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I'm trying to figure out if one SMD capacitor is the same as a through hole decoupling capacitor. With the through hole capacitor the positive current is on one side of the capacitor and the negative current is on the other. And each pin distributes the current to the rest of the circut. It looks as if the capacitor is crossing the connections but everything works as it should. I want to apply the same set up in SMD format.

But I am unsure if I would use one Capacitor with both currents going through it on oposite sides or should I have two SMD capacitors one for positive current and the other for negative?

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    \$\begingroup\$ Surface mount caps are actually better for decoupling than thru hole, because there is less inductance in the leads. Don't worry about the geometry of the cap itself. That is all covered by the specs. As long as it does what it says, you shouldn't care how that is accomplished other than for interest and curiosity. \$\endgroup\$ – Olin Lathrop Oct 4 '12 at 15:55
  • \$\begingroup\$ So one chip can connect to both sides of voltage without any problems? I'm using a 100nF SMD \$\endgroup\$ – Newtech30 Oct 4 '12 at 19:24
  • \$\begingroup\$ @Newtech30 A ceramic decoupling capacitor does not have a polarity, so it will not have issues with the side with positive voltage, and ceramics are very common at that capacitance range. Now someone can write a thorough answer that is helpful. \$\endgroup\$ – Kortuk Oct 10 '12 at 0:11
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I think you have a misunderstanding about capacitors. The physical shape does not change the properties if the ratings are the same. So a 10nf 30V SMD cap is the same as a 10nf 30V through hole cap.

A decoupling cap in power circuits is used to short high frequency noise to ground. You sometimes have a few of them on your board near IC's. This prevents unwanted spurious signals from messing up your digital or analog signals.

A decoupling cap might also be used to filter out the DC component of a signal and only allowing the AC portion through. This is commonly used for analog inputs to amplifiers and RF circuits.

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  • \$\begingroup\$ Your explanation of decoupling cap is inverted. Since it goes across it will shunt AC into ground, you've described it as passing AC through. (-1) \$\endgroup\$ – placeholder Oct 4 '12 at 16:31
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    \$\begingroup\$ @rawbrawb The last paragraph isn't about IC decoupling caps. Though aren't they called coupling caps when used in series for that purpose? \$\endgroup\$ – exscape Oct 4 '12 at 16:36
  • \$\begingroup\$ I missed the 2nd para. but you do contradict yourself. The decoupling cap is also local charge storage for activity on chip. An "AC coupling cap" can also be called a coupling cap or a pass cap in short hand. Why don't you fix your last para then? \$\endgroup\$ – placeholder Oct 4 '12 at 16:41
  • \$\begingroup\$ How exactly would I connected it to make decoupling? Explain it so me in simple terms please like positive goes heat and negative goes there etc. thanks \$\endgroup\$ – Newtech30 Oct 4 '12 at 19:28
  • \$\begingroup\$ Across the rails. i.e from (Vdd to Vss/ power to ground) in parallel to the IC and as close as possible. \$\endgroup\$ – placeholder Oct 4 '12 at 23:31
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You seem a little off in the weeds here :) I think you're saying today you use a through hole cap (maybe a polarized electrolytic), and now you want to use a SM cap? I'm not sure what you mean by SM, since there are several options. You could just use a SM electrolytic that's the similar to what you use today. You might be able to replace it with a similar polarized tantalum cap if the specs match up. If you're really thinking about a tiny surface mount ceramic cap then they are non polarized so you should be fine. Let us know what part number you're using, and share part of the schematic if you can and people here will be better able to assist.

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  • \$\begingroup\$ What I really want to know is how do I attach an SMD capacitor to my circuit properly? It needs to be decoupling. I need to connect it to the positive an negative voltage of a picaxe 08m2. Do I need 2? Meaning one for each side or do I need one meaning both positive and negative passes through the same chip? Sorry I'm not explaining this better. I'm really new at this. \$\endgroup\$ – Newtech30 Oct 4 '12 at 18:23
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Oh I see what you mean, while the leads on through hole capacitor were long enough you could attach them directly next to the power pins, surface mount capacitors don't have any leads. You just need one capacitor, mount it next to the microcontroller with traces designed as on this picture.decoupling cap

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protected by Dave Tweed Nov 30 '14 at 18:06

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