I was wondering if someone knows the part number of the device that drives the outputs on the PCI-6541 board (i.e. during data generation). I've had previous bad experiences with ringing on other NI boards despite following the termination guidelines and want to avoid problems this time. We're interfacing the PCI-6541 with third party boards and need to know how to properly terminate the signals being received from the PCI-6541 so that we do not damage the third party board.
The NI Digital Waveform Analyzer/Generator Help has some good recommendations. I have attached the relevent portions below. If you have already tried this and are still seeing reflection, what is the magnitude of your reflection relative to the magnitude of your signal?
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, 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 will give 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 654X 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 654X.
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: VDUT = Vsource*(Zt/(Zs + Zt)
So, for a 50 Ω termination, programming a generation voltage level of 3.3 V at the NI 654X source produces a 3.3 V x (50/100) = 1.65 V at the DUT. This reduced voltage swing at the DUT should be considered when you create your system.
Depending on voltage swing requirements, you have several generation termination options. The following table lists some of the options for the different voltage swings.
Tip Using a parallel termination resistor is only necessary for applications requiring the highest signal integrity.
Required Voltage Swing at DUT
Termination Options
3.3 V
Generation voltage family = 3.3 V Logic, no termination resistance
2.5 V
Generation voltage family = 2.5 V Logic, no termination resistance
Generation voltage family = 3.3 V Logic, Zt = 156 Ω (Imax = 16 mA)
1.8 V
Generation voltage family = 1.8 V Logic, no termination resistance
Generation voltage family = 3.3 V Logic, Zt = 60 Ω (Imax = 30 mA)
Generation voltage family = 2.5 V Logic, Zt = 129 Ω (Imax = 14 mA)
Thanks for pointing me to the right place! Right now, we are still working the interface to the third party board before testing it out, so we haven't yet looked to see what kinds of reflections are on the lines. Hopefully, the information you sent will help us get the design right so we don't have any problems.
If we do end up seeing large reflections, I will certainly get back to you. Otherwise, you can assume that the system is working well!
