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Peak to Peak voltage (VPP?) discrepancy in U2761A function generator

Question asked by dnao2 on Jun 22, 2012
Latest reply on Jun 26, 2012 by dnao2

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Hi there,

I am using the function generator U2761A. When I use the SCPI command:

APPL:SIN 2000, 1 VPP, 0

as described in the programmer's manual for SCPI commands, the output is a sine wave ranging from +1V to -1V. I believe the definition of VPP (peak to peak) should extend from +0.5V to -0.5V when there is a DC offset of 0V. Can you please offer an explanation of this observation? Does VPP in this case represent "peak-to-peak"? To help out, when I query for the amplitude "high" and "low", I do receive +0.5V and -0.5V respectively. The output load is set to 50Ohm.

Thank you in advance.

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lhornburg

May 11, 2016 10:14 AM

I haven't used this particular function generator, but this is a pretty common function generator question. Usually the cause is the impedance of the measuring device or DUT. DMMs, and most/not all oscilloscopes are high impedance. The output impedance of the function generator is a fixed 50 ohms so when an output is applied to a Hi-Z device, the value appears to be double what you would expect. Here is a previous post for a different fgen I made. The explanation should apply to this function generator as well:

The output Voltage is twice/double the value I set.

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dnao2 Jun 25, 2012 2:51 PM

Yes I believe the mismatch input impedance is the cause. I have been unsuccessful in correcting this at the moment. I can set the U2671a to highZ, I believe the read out value is something like 99E36 ohm, using the command OUTPUT: LOAD?. However, the scope input impedance is 1Mohm, and that is considered "high". In a typical voltage divider, Vout = R2/(R2+99E36), should I expect a 0v output because the R1 (from the function generator) is so much higher now?

Thanks!

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lhornburg

@ dnao2 on Jun 25, 2012 5:57 PM

No,

The function generator output impedance is fixed. You just tell it the load impedance and it does the math for you. The math all works out the same for loads 1Mohm and higher. If you have your function generator set for 50 ohms, the output will be double the value you set. If you set it for Hi-Z, you will get the expected value.

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dnao2 @ lhornburg on Jun 26, 2012 1:10 PM

Hi lhornburg,

You are a lifesaver! This has resolved my issue. Thanks once again!

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4 Replies

lhornburg May 11, 2016 10:14 AM
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I haven't used this particular function generator, but this is a pretty common function generator question. Usually the cause is the impedance of the measuring device or DUT. DMMs, and most/not all oscilloscopes are high impedance. The output impedance of the function generator is a fixed 50 ohms so when an output is applied to a Hi-Z device, the value appears to be double what you would expect. Here is a previous post for a different fgen I made. The explanation should apply to this function generator as well:

The output Voltage is twice/double the value I set.
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dnao2 Jun 25, 2012 2:51 PM
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Yes I believe the mismatch input impedance is the cause. I have been unsuccessful in correcting this at the moment. I can set the U2671a to highZ, I believe the read out value is something like 99E36 ohm, using the command OUTPUT: LOAD?. However, the scope input impedance is 1Mohm, and that is considered "high". In a typical voltage divider, Vout = R2/(R2+99E36), should I expect a 0v output because the R1 (from the function generator) is so much higher now?

Thanks!
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- lhornburg @ dnao2 on Jun 25, 2012 5:57 PM
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  No,
  
  The function generator output impedance is fixed. You just tell it the load impedance and it does the math for you. The math all works out the same for loads 1Mohm and higher. If you have your function generator set for 50 ohms, the output will be double the value you set. If you set it for Hi-Z, you will get the expected value.
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  - dnao2 @ lhornburg on Jun 26, 2012 1:10 PM
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    Hi lhornburg,
    
    You are a lifesaver! This has resolved my issue. Thanks once again!
    Like • Show 0 Likes0
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Peak to Peak voltage (VPP?) discrepancy in U2761A function generator

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