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ALLPCB TEAM Jan 20, 2018
The way of connecting decoupling capacitor when VCC and GND pins aren't close
I want to make a pcb and it host an ATmega 162 microcontroller in PDIP package. But VCC and GND pins are diagonally arranged. Then the capacitors should be very close to the pins for maximum effect. Now there are 3 ways to connect the capacitors. 1.Place capacitors near ground and run wire to VCC, 2.Run wires to the capacitors to make sure that the distance from both pins is equal, 3.Place capacitors near VCC and run wire to ground. There's always the "none of the above" option. Can you give me some advice in making right decision?
A ground plane is always used by me and for chips with opposite power pins I place a cap next to the power pin and ground the other end. The ground plane has a low inductance which reduces the effect compared to wiring a single trace to Vss. The decoupling cap is to provide a local current source for the chip, so this works well.
You should use at least two equal bypass capacitors, one at each side of the IC,one near ground and one near VCC for this type of package. The parallel inductance of two traces to two different caps lowers the total trace inductance and the current flowing from each bypass cap in opposite directions helps cancel out EMI.
The decoupling capacitor should be very close to the power pin, as the power line has a higher impedance than the ground reference. For providing a very low impedance path,a large ground plane is necessary. Multilayer designs, which refers to 4+ layers,will always use a power plane. Among other things, a low impedance source.
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