Source Impedance

The NI 655x data, clock, and event generation channels have a 50 Ω source impedance. For applications where the full voltage swing is required at the DUT, a parallel termination resistance of 1 kΩ to 10 kΩ is recommended. With a system terminated by 10 kΩ, the majority of the signal reflections are eliminated by the source 50 Ω termination and the parallel termination, and the voltage seen at the termination resistor is 99.5% of the configured voltage.

Because the NI 655x interface cable (NI SHC68-C68-D2) is a 50 Ω transmission line, it is possible to build matched impedance systems with a 50 Ω parallel termination as the load. While a matched system is beneficial because all reflections are eliminated, the voltage at the termination is a voltage division of the voltage generation of the NI 655x, as illustrated in the following figure.

Use the following formula to calculate the voltage sensed at the termination point, V_TERM.

where    V_O is the voltage driven by the NI 655x,

R_Term is the termination impedance

R_Source is the source impedance

For example, if R_SOURCE = 50 Ω and if the termination resistance is also set to 50 Ω, then the voltage level seen at the termination is one-half the source voltage.

Always calculate the maximum current that the NI 655x in your test system can source and sink. You can calculate the maximum current using the following formula:

Max current = max{|V_OH|, |V_OL|}/(50 + R_Term)

Note

Refer to the NI 655x specifications for details on the maximum current that the NI 655x can source.

The NI 655x generation lines can be programmatically set to a high-impedance (tristate) state when not in use. Upon power up, DIO<0..19> and PFI <0..3> are set to high-impedance and remain in that state until configured for generation.

Refer to Termination and Terminating Your Module for more information about signal reflections and termination.