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Do we need decoupling capacitors for each IC?

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ericmar

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As above.

Besides, I'm using a voltage regulator (7805) in my circuit, do I need to put decoupling capacitors too? I found some circuits which always use few capacitors on the output voltage side of the voltage regulator, what are they used for?

Thanks!
 

Hi

Its depends on your Vcc trace length. Regulator manufacturesrs recommend using this capacitor if your Vcc trace length greater than 1 inch. Not using this capacitor increases chance of oscillation in Vcc.

Regards
 

Hi,

Decoupling capacitors are needed by the regulator itself to avoid output voltage oscillations, but also between vcc and gnd pins of analog and digital circuits.

For analog circuits, Vcc and Gnd must be shorted for AC signals for a very large bandwith. Therefore, you can see high value electrolitic capacitors in parallel with small ceramic ones after the regulator between Vcc and Gnd.

For digital circuits, a small 47 to 100nF ceramic capacitor is connected between Vcc and Gnd pins (as close as possible) of each circuit to provide the current pulse needed during circuit state transition from 0 to 1 or from 1 to 0.
 

Decoupling capacitors are needed for an IC depending on the power rating/switching/ type.
For sensitive circuits you stabililze voltage by using the capacitors.
For switching you provide small and fast "power supplies" when using capacitors. If lot of switching is not present no needof capacitors.
B R M
 

I would always decouple Ics as closeas you can get them to the IC and always on switching Ics
Barrybear
 

I think the output capacitors are used for transient response. The question is why are they external to the IC? I love to see how people will load a regulator with a capacitor. Decoupling capacitors are used because of RF noise.
 

In this I.C.s 78xx, 79xx, is very important that de output capacitor were minor than 1 microfarad. If you need more, you must put a 1n4007 in antiparalel between imput and ouput on the IC.
:)
 

Decoupling capacitors should not be necessary if good power and ground planes are provided. I mean they extend over all the board. Otherwise 100nf ceramic capacitor per power pin could be enough. In case of doubt, follow reference design for the chip in question.
 

You can use one capacitor on 4-5 IC but then you have to carefully route your PCB. You can buy IC bottom with build in capacitor for decoupling, so you dont have to worry about this.


Mr.Cube
 

I think they are useful because it can obliterate the noise generated in your circuits especially in PA design.
 

yes. it is required for each combinations of vcc and gnd , if multiple vcc pins are there
 

decoupling cap use on reg cct is basically to avoid oscillation.
But, other than this, it used to filter out the high freq noise.
Normally, there are 2 types of capacitor using at the reg output for decoupling purpose:
1. ceramic capacitor (non-polarity, smaller capacitance--> abt 1uF or less)
2. electrolytic capacitor (polarised, larger capacitance--> abt 1uF or more)
The purpose of the ceramic cap using in a reg cct is to filter out the high freq noise. U can found this kind of decoupling cap in almost all Audio cct. bcos the interferece noise might interrupt ur audio cct through it's power supply.
The purpose of using the electrolytic capacitor is to stabilize the oscillated o/p voltage. The larger capacitance will brings better performance in this issue. But provided ur regulator cct is not using for fast speed transmittion cct during the initial stage (within few msec just after u power up ur cct). This is bcos the large capacitance between the Regulator line and gnd will cause the delay in output voltage due to charging of the cap.
It also help on cancelling the "spike" coming from ur reg cct while u turn on the reg cct.
 

Depending on your IC speed, you need at least one decoupling cap per Vcc pin. You might wanna use two or three types of caps per Vcc pin contingent on your frequency elimination need.
 

As much as you use bypass caps between Vcc and GND as less as you see noise on the power lines.Each IC can contribute its own noise to the traces . I thnik it is a must to use these caps(10uF // 100nF) One for high frequency and one for low frequency filtering especially for microcontrollers if you want to increase the reliabity of your circuit.
 

djalli said:
What is the frequency (or range of frequencies) you will be operating? Then I tell you the values of caps you need.
Click to expand...
Do u mean the crystal/oscillator? I might use a 4MHz or 8MHz one!
 

My advice is not to spare on capacitors.
 

ericmar said:
djalli said:
What is the frequency (or range of frequencies) you will be operating? Then I tell you the values of caps you need.
Click to expand...
Do u mean the crystal/oscillator? I might use a 4MHz or 8MHz one!
Click to expand...

You need to put by-pass capacitors under the IC's is that what you want?
 

djalli said:
ericmar said:
djalli said:
What is the frequency (or range of frequencies) you will be operating? Then I tell you the values of caps you need.
Click to expand...
Do u mean the crystal/oscillator? I might use a 4MHz or 8MHz one!
Click to expand...

You need to put by-pass capacitors under the IC's is that what you want?
Click to expand...
Yes. But how do I know wat frequency my circuit will be operating?
 

I believe this is a very wise advice. Not only in order to reduce spikes caused by digital logic circuits, or for safe RF decoupling, but also to reduce EMI from the board. Today's EMC regulations requires low radiated emission levels from all electronics manufactured. My own experience is that it's more easy to get your electronic design EMI approved if there are plenty of decoupling capacitors on the PCB's. In addition to careful and intelligent PCB layout design.

pixel said:
My advice is not to spare on capacitors.
Click to expand...
 

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