
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
s
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
R
S
50Ω+
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