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The NI 656x requires a differential termination at the destination of 100 Ω to properly drive the LVDS logic levels and to maintain signal integrity.
LVDS is a current-driven technology. That is, a logic state is derived from the differential voltage generated by forcing a current through a known impedance. Forcing current one direction signifies a logic high level, and forcing current the alternate direction signifies a logic low level. As such, it requires that the current path be completed, through a 100 Ω resistor, at the destination. The differential voltage then seen at the receiver is a function of this resistor. LVDS levels are generated using a 100 Ω resistor at the receiver.
For applications requiring the highest levels of signal integrity and timing accuracy, NI strongly recommends carefully controlling the termination impedance at the end of the transmission line. In a differential environment, there is an effective virtual ground at the midpoint of there terminating resistor. In a 50 Ω single-ended environment, the transmission line is effectively matched with the 100 Ω differential impedance caused by this virtual ground effect.
A common configuration for your NI 656x is to configure the terminals for single-ended mode and connect them directly to your device under test (DUT). Most digital logic inputs have an input impedance of 1–10 KΩ. Therefore, connecting the NI 656x output terminals directly to the input of your DUT effectively creates a source-terminated configuration because the PFI channels of the NI 656x have a 50 Ω source impedance when in single-ended mode.
While this source configuration does not provide the absolute highest level of signal quality, there are many advantages to a source-terminated configuration. First, very good signal levels are possible if you ensure that you have the cleanest possible 50 Ω characteristic impedance transmission line. Second, this source-terminated configuration allows you to directly wire to your DUT without the need for additional termination resistors. Lastly, given that at DC there is effectively a voltage divider between the 50 Ω Zs resistance and the high-impedance Zt of your DUT, having a source-terminated load preserves the largest possible voltage swings at the DUT according to the following formula: Vt = Vs×(Zt/(Zs + Zt))
Therefore, for a DUT with an input impedance of 1 kΩ, programming a generation voltage level of 3.3 V at the NI 655x source produces a 3.3 V × (1000/1050) = 3.14 V swing.
The source-terminated load generates reflections in the transmission line. These reflections, however, are absorbed at the source and not re-reflected back to the load, thus preserving the signal integrity. Practically, the source impedance does not perfectly match the transmission line impedance; therefore, a small fraction of the reflected wave is re-reflected back toward the load. This second reflection creates small signal aberrations and a low level of inter-symbol interference.
For example, a 5% mismatch at the source results in a 2.5% re-reflection back at the load:
Γs = (1.05 - 1)/(1.05 + 1) ≈ 2.5%
The source-terminated load configuration is easy to use with a terminated source, such as the NI 656x, and is recommended for all applications except the most demanding in regard to timing precision or signal integrity. For applications demanding the highest levels of signal quality and timing precision, NI recommends that you seriously consider following the recommendations of the terminated load configuration.
For applications requiring the highest levels of signal integrity and timing accuracy, NI strongly recommends carefully controlling the termination impedance at the end of the transmission line. To control the termination impedance, add a parallel termination resistor to ground as close as possible to the digital input pin of the device under test (DUT). In this configuration, the transmission line is terminated at both ends of the transmission line, which produces the highest possible signal integrity.
Ideally, the source impedance, ZS, and the characteristic impedance of the transmission line, Z0, should be kept as close as possible to 50 Ω as this gives you the best possible signal quality.
However, depending on your NI device, having all the lines terminated into 50 Ω may violate the maximum current specifications. Refer to the NI 656x specifications for more information about the maximum current for your device to determine how many lines you can simultaneously terminate into 50 Ω.
While a Zt of 50 Ω is ideal, you can also use values as high as 300 Ω without significantly affecting signal quality. Using this higher resistance value enables you to increase the voltage swing across the DUT and decrease the drive current requirements on your NI 656x.
Given that at DC there is effectively a voltage divider between the 50 Ω ZS resistance and the termination resistance, having a terminated load reduces the largest possible voltage swings at the DUT according to the following formula: Vt = Vs×(Zt/(Zs + Zt))
Therefore, for a 50 Ω termination, programming a generation voltage level of 3.3 V at the NI 656x PFI source produces a 3.3 V x (50/100) = 1.65 V swing at the DUT. This reduced voltage swing at the DUT should be considered when you create your system.