NI 656x Acquisition Termination

DIO, STROBE, LVDS PFI Channels

The NI 656x devices employ a single differential 100 Ω terminating resistor located at the differential receiver. This 100 Ω resistor guarantees signal quality in a 100 Ω differential environment and induces the correct voltage levels required by the LVDS standard.

This impedance is always present and is not software selectable.

Single-Ended PFI Channels

For single-ended trigger operations, the NI 656x can only be used in a high-impedance load configuration because the input impedance of the NI 656x is set to 10 kΩ, when in single-ended mode. The high-impedance load configuration is easy to drive since it does not present a significant DC load to the source and preserves the signal amplitude. It is very important that you follow the recommendations in this section to achieve the highest level of AC signal quality.

The same transmission line considerations discussed for the generation case are applicable to acquired signals. The input impedance of 10 kΩ implies a reflection coefficient Γ_t of 0.99, or nearly full reflection.

With all high-impedance load transmission lines, it is essential that you take care to match the source impedance of the transmission line to the characteristic impedance of the transmission line. The source matching in this configuration is particularly important, since there are significant reflections from the high-impedance load (input of NI 656x in this case).

You should take care to ensure that your transmission line has a characteristic impedance of as close to 50 Ω as possible.

Therefore, Z_s (external source output impedance) should match Z₀ = 50 Ω (the cable impedance) for 10 kΩ input configuration. To achieve this Z_s = 50 Ω, you must determine the output impedance of your digital driver and add a series resistor as close as possible to the driver pin such that the output impedance of your buffer plus the value of the series resistor equal 50 Ω.

If you require 50 Ω termination for your acquisition application, consider using one of the NI 655x products.