Question
Why do CMOS digital integrated circuits have high immunity to noise on the input signal or power supply?
Answer Options
- A) Large bypass capacitance is inherent
- B) The input switching threshold is about twice the power supply voltage
- C) The input switching threshold is about half the power supply voltage
- D) Bandwidth is very limited
Correct Answer: C
Explanation
Noise immunity refers to a digital circuit’s ability to tolerate voltage variations or glitches on its input or power supply without accidentally changing its output state. CMOS (Complementary Metal-Oxide-Semiconductor) is known for excellent noise immunity, making it reliable in electrically noisy environments.
CMOS achieves this high immunity because the input switching threshold is about half the power supply voltage (e.g., 2.5 \text{ V} in a 5 \text{ V} system). This provides a very large ‘buffer zone’ or noise margin: noise must pull a signal down from 5 \text{ V} to less than 2.5 \text{ V}, or push a 0 \text{ V} signal up past 2.5 \text{ V}. This large margin makes accidental state changes from small noise pulses highly unlikely.
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