www.fairchildsemi.com

8

FACT Noise Characteristics

The setup of a noise characteristics measurement is critical

to the accuracy and repeatability of the tests. The following

is a brief description of the setup used to measure the

noise characteristics of FACT.

Equipment:

Hewlett Packard Model 8180A Word Generator

PC-163A Test Fixture

Tektronics Model 7854 Oscilloscope

Procedure:

1. Verify Test Fixture Loading: Standard Load 50 pF,

500

Ω.

2. Deskew the HFS generator so that no two channels

have greater than 150 ps skew between them. This

requires that the oscilloscope be deskewed first. It is

important to deskew the HFS generator channels

before testing. This will ensure that the outputs switch

simultaneously.

3. Terminate all inputs and outputs to ensure proper load-

ing of the outputs and that the input levels are at the

correct voltage.

4. Set the HFS generator to toggle all but one output at a

frequency of 1 MHz. Greater frequencies will increase

DUT heating and effect the results of the measure-

ment.

5. Set the HFS generator input levels at 0V LOW and 3V

HIGH for ACT devices and 0V LOW and 5V HIGH for

AC devices. Verify levels with an oscilloscope.

FIGURE 1. Quiet Output Noise Voltage Waveforms

Note 20: Input pulses have the following characteristics: f

= 1 MHz,

• Determine the quiet output pin that demonstrates the

greatest noise levels. The worst case pin will usually be

the furthest from the ground pin. Monitor the output volt-

ages using a 50

Ω coaxial cable plugged into a standard

SMB type connector on the test fixture. Do not use an

active FET probe.

worst case transition for active and enable. Measure

case transition.

• Verify that the GND reference recorded on the oscillo-

scope has not drifted to ensure the accuracy and repeat-

ability of the measurements.

• Monitor one of the switching outputs using a 50

Ω coaxial

cable plugged into a standard SMB type connector on

the test fixture. Do not use an active FET probe.

output begins to oscillate or steps out a min of 2 ns.

Oscillation is defined as noise on the output LOW level

the output begins to oscillate or steps out a min of 2 ns.

Oscillation is defined as noise on the output LOW level

• Verify that the GND reference recorded on the oscillo-

scope has not drifted to ensure the accuracy and repeat-

ability of the measurements.

FIGURE 2. Simultaneous Switching Test Circuit