HD74AC Datasheet, PDF (23/52 Pages) Hitachi Semiconductor – HD74AC Series Common Information

Definition of Specifications
Test Load
DUT
50pF
500Ω
450Ω
t_ZH
OPEN ^tHZ
2 × V_CC t_ZL
t_LZ
t_r = 3.0 ns
t_f = 3.0 ns
50Ω Scope
Figure 1 AC Loading Circuit
3. AC Loading and Waveforms
3.1 Loading Circuit
Figure 1 shows the AC loading circuit used in characterizing and specifying propagation delays of all
FACT devices (HD74AC and HD74ACT) unless otherwise specified in the data sheet of a specific device.
The use of this load, differs somewhat from previous practice provides more meaningful information and
minimizes problems of instrumentation and customer correlation. In the past, +25°C propagation delays
for TTL devices were specified with a load of 15 pF to ground; this required great care in building test jigs
to minimize stray capacitance and implied the use of high-impedance, high-frequency scope probes. FACT
circuits changed to 50 pF of capacitance, allowing more leeway in stray capacitance and also loading the
device during rising or falling output transitions.
This more closely resembles the inloading to be expected in average applications and thus gives the
designer more useful delay figures. We have incorporated this scheme into the FACT product line. The
net effect of the change in AC load is to increase the average observed propagation delay by about 1 ns.
The 500 Ω resistor to ground can be a high-frequency passive probe for a sampling oscilloscope, which
costs much less than the equivalent high-impedance probe. Alternately, the 500 Ω resistor to ground can
simply be a 450 Ω resistor feeding into a 50 Ω coaxial cable leading to a sampling scope input connector,
with the internal 50 Ω termination of the scope completing the path to ground. This is the preferred scheme
for correlation. (See figure 1.) With this scheme there should be a matching cable from the device input
pin to the other input of the sampling scope; this also serves as a 50 Ω termination for the pulse generator
that supplies the input signal.
Shown in figure 1 is a second 500 Ω resistor from the device output to a switch. For most measurements
this switch is open; it is closed for measuring one set of the Enable/Disable parameters (low-to-off and off-
to-low) of a 3-state output. With the switch closed, the pair of 500 Ω resistors and the 2 × V_CC supply
voltage establish a quiescent high level.
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