Fanout vs. Noise Margin: Key Differences Explained

This article clarifies the core differences between Fanout and Noise Margin, two crucial concepts for understanding the performance of logic gates. We’ll explore what they are, why they matter, and how they’re calculated.

Fanout

Fanout refers to the maximum number of logic gates that a single logic gate output can reliably drive without compromising its performance. Think of it as the “load capacity” of a gate. A higher fanout is generally desirable, as it reduces the need for extra driver gates to handle a larger number of connected gates.

Image alt: Fan Out

As shown in Figure 1 (implied from the text), the driver logic gate and the gates it drives should ideally be of the same type for optimal performance.

The formulas for calculating Fanout are:

• Fanout (High): I_OH(max) / I_IH(max)
• Fanout (Low): I_OL(max) / I_IL(max)

Where:

• I_OH(max) = Maximum output current in the high state.
• I_IH(max) = Maximum input current required to maintain a high state.
• I_OL(max) = Maximum output current in the low state.
• I_IL(max) = Maximum input current required to maintain a low state.

The overall Fanout is the lower value between Fanout (High) and Fanout (Low). This ensures that the gate can reliably drive the specified number of loads in both the high and low states.

Noise Margin

Noise margin quantifies a logic circuit’s ability to tolerate noise without causing unwanted changes in the output voltage. In other words, it represents how immune a circuit is to spurious signals that might trigger a false switching event.

Image alt: Noise Margin

A quantitative measure of noise immunity is known as Noise Margin. The higher the noise margin, the more robust the circuit is against noise.

We define the high-state and low-state noise margins as follows:

• High-State Noise Margin (V_NH): V_OH(min) - V_IH(min)
• Low-State Noise Margin (V_NL): V_IL(max) - V_OL(max)

Where:

• V_OH(min) = Minimum output voltage in the high state.
• V_IH(min) = Minimum input voltage required to be recognized as a high state.
• V_IL(max) = Maximum input voltage allowed to be recognized as a low state.
• V_OL(max) = Maximum output voltage in the low state.

In a typical logic family, the following relationship holds true:

V_OH(min) > V_IH(min) > V_IL(max) > V_OL(max)

Just like with Fanout, the overall noise margin is the lower value between the high-state noise margin and the low-state noise margin. This represents the circuit’s weakest point in terms of noise immunity.