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Jorge Vega

# Understanding Harmonic Distortion Versus Amplitude in Operational Amplifiers, Part 1

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Brad Albing
6/24/2013 4:08:13 PM
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Re: SNR
Jorge - thanks for the additional answers/information to clarify the graphs.

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Jorge Vega
6/10/2013 1:31:51 PM
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Re: SNR
Hello Amrutah, THD+N versus frequency is a graph that usually always accompanies THD+N versus Amplitude.  I didn't include it here just because of topic constraints but you can check out this reference to: http://www.en-genius.net/includes/files/acqt_013012.pdf.  It goes in depth on the topic.  Let me know however if it is still unclear.

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Jorge Vega
6/10/2013 1:19:09 PM
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Re: Noise
Hi Amrutah,

You are correct, one over x is representative of THD+N only in the noise dominated region.

Sure thing, the other dimension to this graph is one where the amplitude is fixed and a frequency sweel is performed.  So, you can actually cross-verify that both of the graphs match each other.

For example, if you look at the datasheet of OPA1604 (http://www.ti.com/lit/ds/symlink/opa1604.pdf), figures 7 and 11, you can cross check them.  Figure 7 shows the over-frequency graph with a fixed amplitude of 3VRms.  At 1Khz, THD+N for the 2KΩ load is btw 0.00002% and 0.00003%, now go to Figure 11 (THD+N versus Amplitude).  Here you will see that THD+N at 3Vrms is also btw 0.00002% and 0.00003%.  They match and now you know that the data presented is consistent.  This link shows more on THD+N versus Frequency: http://www.en-genius.net/includes/files/acqt_013012.pdf and also http://www.en-genius.net/includes/files/acqt_042312.pdf

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Jorge Vega
6/10/2013 12:57:42 PM
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Re: SNR
Thanks for your question on Figure 3. The measurement for this figure was obtained using an Audio Precision instrument for THD+N measurements. The settings on the instrument were such that it collected only the harmonics with the fundamental of the signal notched out. You can see the notch right 1Khz (counting the hairlines). Thus, there is no signal only noise on the first two upper graphs in figure 3.  The third graph however does begins to show discernible harmonics and thus THD components.  So we shouldn't be seeing any signal because the nature of the measurement notches it out leaving only the distortion components in addition to noise.

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amrutah
6/9/2013 7:46:45 PM
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Re: SNR
DaeJ,

I agree. I also think a THD+N Vs. Freq should also accompany a figure of Noise Vs. Freq. so that the contribution of Noise for particular freqency with THD is high at 4V.

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amrutah
6/9/2013 7:43:08 PM
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Noise
Jorge,

Thanks for the discussion on THD+N topic and looking forward to learn more here.

In the Figure 4 you meant:

"The function f(x) is representative of THD+N in the noise-dominated region".

At low amplitudes of "x", the f(x) is a inverse function of x and fits into the curve as shown in Fig.1.  There is no information about the frequency.  Do you think it makes sense to add the frequency as another dimension

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DaeJ
6/7/2013 11:07:45 PM
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SNR
Noise was described in the Figure 3. I am wondering where is signal in the Figure 3. I think that red line is noise and Figure 3 come from DUT diagram. To get clarification, Time domain is also a good presentation for SNR.

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