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Nora
Joined: 31 Mar 2006
Posts: 49
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 12 Aug 2008 13:45 Capacitors between power and ground |
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Hello-
What is the reason to use 2 capacitors in parallel between ground and power instead of just one?
It is 2 0.1uF caps.
Thanks!
N_N
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seadolphine2000
Joined: 12 Apr 2005
Posts: 563
Helped: 31
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| 12 Aug 2008 13:51 Capacitors between power and ground |
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Capacitors between power and ground is used to suppress spikes. These spikes can damage the board, or at least, the sensitive components.
The larger the value of the capacitor, the better the protection.
Hope this helps.
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Stewie911
Joined: 12 Aug 2008
Posts: 12
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| 12 Aug 2008 13:58 Re: Capacitors between power and ground |
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What is your application/circuit?
If it's on a long power line, it could be to just make sure that all AC signals are bypassed.
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Old Nick
Joined: 14 Sep 2007
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| 12 Aug 2008 14:14 Re: Capacitors between power and ground |
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There is usually one 0.1uF cap per power pin on an IC, (the cap being placed as close to the pin as is physically possible. There Usually is a 10u 47u and 1uF decoupling the supply aswell (however only one per power-plane is usuall).
Hope this answers your question
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LvW
Joined: 07 May 2008
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| 12 Aug 2008 16:02 Re: Capacitors between power and ground |
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| Nora wrote: |
What is the reason to use 2 capacitors in parallel between ground and power instead of just one? |
I think, an answer concerning the parallel connection is not yet given.
Therefore: The high value cap has to short the very low frequencies as good as possible. However, as these capacitors have bad high frequency properties and exhibit a relatively high series parasitic resistance, a high quality low value capacitor is placed in parallel.
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penrico
Joined: 28 Aug 2001
Posts: 225
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| 12 Aug 2008 19:27 Re: Capacitors between power and ground |
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Possibily, the board uses so many .1uF capacitors. So, if in a board the manufacturer needs a .2uF, it's cheapest and easiest put 2 capacitors in parallel to get .2uF that buy a reel to use just one capacitor of .2uF in a board that uses so many .1uF capacitors.
When you manufacture in long scale, it's common to unify all the values to some few. Then in the pick and place machine, you will have few reels. And manufacturing is cheapest.
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Nora
Joined: 31 Mar 2006
Posts: 49
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| 12 Aug 2008 19:39 Re: Capacitors between power and ground |
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Wow!
Great answers everyone.
Thanks!
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389iaB4Yg6Xn
Joined: 26 Jun 2008
Posts: 5
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| 12 Aug 2008 20:17 Re: Capacitors between power and ground |
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| also sometimes it is important for these blocking capacitors to be located near the IC, they are suppose to protect against noise in Vcc. that is why they might be connected on same lines but are dislocated on PCB...
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subharpe
Joined: 09 Jan 2008
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| 13 Aug 2008 6:37 Re: Capacitors between power and ground |
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| The two capacitors are one electrolytic capacitor (polarised) (typical value 6.8uF)and the other ceramic non-polarized (typically 100nF). The electrlytic capacitor has high current capacity so that it can carry large spike current when there is any spike in the power supply line. But the frequency responce of the capacitor is less which allows the spike to be present to some extent. The ceramic capacitor is good at frequency response, so it blocks the spike at the output.
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xulfee
Joined: 27 May 2008
Posts: 230
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Location: Pakistan
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| 13 Aug 2008 7:51 Capacitors between power and ground |
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capacitors r mostly used in this configuration as coupling & bypassing compunents to protect ur circuit.
where u need to block dc (amplifire input) u put capacitors in series & where u need to pass dc (baising for transistors) u put it in parallel
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melad
Joined: 06 Jan 2006
Posts: 10
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| 14 Aug 2008 13:33 Capacitors between power and ground |
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1. the large capacitor is the normal ripple bypass capacitor which is intended for power line filtering
2. due to the high internal inductance of the large electrolytic capacitors it will has a bad high frequency filtering capabilites so we put a low capacitance ceramic capacitors which has a low internal inductance , this capacitor will do the high frequency rejection properties which may be introduce from switching spikes and so signals
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RF-OM
Joined: 03 Aug 2007
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Location: The Earth
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| 14 Aug 2008 17:21 Re: Capacitors between power and ground |
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| There may be much more than two caps in parallel. The big electrolytic and ceramic caps usually installed near power source for filtering purpose. Then a few caps may be installed near active circuit (IC, transistor and so on). The purpose of these caps is to bypass (shorten) power supply line to ground and have minimally possible impedance between power pin and ground. If you check the impedance plot for capacitor you'll see that this plot has deep minimum at the self-resonance frequency of cap. This is the frequency of the best bypassing. One cap may provide good bypassing only in relatively narrow region, but when you install a few of caps in parallel and they have different capacitance such a network will provide low impedance bypassing for wide frequency range. In this case the smallest cap must be installed as close to the power pin as it possible.
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arman_arian2005
Joined: 20 Oct 2006
Posts: 33
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| 15 Aug 2008 17:08 Re: Capacitors between power and ground |
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| LvW wrote: |
| Nora wrote: |
What is the reason to use 2 capacitors in parallel between ground and power instead of just one? |
I think, an answer concerning the parallel connection is not yet given.
Therefore: The high value cap has to short the very low frequencies as good as possible. However, as these capacitors have bad high frequency properties and exhibit a relatively high series parasitic resistance, a high quality low value capacitor is placed in parallel. |
This is Correct!
As is well demonstrated electrolytic capacitors have bad frequency response. One may model them roughly by a capacitor and an inductor in series. What I here intend to add is to keep in mind to place a 100nF capacitor near every block of circuit which draws more than 1mA current in high frequency circuits.
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M!k
Joined: 22 Apr 2002
Posts: 812
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| 15 Aug 2008 19:30 Re: Capacitors between power and ground |
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| Nora wrote: |
Hello-
What is the reason to use 2 capacitors in parallel between ground and power instead of just one?
It is 2 0.1uF caps.
Thanks!
N_N |
Could also be that the layouter didn't know what he was doing. I've seen it many times that the caps were not correctly placed near an ic power pin 
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insrusingh
Joined: 08 Jul 2008
Posts: 16
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| 19 Aug 2008 13:52 Re: Capacitors between power and ground |
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Hi
Two capaciors in parallel are required as all he capacitor are not truly capcitive it has some inductance due to its lead length or when we use SMD components for decoupling we use ceramaic capacitorshaving dielectric(X5R,X7R,NPO,COG).so when added in parallel effective inductance becoms less(calculate same as the resistance) so it functions more as capacitors.So. it will not effect the resonance frequency.Afterall our purpose is to increase the resonance frequency point.
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