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

Design Considerations
flowing back down the line equal to the current flowing in the line prior to the opening of the switch. This
current wave will propagate down the line where it will encounter the high impedance tester load. This will
cause the wave to be reflected back down the line toward the DUT. The current wave will continue to
reflect in the transmission line until it reaches the voltage applied to the tester load. At this point, the
current source impedance decreases and it will dissipate the current. A typical waveshape on a modern
ATE is depicted in figure 27.
5
OE
4
3
2
OUTPUT
1
0
1 2 3 4 5 6 7 8 9 10 11 12
Time (ns)
Figure 27 Typical ATE 3-State Waveform
Transmission line theory states that the voltage level of this current wave is equal to the current in the line
times the impedance of the line. With typical currents as low as 5 mA and impedances of 50 to 60 Ω, this
voltage step can be as minimal as 250 mV. If the comparator was programmed to the 10% point, it would
be looking for a step of 550 mV at 5.5 V V_CC. Three reflections of the current pulse would be required
before the comparator would detect the level. It is this added delay time caused by the transmission line
environment of the ATE that may cause parts to fail customers’ incoming tests, even though the device
meets specifications. Figure 28 graphically shows this stepout.
5
4
3
2
1
0
1 2 3 4 5 6 7 8 9 10 11 12
A
BC
Time (ns)
Figure 28 Measurement Stepout
Point A represents the typical 50% measurement point on tester driven waveforms. Point B represents the
point at which the delay time would be measured on a bench test fixture. Point C represents where the
delay time could be measured on ATE fixtures. The delay time measured on the ATE fixture can vary
from the bench measured delay time to some greater value, depending upon the voltage level that the tester
is set. If the voltage level of the tester is close to voltage levels of the plateaus, the results may become
non-repeatable.
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