Reducing noise in sequential circuits

Reducing noise in sequential circuits

When designing a sequential circuit with one or more clock signals, particularly circuits with either high-frequency clock signals, lots of wires, or both, noise can be a big problem! The circuit below from a past Digital Systems Design Project Showcase shows a good example of a circuit with lots of unwanted inductance caused by many loops of wire.

The symptoms of a noisy circuit can be vague and ill-defined. Typically students will state that their circuit “just doesn’t work right,” but there is no consistent problem. Note that this is different from the problem caused by problematic output voltages from a 555 timer, which typically has a well-defined cause and also has a simple solution. There are a couple of effective solutions that may (and typically do) fix the problem of noise in large sequential circuits.

Better ground

Ground is arguably the most important signal on a digital circuit. However, many looped ground wires on a breadboard can be an issue. When placing ground wires, it may be worth your time to cut and strip ground wires to the length you need rather than relying on jumper wires. I keep a supply of wires that you can cut to length in the lab so that you can use them on your design projects. It can be a large expenditure of time to do this, but it is worth doing it for ground wires. The circuit below shows regular jumper wiring, but cut to length ground wires.

Bypass capacitors

Placing bypass capacitors between VCC/GND on every chip will eliminate much of the noise that could be present on the power pins of those chips. Use a 100 nF (0.1 uF) ceramic capacitor. Place it between the VCC pin (pin 14, 16, 18, etc., depending on the chip) and ground on every chip. This will ensure that each chip receives a constant, smooth, 5 V supply voltage. The images below shows the proper placement of these capacitors.