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Measurements of 19+20kHz IMD of various opamps at gain 40dB

HpW

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#5
May you consider to use the new TI OPA1656 :D as "SoundPlus™ ultra-low noise and distortion, Burr-Brown™ audio operational amplifier"
 

Blumlein 88

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#7
All of your graphs show something at 8, 9 and 10 khz. Is this an aliasing/imaging product from using digital test gear? Looks like you used a 32 k FFT and 48 khz sampling.

Thanks for doing and sharing this btw. Good info.
 
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March Audio

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#8
All of your graphs show something at 8, 9 and 10 khz. Is this an aliasing/imaging product from using digital test gear? Looks like you used a 32 k FFT and 48 khz sampling.

Thanks for doing and sharing this btw. Good info.
That's why I asked for a loopback, I wouldn't expect those products, especially at exactly the same level in each plot.
 
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pma

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Thread Starter #9
Can you post the loopback of the measurement system?
The measurements were done in January 2013. I may try to re-arrange the system, but I am not sure if I can replicate it perfectly. 2nd issue is that the DUT had 40dB gain so the DAC output is small and less distorted than if it works in loopback without gain. My guess is that the system distortion limit was close to OPA637 result.

All of your graphs show something at 8, 9 and 10 khz. Is this an aliasing/imaging product from using digital test gear? Looks like you used a 32 k FFT and 48 khz sampling.

Thanks for doing and sharing this btw. Good info.
Thank you. You are right, the 8,9,10kHz lines at -105 dBr were the soundcard artifacts present in every CCIF measurement with that card.

@restorer-john Yes +/-15V supply, 3.4Vp-p output as below
OZ_CCIF2_30January2013.PNG


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This was the test circuit. Double-sided PCB with ground plane.
opamp_40dB_test.png

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I have one more comment on 19+20kHz. Sometimes it is correctly stated that music does not contain such frequencies in high amplitudes. This is of course true, but the point of the 19+20kHz is different. Let's not forget that this test also shows static distortion of the transfer function. So if the DUT has frequency independent distortion (= static distortion only), it is also shown in the plot. H2 is shown as a difference tone, H3 as two lines at the side of base frequencies. Same for further even harmonics (they follow diff. tone) and odd harmonics ("skirts"). The bonus is that the 19+20kHz test also shows dynamic distortion, i.e. that related to higher frequency non-linearity or slew rate limit. These dynamic distortions remain almost invisible in the 1kHz sine spectrum test. So I suggest 19+20kHz test even at higher amplitudes (than those in natural music) for the test reasons to reveal dynamic behavior of the amp.

Below is the 19+20kHz test, simulated results, on a purely static DC nonlinearity, -80dB (re FS) H2 and H3. Original and distorted.

ccif_orig.png


ccif_dist.png
 
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