NI 655x Acquisition Termination
Unterminated Acquisition Configuration
For acquisition operations, an unterminated configuration essentially implies that you set the input impedance of the NI device to 10 kΩ using NI-HSDIO. The unterminated configuration is easy to drive since it does not present a significant DC load to the source and preserves the signal amplitude. As a result, this is the recommended configuration for most applications. When you use this mode, however, 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. Programming an input impedance of 10 kΩ implies a reflection coefficient Γt of 0.99, or nearly full reflection and is effectively an unterminated input.
With all unterminated 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 unterminated load (input of NI 655x in the case).
First, you should take care to ensure that your transmission line has a characteristic impedance of as close to 50 Ω as possible.
Therefore, Zs (external source output impedance) should match Z0 = 50 Ω (the cable impedance) for 10 kΩ input configuration. To achieve this Zs = 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.
A 10 kΩ input configuration is not recommended for use with drivers that are not matched to the cable impedance at 50 Ω.
Terminated Acquisition Configuration
Input termination of 50 Ω is recommended for best signal integrity since there are no reflections back to the signal source, provided that the signal source can drive this load.
The 50 Ω input configuration reduces the signal swing seen by the NI 655x input comparators by half, assuming the signal source is 50 Ω. You should consider this amplitude reduction when configuring the input thresholds.
For example, a 50 Ω source with a 0 to 5 V step generation (into high-impedance) is seen as a 0 to 2.5 step source at the NI 655x acquisition comparators when you configure the NI 655x for 50 Ω input impedance.