AWG G3 PAD FOR PS3 Dokumentacja Pobierz pdf (Strona 127)

DUT Board Design for Mixed-Signal Tests 5 DUT Board Performance Considerations

System Reference, January 2001

127

Matching the impedance of the signal source should be a

matter of course. This avoids reflections and is essential

for all high frequency analog signals. Low voltage ECL

signals, too, should only be transported by impedance

matching coaxial cables.

The cable shield is normally connected to the respective

analog or digital ground, except for DC signals to to from

the PMU. For the PMU force and sense lines, the cable

shield should be connected to guard connectors.

Notice that if you connect a coaxial cable to a high imped-

ance input, the cable has a capacitance (depending on its

dimensions, typically 100 pF/m). In high frequency appli-

cations, you may need to ensure that the signal source can

drive that capacitive load.

If the requirements are not that stringent, you may also

consider using twisted-pair cables or a printed circuit

stripline approach (implemented by a broad ground

conductor running physically just underneath the signal

line).

It is always a good idea to mount and mark some test pins

on the DUT board where the probe of an external instru-

ment or oscilloscope can be attached. This will ease trou-

bleshooting.

Filtering Supply and DC Voltage Real world power supplies usually have a rather high

impedance for high frequency load changes. That means a

sudden current change of the DUT will cause a voltage

spike (or drop) at the DUT supply pin.

Also, changes of the supply voltage will influence the

generated or received signal and thus increase the noise

level.

Use lowpass filters or at least bypass capacitors to reduce

noise.

• A lowpass filter can be directly inserted into the supply

lines.

1 2 ... 122 123 124 125 126 127 128 129 130 131 132 ... 284 285

Brak uwag

AWG G3 PAD FOR PS3 Dokumentacja Strona 127