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Correctly Terminating Signal Lines 5 DUT Board Performance Considerations

System Reference, January 2001

121

Terminating Output Pins

Output pins are not automatically provided with termina-

tion. If you believe that the outputs from a DUT are oper-

ating fast enough to cause transmission line effects, you

can add a 50 Ω termination load.

When you select termination for your DUT output, the

load is provided by the driver-half of the IO channel.

Setting Termination Voltage In general, if you use ECL-type levels, (usually –1.7 V low

and –0.8 V high), set the driver (V

term

) to –2 V in the Level

setup.

Special Considerations for

CMOS Outputs

Some devices (notably high speed CMOS) cannot drive

into the 50 Ω provided by the coaxial cable with 50 Ω

termination at the pin electronics. The solution is to use a

resistive divider:

Connect a load resistor R

directly in series with the DUT

output. The output voltage of the DUT is then scaled down

by a factor of:

Terminating Bidirectional Pins

If you are also testing bidirectional pins at high speeds, the

considerations are very similar to those for an output pin.

During output cycles (DUT pin sends), termination is

provided by the driver impedance and level of the IO

channel connected to that pin—for as long as it is not

switched to tristate. If the DUT expects other levels than

the driver levels, the termination is obtained by using the

active load and switching the driver to Hi Z.

50Ω

50Ω+

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AWG G3 PAD FOR PS3 Especificações Página 121