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Pxie-6556 - How to control the rise time in the digital lines (hsdio)

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Is there any way to control the rise time in the PXIe-6556 digital lines?

Even in a low frequency signal 10kHz the rise time is about 2ns.  




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I use the PXIe-6556 in the lab.

The system works well for high bandwidth applications.

But for low bandwidth systems (a simple 10kHz square wave through a coaxial cable) can not be generated by the pxi without a high distorsion in the signal edge due to the 2ns rise time.

I would like to solve this problem just increasing the rise time as I can do with a function generator


Is there a solution to use the pxie-6556 in this setting?


Please, see attached the waveforms.

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Accepted by topic author engfpe

Hello engfpe,


The PXIe-6556 is a 50 Ohm system, meaning the output is source series terminated to be 50 Ohms and all our cables and accessories are 50 Ohms.  With this setup regardless of the data rate you should have a clean rising or falling edge regardless of the data rate. The quality of the output (rising or falling edge) at your device will be related to the impedance matching of your cables. 


The PXIe-6556 cannot adjust the slew rate by itself.  However you can insert some simple passive components to do this for you.  I've attached below to images.  The first is a schematic showing one way of slowing down the edge.  The second is the a simulated waveform showing the original edge rate before slowing it down, and the slowed down edge.   This simulation is not of the PXIe-6556 but instead a generic digital output for concept.  In the schematic R1 is set to 34 Ohms because U1.8 has the additional 16 Ohms inside it.  TL1 is the 50 Ohm output simulating the cable on the PXIe-6556.  R2, R3 and C1 are components you could insert after the PXIe-6556 cable before it goes to your device/cables.  In this setup the cable going to your device is TL2 which is also 50 Ohms but it could be another impedance in which case you would change R3 to match.


You can see in the attached images you can significantly slow down the edge with this setup by altering C1.  I hope this helps,

Jesse O. | National Instruments R&D
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Have you tried terminating your cable with a matched load (50 ohms)? That should give you a clean signal without ringing and overshoot, although at the cost of reduced amplitude.



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Hi, Jesse_O / johnsold


I think I will use those "hardware" solutions as a workaround.


And also I will include the rise time control in the product new features wish list 🙂



Thanks for your help,



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