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R series: output impedance matching versus output current

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Hi,

 

I am wonder about output impedance specified for PXie-782xR and others l ike 6581 module adaptater.

 

The output impedance specified is 50 Ohm. To get the best signal integrity, it is mandatory to adapt the impedance using a cable (line transmission) with a 50 0hm caracteristic impedance and a load impedance of 50 Ohm.

But I don't understand how this is possible, because with a high level of 3.3V the output current sourced by the pin should be 66 mA. The 7821R max output current per pin is 4 mA.

 

This FPGA offer 128 pins, so the maximum total output current should be 8.5 A, so 28 W. This is not realistic !

 

Someone could explain me why output are 50 Ohm ?

And how should I adapt the line and termination ?

 

Thanks and best regards,

Benoit Chantepie

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Hello,

Why 50 Ohm? -> http://www.microwaves101.com/encyclopedias/why-fifty-ohms

 

Your calculation (3.3V/50Ohm= 66mA) means that you ask for a High state on output AND you wire the ouput to GND, that's not a good idea! That is called a short circuit.

Actually, devices are trying to have a maximal input impedance in order to minimize voltage loss.

So you will never have 3.3V/50 Ohm, but 3.3V/(50Ohm + Zinput).

Best regards,

Jovan N. - Application Engineering
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Hi Johan,

 

Concerning my 1st question: I would understand why the output is 50 Ohm with regards to the current limitation. This was not clear. Sorry.

 

Wire the output to GND through a 50 Ohm is not a short but impedance matching.

This is very usual, specially to adapt an AWG which has an output impedance of 50 Ohm for example. You can also do it in the input stage of a scope.

 

See your products: PXIe-5114, PXIe-5162, PXIe-5122, PXIe-5105, PXIe-5186, PXI-5922

Each of this input can be set 50 Ohm or 1 MOhm. Are you telling me that if you set the input to 50 Ohm you have a short to GND. Impedance matching is one the most important parameter to certify the signal integrity.

 

Secondly:

if you adapt the line with a 50 Ohm to GND near the DUT input and you consider the input impedance of your DUT, to result impedance viewed by the instrument will not be (50 Ohm + Zinput) but (50 Ohm // Zinput).

If Zinput = HiZ, the result impedance will be 50 Ohm. The transmission is so adapted.

 

My questions are still the same:

 - Is somebody could explain how to adapt the transmission line connected to a 50 Ohm output of the R series?

 - Why the output is specifiy 50 Ohm with a limited current to 4 mA and the impossibility to adapt the transmission line with a 50 Ohm static load ?

 

I am sure I missunderstand something but I don't know what.

 

Thanks and best regards,

Benoit.

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I agree that this is a misleading specification. Almost anyone who sees a 50 ohm output impedance specification will expect the device to be able to drive a 50 ohm load over the entire output voltage range.  Clearly these devices cannot do that.

 

On a more subtle, techincal view, it is conceivable that the specification means that the voltage drop at the output is equivalent to the drop from an ideal voltage source in series with a 50 ohm resistor, up to the current limit.  However, this interpretation is not supported by Table 2. Digital Ouput Logic Levels from the NI PXIe-7821R Specifications document.  It is close for output low levels but has subtantially larger differences for output high levels.

 

Jovan, Ask an RF engineer to read that specification and then get the documentation people to fix it.

 

Lynn

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Hi Lynn,

Thanks for your intervention.

 

Hi NI,

9 days from my questions to NI France support and NI community and still no correct answer.

I am waiting for an answer before I buy instruments to set a new testbench and I have not planned the delay due to the non-support from NI.

 

I need to know how the instrument works before buy it.

Please answer.

 

Thanks and best regards,

Benoit

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Accepted by topic author benc13

Benoit,

 

After some further thought, the whole concept of output impedance is really not applicable to digital logic outputs because they spend most of their time in saturated states and the time spent in transient states is usually not specified except for timing limits.

 

As far as your purchase process is concerned I would ignore the output impedance specification. Look at the voltage and current limits. If those do not meet your requirements, then consider external buffers. You can specify the buffers to meet the requirements of your testbench (including fault protection if needed) and to satisfy the specifications of the NI devices.

 

Lynn

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Hello Benoit,

 

Hopefully I can provide a little help.  The PXIe-7821R has both a 50 Ohm output and a 4mA current limit.  Let me try and explain both of these specifications.  

 

First the 50 Ohm output.  When driving long cables with fast rising edges there needs to be termination.  The most common for CMOS logic levels is source series termination, and a high impedance load.  This allows a 3.3V output to communicate with a 3.3V device under test (DUT).  There is a little information about source series termination here.   With the output being 50 Ohm, it is important to have a 50 Ohm characteristic impedance cable.  This setup will require the FPGA drives 3.3V / (50 Ohm output + 50 Ohm cable) = 3.3V / 100 Ohm = 33mA.  However this current is only required for a short duration. The duration of this 33mA is defined by the length of the cable attached.  For example if you have a 1m cable, the time it takes for the initial 0->3.3V rising edge to propagate from the FPGA down the cable and back is about 10ns.   Since the cable is high impedance to DC voltages, after this initial 10ns, the cable and device all see 3.3V and no more current flows.  If the digital output was generating a 1MHz square wave, it would toggle back from 3.3V -> 0V after 500ns, meaning the FPGA is only driving for 10ns/500ns = 2% of the time.   This is all assuming the load is not terminated with 50 Ohms.

 

Next the 4mA output current specification.  This is more the DC drive strength limit, and not the transient current.  The output can drive more current temporarily to allow for driving cables as described above.  What this 4mA spec is saying is that you cannot drive a DC current of more than 4mA.  So it is not possible to drive a 50 Ohm load as it would require a DC current of 33mA (3.3V / [50 Ohm output + 50 Ohm load] = 33mA).  

 

For your device / cabling with the PXIe-7821R I would recommend 50 Ohm  impedance cables connecting to a high impedance DUT.  This will allow for the higher transient currents caused by driving the cable, but keep the current below the 4mA DC specification. 

 

I hope this helps answer your questions.  If not please let me know,

Jesse O. | National Instruments R&D
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Jesse O.,

 

Thank you for the explanation.  

 

Please ask the documentation people to include that information in the specifications documents.  Even a footnote to the 50 ohm spec which says something like "This can drive 50 ohm cables with series source termination but cannot drive a 50 ohm resistive load" would eliminate a lot of confusion.

 

Lynn

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Hi Jesse,

 

Thanks for your precise answer. It makes sense for me.

I understand know how to adapt the line.

 

Best regards,

Benoit C.

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I understand "now" of course 🙂

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