Bypass capacitors

Precautions need to be taken to avoid the risk of erroneous digital transitions in digital circuits.When an IC output state changes short spikes are created on the ground and +ve lines and other ICs nearby may see these spikes as a logic change on their digital input resulting in disaster.

The best methods for protecting against this scenario are

1. Use a ground plane on the PCB (lower inductance and resistance means smaller spikes)
2. Use bypass capacitors liberally throughout the circuit

There are two types of bypass capacitors that I use in my designs

1. 0.1 uF (100nF) 25V ceramic disc capacitors across the +ve and GND pins of each IC (08-0235). Place one across the +ve and GND pins as close as possible.
2. Larger 10uF 25V tantalum capacitors to bypass the power supply on the pcb to provide a local reservoir of energy (11-0666 ). Place one across the power supply rails as close to the power connectors as possible.

Note for PCB designs use radial rather than axial capacitors as the leads come out of the same end and require less space on the pcb.

Use capacitors with a voltage rating at least double the expected voltage. For 5V CMOS digital the 25V capacitors above are what I had and capacitors with a lower voltage rating can be used.