HD74AC datasheet(52/52 Pages) HITACHI

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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.

OUTPUT

Time (ns)

6789 10 11 12

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.

Time (ns)

9 10 11 12

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.

HD74AC Datasheet(PDF) 52 Page - Hitachi Semiconductor