Question

1. Discuss how, for digital logic,

the concepts of noise margins and fully restored logic help with both the manufacturing yields and the useful working lifetime of modern digital electronics.

How do the rail voltages and noise margins determine the selection of integrated circuits for use in or near environments that are electrically noisy, exposed to high levels of ionizing radiation, or subjected to large temperature changes?

Answer 1

Noise margin is a measure of design margins to ensure circuits functioning properly within specified conditions. Sources of noise include the operation environment, power supply, electric and magnetic fields, and radiation waves. On-chip transistor switching activity can also generate unwanted noise. To ensure that transistors switch properly under specified noisy conditions, circuits must be designed with specified noise margins.

The below figure illustrates noise margin and the terms, assuming that the signal generated by the driving device is wired to the input of the receiving device and that the wire is susceptible to noise. The minimum output voltage of the driving device for logic high, VOH min, must be greater than the minimum input voltage, VIH min, of the receiving device for logical high. Because of noise being induced on the wire, a logic high signal at the output of the driving device may arrive with lower voltage at the input of the receiving device. The noise margin, NMH = |VOH min – VIH min|, for logical high is the range of tolerance for which a logical high signal can still be received correctly. The same can be said with noise margin, NML = |VIL max – VOL max|, for logical low, which specifies the range of tolerance for logical low signals on the wire. Smaller noise margins mean circuits are more sensitive to noise.