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


Permlink Replies: 4 - Pages: 1 - Last Post: Jun 26, 2012 2:10 PM Last Post By: dnao2 Threads: [ Previous | Next ]
dnao2

Posts: 9
Registered: 04/22/12
Peak to Peak voltage (VPP?) discrepancy in U2761A function generator
Posted: Jun 22, 2012 1:58 PM
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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.
lhornburg

Posts: 475
Registered: 01/15/10
Re: Peak to Peak voltage (VPP?) discrepancy in U2761A function generator
Posted: Jun 22, 2012 2:45 PM   in response to: dnao2 in response to: dnao2
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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:

http://www.home.agilent.com/owc_discussions/thread.jspa?messageID=99293&#99293
dnao2

Posts: 9
Registered: 04/22/12
Re: Peak to Peak voltage (VPP?) discrepancy in U2761A function generator
Posted: Jun 25, 2012 3:51 PM   in response to: dnao2 in response to: dnao2
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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!
lhornburg

Posts: 475
Registered: 01/15/10
Re: Peak to Peak voltage (VPP?) discrepancy in U2761A function generator
Posted: Jun 25, 2012 6:57 PM   in response to: dnao2 in response to: dnao2
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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.
dnao2

Posts: 9
Registered: 04/22/12
Re: Peak to Peak voltage (VPP?) discrepancy in U2761A function generator
Posted: Jun 26, 2012 2:10 PM   in response to: lhornburg in response to: lhornburg
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Hi lhornburg,

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

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