Noise
Noise presents a notable challenge in the design of digital circuits due to its potential to disrupt the intended circuit behavior. The behavior of a manufactured digital circuit often deviates from the expected response, and this discrepancy can be attributed to several factors, including variations in the manufacturing process and the presence of unwanted noise sources.
Manufacturing variations introduce fluctuations in the dimensions, threshold voltages, and currents of MOS transistors, both between different runs and even within a single wafer or die. These variations can have a profound impact on the electrical behavior of the circuit, leading to unpredictable responses.
Noise, in the context of digital circuits, refers to unwanted variations in voltages and currents at logic nodes. It can be introduced by internal or external sources. Internally generated noise, often proportional to the signal swing, includes capacitive and inductive crosstalk as well as power supply noise. Externally generated noise, such as input power supply noise, is unrelated to signal levels and is typically expressed directly in voltage or current units.
Noise sources can interact with each other and propagate through the circuit, potentially corrupting the signal integrity and causing errors. For instance, adjacent wires can act as coupling capacitors or mutual inductances, leading to signal interference. Moreover, noise on power and ground rails can influence signal levels within gates, further complicating circuit operation.
Effectively managing noise sources and their impact on digital circuits is a significant challenge in high-performance circuit design. Engineers must devise strategies to mitigate noise-induced deviations and ensure that the circuit maintains its desired functionality despite variations and disturbances. This underscores the importance of robust design practices to address the intricate interplay between noise sources, manufacturing variations, and signal integrity.
How can noise signals enter a digital circuit?
Noise signals can enter a digital circuit through various means, including coupling between wires forming capacitors and mutual inductance, noise on power and ground rails, capacitive and inductive cross talk, and internally generated power supply noise.
How is the continuous range of values of an electrical node voltage transformed into a discrete variable in digital circuit representation?
The continuous range of values of an electrical node voltage is transformed into a discrete variable by associating nominal voltage levels (VOH for 1 and VOL for 0) with logic states. Applying VOH to the input of an inverter, for example, yields VOL at the output, and vice versa. The difference between these levels is known as the logic or signal swing (Vsw).