I have often seen decoupling capacitors "hugging" the IC that they're protecting - but that's on '80s era through-hole boards, and things have changed.

It also doesn't help that the design of ICs, for some reason, puts Vcc and GND on diagonally opposite pins!

I know that decoupling capacitors should be "as close as possible" to their ICs, but I assume that the Vcc pin is more important? With (two layer) circuit board designs often having large ground areas, it implies that any ground is a good ground, and you can get better track layouts if you mount the cap perpendicular to the power pin, with the ground side "hanging in the breeze" on a convenient ground pad. I assume that running a specific ground rail to the IC's ground pin(s) is not necessary?


OK, there seems to be some confusion (perhaps on my side!). Assume a one layer board, with an IC and capacitor with the following pinout:

14 13 12 11 10  9  8
                  V+       O Decoupling
V-                         O capacitor
 1  2  3  4  5  6  7

I can either do this (option 1):

14 13 12 11 10  9  8
 1  2  3  4  5  6  7

or this (option 2):

   14 13 12 11 10  9  8
    1  2  3  4  5  6  7

or this (option 3):

14 12 12 11 10  9  8
                      O----(some adjacent ground path)
 1  2  3  4  5  6  7

I am inferring that #3 is a bad idea?


Or maybe, since the decoupling is so important, I could do this (for a huge DIP40 chip)?

+-- (some convenient Vcc path)
|   40 39 38 37 36 ... 22 21
O                          +--O
O---+                         O----(some convenient ground path)
    1   2   3   4  ... 19 20
  • 1
    \$\begingroup\$ The diagonal power pins are a hold over from 2-layer through-hole boards laid out in a grid. Chips without power pins at the corners (eg SN5490) were actually an annoyance. \$\endgroup\$ – Spehro Pefhany Oct 1 '16 at 12:43
  • \$\begingroup\$ @SpehroPefhany I can understand that chips with power pins in the middle of a side were a problem - but why didn't they put both the supply pins on the same side of the chip? On a DIP14, why use pins 1&8/7&14, rather than 1&14 or 7&8? \$\endgroup\$ – John Burger Oct 1 '16 at 12:58
  • \$\begingroup\$ I think the routing channels would be more constrained if they were both on the same side so the board would end up bigger but it's been a long time since I dealt with anything resembling the old grid layouts. Note the disk caps that fit nicely across the ends without obscuring any pins when the power pins are on opposite sides. Not like today with SMD parts with many power rails. \$\endgroup\$ – Spehro Pefhany Oct 1 '16 at 13:18
  • \$\begingroup\$ I've edited the question to assume a one-layer board \$\endgroup\$ – John Burger Oct 1 '16 at 13:30
  • \$\begingroup\$ @SpehroPefhany That picture is... exactly what I am familiar with! Ah, the memories - and the realisation that the 80s were a long time ago! \$\endgroup\$ – John Burger Oct 1 '16 at 14:08

The objective is always to minimise current loop areas, and yea the ground side matters just as much as the VCC side.

These days edge rates are high enough that my usual default is a 4 layer stackup so I can have a solid internal power and ground plane, two layer is just a pain if dealing with anything faster then maybe HC series (And even there you need to think about it).

  • 2
    \$\begingroup\$ Hi Dan, thank you for your answer. I edited it to remove the signature, the format of this board differs from a forum and the signature, both in questions and answers, is to be avoided. \$\endgroup\$ – Vladimir Cravero Oct 1 '16 at 11:35
  • \$\begingroup\$ @DanMills Thanks for that - but how is that possible? The manufacturers actively conspire to prevent this, by making ICs have their power on diametrically opposed pins! You need to put the cap on one side or the other - surely Vcc is best? \$\endgroup\$ – John Burger Oct 1 '16 at 11:50
  • \$\begingroup\$ @JohnBurger - perhaps years ago, when data buses were "wire-or" ground integrity was more important than Vcc integrity. These days with tri-state and CMOS, both ground & Vdd are equally important. \$\endgroup\$ – glen_geek Oct 1 '16 at 14:13

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