontroller (which is purely working on the Lows and Highs) may not ignore this noise and will change some of its function as output which is not desired at all. Just to eliminate this Noise on the DC supply a Decoupling Capacitor is used, as the AC ( a noise in this case) can pass through the capacitor and DC remains blocked. Here with the Decoupling capacitor (some times called Bypass Capacitor) the noise passed to the return path (Ground) without disturbing the working of the Microcontroller.
This is in short why the decoupling capacitors are used.
Hope it explains a bit.