HD74AC datasheet(26/52 Pages) HITACHI

26 / 52 page

Definition of Specifications

Consider the input. If the internal ground changes, the input voltage level appears to change to the DUT. If

the input rise time is slow enough, its level might still be in the device threshold region, or very close to it,

when the output switches. If the internally-induced voltage is large enough, it is possible to shift the

threshold region enough so that it re-crosses the input level. If the gain of the device is sufficient and the

input rise or fall time is slow enough, then the device may go into oscillation. As device propagation

delays become shorter, the inputs will have less time to rise or fall through the threshold region. As device

gains increase, the outputs will swing more, creating more induced voltage. Instantaneous current change

will be greater as outputs become quicker, generating more induced voltage.

Package-related causes of output oscillation are not entirely to blame for problems with input rise and fall

time measurements. All testers have V

CC and ground leads with a finite inductance. This inductance needs

to be added to the inductance in the package to determine the overall voltage which will be induced when

the outputs change. As the reference for the input signals moves further away from the pin under test, the

test will be more susceptible to problems caused by the inductance of the leads and stray noise. Any noise

on the input signal will also cause problems. With FACT logic having gains as high as 100, it merely takes

a 50 mV change in the input to generate a full 5 V swing on the output.

3.4 Propagation Delays, f

max, Set and Hold Times

A 1.0 MHz square wave is recommended for most propagation delay tests (figures 3 and 4) The repetition

rate must necessarily be increased for testing f

max. A 50% duty cycle should always be used when testing

max. Two pulse generators are usually required for testing such parameters as setup time, hold time,

recovery time etc.

DATA

OUT

tpxx

Vmi

Vmo

Vmi = 50% V

CC for HD74AC devices; 1.5 V for HD74ACT devices

Vmo = 50% V

CC for HD74AC/HD74ACT devices

Figure 3 Waveform for Inverting and Non-Inverting Functions

HD74AC Datasheet(PDF) 26 Page - Hitachi Semiconductor