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Should bypass caps be closer to ground or Vcc?

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pigtwo

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Hello all,

Suppose I have, for example, a SSOP-24 IC with Vcc on pin 1 and ground on pin 13(IE far from each other). If I want to place one bypass cap across Vcc and ground near this IC I should try to place the IC as close to the Vcc and ground pins as possible but in this case the pins are far from each other.

Let's assume I have a ground plane and Vcc plane under this IC. What I'm wondering is in this case what is the preferred approach. Does it matter whether the cap is next to the Vcc pin or next to the ground pin? My first thought is that it shouldn't matter because the loop area should be(mostly) the same in both cases. But maybe I'm not thinking of something.

Thanks for your input!
 

Hi,

Maybe not so, just a thought... What else is on the board? Would any momentary voltage drop(s) caused by other devices current draws affect Vcc or ground?

For the very simple circuits I make which have a solid ground plane, with pinouts as you describe I put the capacitor(s) as close to the IC's positive supply pin as I had understood that the return path from the IC gnd pin to circuit ground is as short as possible anyway.
 

Hi,

Theoretically with identical VCC and GND planes..it makes no difference.
But it's good habit to place the capacitor close to the VCC pin.

I say it's way more important to have a good GND plane than two mediocre VCC and GND planes.

Klaus
 

@d123 That's something I didn't really think about. I might be overthinking it but it seems like it should be symmetric. So the opposite situation could happen where something else on the board dumps a lot of current into the ground thus raising the ground voltage in the vicinity. It seems like in this case you would want the cap closer to the ground pins. I'm not disagreeing, more just thinking out loud. I do generally put the bypass caps near the Vcc pin, I was just working on a layout where there was very little room near the Vcc pin but a lot of room near the ground pin. It sounds like it's mostly ok to do either.

@KlausST Ok, I do try to put it near the Vcc pin but mainly out of habit/convention. But I had to put one near the ground pin instead which got me thinking about this. When you say a 'good ground plane', what makes a ground/Vcc plane good? Is it just having a very solid ground plane without very narrow connections and also being a relatively large?
 

This is strictly a local matter: what matters is the pot diff between Vcc and ground. A good ground plane is NO good if the Vcc is not stable locally.

It will be a good habit to place the cap close to the Vcc pin because we are implicitly assuming that the gnd is at GND potential.
 

    V

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Hi,
A good GND plane is low impedance at high frequencies, solid.

Klausk
 

My first thought is that it shouldn't matter because the loop area should be(mostly) the same in both cases.

It's not the "loop area" that is of concern. It is the momentary glitch in VCC when internal parts of the IC switch (especially bad for digital ICs). You want your capacitor to be as close to the VCC pin as you can get it. The cap is there to provide enough energy to minimize the transients at the source - the VCC line of the IC.

So, mount your cap near your IC's VCC pin. Just make sure that the ground that you bring to the cap is low impedance, or you will introduce other problems.
 

    V

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