PCI-6541 voltage answers and a question about schottky diodes

[mlloyd]

I am using the PCI-6541 with a third party card that runs at 3.3V.

For transmission to the third party device, I can use a 50 ohm parallel termination as the load as stated in the documentation, to create a matched impedance system.

For receiving data from the third party device

1. How can I make a matched impedance system? 
2. The third party device has a 50 ohm series resistor, then the cable is 50 ohms, but does the PCI-6541 have the parallel 50 ohm resistor? 
3. Would the 6541 be able to read a voltage that is half of 3.3V if it is set to run using the 3.3V logic family?

Thank you very much for your assistance with these questions!

Regards,

mlloyd

[mlloyd]

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@mlloyd wrote:

For receiving data from the third party device

1. How can I make a matched impedance system? 
2. The third party device has a 50 ohm series resistor, then the cable is 50 ohms, but does the PCI-6541 have the parallel 50 ohm resistor? 
3. Would the 6541 be able to read a voltage that is half of 3.3V if it is set to run using the 3.3V logic family?

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I think I've answered my own questions after looking at the detailed channel electronics information, but please let me know if I'm incorrect...

1. I don't think you can have a matched impedance system for data acquisition.
2. There is a parallel 10 k-ohm resistor, which will allow the 6541 to receive the full voltage value being sent by the peripheral device.  The Schottky diodes protect the 6541 from overshoot and ringing.
3. If the input voltage to the 6541 were half of 3.3V, or 1.65V, acquisition could be set to the 1.8V logic family.  The minimum high voltage threshold is 1.35V.

When I used another NI card, there were specific recommendations made for Schottky diodes that the peripheral device should have.  Is there a specific diode a third party device should have when connecting to the PCI-6541?

[RyanB]

Your statements are correct. 

Regarding Statement 1&3:

The NI 6541 has a 50 Ohm output impedance.  This will allow you to create a fully matched impedance system when using the 6541 to generate data by adding parallel 50 Ohm termination at the load, as you said.  (Note: The NI 6541 cable and accessories have a characteristic impedance of 50 Ohms).  Be aware of the 6541 output current limitation when driving a 50 Ohm load.

The NI 6541 inputs have a 10kOhm parallel termination resistor.  The 6541 does not have the ability to present a 50Ohm load for acquisitions.  If this is a requirement for your application, the NI 6551 can be configured in either 50 Ohm or High-Impedance (10kOhm) mode for acquisition.  For most applications, having only source termination is sufficient.

For your acquisition, you have a 50 Ohm source impedance driving a high-impedance or open load (via a 50 Ohm transmission line).  This would be a source-terminated system.  The open load would cause signals to reflect back to the source, however as the source impedance matches the cable impedance, the reflections would be absorbed by the 50 Ohm source termination. 

The NI HSDIO Help file includes termination information and recomendations.  Specifically these sections:

Fundamentals->Termination

Integration and System Considerations -> Terminating your module -> NI 654x

Regarding Statement 2:
The 6541 I/O are protected with clamp diodes to +5V and Ground.  You do not need to add external protection circuitry for systems within the legal voltage range (like your 3.3V application).

Thanks and good luck,
RB

[mlloyd]

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@RyanB wrote:

Be aware of the 6541 output current limitation when driving a 50 Ohm load.

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If I perform the calculation for maximum current as suggested in the help files, it appears that NONE of the voltage families could EVER be run in a matched impedance system!  For example, at 3.3V, the maximum DC drive strength is +/- 32mA.  The maximum current required would be

I max = V OH (max)/ (100 ohms) = 3.3V / 100 ohms = 33 mA,

which means a matched impedance system wouldn't work. 

Am I reading the max current the 6541 can source correctly?  Am I correctly performing the calculation?  If I am correct on these two points, do you know how I can create a matched impedance system If the maximum current is always going to be greater than the allowable current?

Thank you very much for your help on this issue.  I hope there is a solution!

Regards,

mlloyd 

[Allen H]

Hi mlloyd,

You are doing your calculations correctly.

I would like to concentrate more on the termination scheme.  The 6541 provides an output impedance that is very close to 50 Ohms, and the SHC68-C68-D2 cable is also matched to 50 Ohm characteristic impedance.  In almost all applications you can generate data from the 6541 into a high-impedance load and your signals will be fine.  Even though you will get reflections at the receiver, those reflections will be absorbed by the matched source impedance at the 6541.  You wind up with signals that are still very clean.

In other words, you should be able to use high-impedance without sacrificing your signal quality.

I hope that helps,

Allen