Q. I have been studying digital ICs. The lessons speak about noise immunity, and how the threshold values for voltages that represent HI and LO are separated to make noise immunity better. Why is noise immunity so important?
A. On a circuit board there is some capacitive coupling between the conductors. Pulses on one conductor can get coupled onto another, pulling the voltage away from its intended HI or LO value.
Also, when an output changes state, it does so very quickly. The faster the voltage changes, the more current the capacitance will draw from the output. This current must flow through the popwer supply and ground pins of the IC.
The current can cause a brief voltage drop to appear across the foil traces that supply power and ground to teh IC. While "de-spiking" capacitors can be connected between the supply and ground pins of ICs to help reduce this problem, false signals can still occur. By separating the thresholds for HI and LO voltages to which the inputs will respond, the input voltages can vary over a wider range without causing the IC to "misinterpret" a voltage.