How to read datasheets
Which is the common definition of the propagation delay tPLH in a logic IC datasheet?
- AThe time it takes for the output to go from 10% to 90% of its swing when changing from L to H
- BThe time from the input changing from L to H until the output reaches the fully settled 100% point
- CThe time from an input change until the output changes from L to H, measured between the 50% points of the swings
Show answer and explanation
Answer: C. The time from an input change until the output changes from L to H, measured between the 50% points of the swings
tPLH is the propagation delay when the output changes from L to H, and tPHL is the delay when it changes from H to L; the two are generally not equal. The reference points are usually 50% (VDD/2) of the input and output swings respectively, though 1.5 V is sometimes used for TTL families. The time to go from 10% to 90% of the swing is the rise time, a separate parameter from propagation delay. Delay varies with load capacitance, supply voltage and temperature, so read it together with the test conditions in the table.