PXI 5412 Load Impedance

[Guy04]

I'm using the PXI-5412 and I'm getting some results that are not making sense.  I'm generating a 0-5V square wave.  I have to first normalize my waveform to be 0-1V and then I'm applying a gain of 5 by using the "niFgen Configure Arbitrary Sequence.vi".  Since my Load resistance is set to 50 Ohms I have terminated the output Load resistance to also be 50 Ohms.  Therefore, by using the Votage divider equation I should see only 2.5 Volt max but I'm actually getting 5 Volts.  Can you please explain how this is happening.  Is the 5412 doing some internal compensation?

Also, when I terminate the output load impedance to 1M Ohm (RL) and keep the 5412 to 50 Ohm (Ro), I get a max of about 10 Volts.  So it seems to me that the function generator is multiplying the gain by a factor of 2 somehow.  Can someone please clarify?

Thanks.

Message Edited by Guy04 on 04-13-2006 12:35 PM

[JerryAL]

Hi Guy

The default setting for load impedance for the NI 5412, and all NI signal generators is 50 ohms.  This applies to the hardware as well as the driver.  When the actual load is 50 ohms and niFgen is configured for 50 ohms load impedance, you will get 5 V out in your case.  As you saw, when going to a high impedance load, you saw 10 V.  With no negligable current, the entire inner driving voltage is dropped across the load.

Refer to the NI Signal Generators Help file for more information:

NI Signal Generators Help/Devices/NI 5412/NI PXI-5412/Front Panel Connectors/CH0

Jerry

[Guy04]

I'm sorry but I still don't understand.  I saw the help file and it has the voltage divider equation, so when I calculate the Vout I get:

Vout =  (50 / (50+50) ) x 5V = 2.5Volts

and

Vout =  (1M / (1M+50) ) x 5V ~ 5Volts

So I still don't understand how I'm seeing the 5 Volts and 10 Volts because by these equations I should see 2.5 Volts and 5 Volts respectively.

[JerryAL]

Vout =  (50 / (50+50) ) x 10V = 5Volts

and

Vout =  (1M / (1M+50) ) x 10V = 10Volts

J

[Guy04]

Jerry,

Thanks for your help.  I really appreciate it.

If I'm applying a gain of 5 then shouldn't I be multiplying by 5.  Where does the 10 come from?

Thanks.

[JerryAL]

The hardware is designed to work with a 50 ohm load.  So it is designed to generate 10 V internally so that there will be 5 V at the output due to the output and load impedance voltage divider.

J

[Guy04]

Thanks for help.

[NeilF]

Remember, you don't have to do this manually yourself.  Just set the Load Impedance attribute to the actual impedance for your load, and NI-FGEN will do the calculation automatically for you and give you the voltage you are looking for.  This was added in NI-FGEN 2.0.

Neil F.

[Guy04]

The 5412 only lets you change the value to 50 Ohm or 75 Ohm, so when the Load impedance is lets say 2k then I would actually have to do this manually right?

[NeilF]

You're confusing the Load Impedance and Output Impedance.  The output impedance can only be set to 50 or 75 ohms (the 75 ohm choice is for video applications).  The load impedance can be any value.

Neil