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Old 12th December 2007, 04:23 PM   #1
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Default Question about testing DACs dynamics range

Hello, it might sound stupid question but I could not find the answer so far...
I tried to understand why the DAC THD+N is being tested using 991kHz test tone. Same with 4x and 8x oversampling.
Trying to measure DAC dynamic range with 1kHz test tone gives lousy results: high values for 2nd and 3rd harmonics (for high quality multibit DACs).
Please, advise
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Old 13th December 2007, 07:40 PM   #2
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What I noticed replacing the 1kHz test tone with 991Hz, it gives much more clear DAC output spectra, actually meeting the datasheet dinamics spec.
1kHz produces random placed on the x axis spikes... The FFT requirement for complete signal period is not fulfilled for both 1kHz and 991Hz for sampling frequency 44,1kHz...
I'm really brainteased with this puzzle
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Old 14th December 2007, 07:09 AM   #3
oshifis is offline oshifis  Hungary
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Then you get an integer number of samples in a full sine period. For example there are 45 samples in 44.1 kHz sample periods. The individual samples in each consecutive sample period fall at the same point of the sine wave. E.g. if the first and 23rd is at zero crossing, then the 46th is again at zero crossing etc.
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Old 14th December 2007, 07:18 AM   #4
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The FFT requrement for the test signal is just what you say, but for the given example I can not fully agree
The period of 991Hz does not contain integer number of 44100Hz periods (samples), but some 44,50050454086781029263370332997.
I checked this before posting my question above...or my calculator is not trusty anymore
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Old 14th December 2007, 10:31 AM   #5
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Default Re: Question about testing DACs dynamics range

Quote:
Originally posted by beauty_divine

Trying to measure DAC dynamic range with 1kHz test tone gives lousy results: high values for 2nd and 3rd harmonics (for high quality multibit DACs).
And how bad is it ?
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Old 14th December 2007, 12:09 PM   #6
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3rd harmonic at -65dBc and 2nd around -75dBc.
When switched to 991Hz the spurious free dinamic range widened to -110dB The pair of DACs I measure is PCM63 K-grade i.e. exceptionally good ones.
So far no explanation about this phenomena :
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Old 14th December 2007, 12:44 PM   #7
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When I measure 1 kHz it is spurious free to the noise floor of my analyzer (-94dB)

But I also have bad test CDs that show some harmonics. Maybe old ones recorded fom sine generator.

Nearly any DAC is good at 0 dB. Try -60dB 1 kHz signal.
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Old 14th December 2007, 12:59 PM   #8
4real is offline 4real  Netherlands
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How about looking at the generated 1 Khz digital file and do a FFT on it. I bet the file is simply very bad...
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Old 14th December 2007, 01:15 PM   #9
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I generate 1kHz sin using Adobe audition. The noise floor of the generated signal is -136dB as reported by Adobe and Cooledit. The same signal recorded on CD and extracted from it, again spectrum analized gives the same -136dB noise floor and the respective 1kHz fundamental tone magnitude (-10, -20 ... -60dB as recorded on diffrenet tracks). So I do not suspect the recorded test tone signal, neither the generation process itself but a test problem related to FFT analyses (or whatever they use in spectrum analysators) ... After a day of thinking why so extremely good sounding DACs measure so bad spectrum I noticed Burr Brown test condition: 991Hz test tone, what proved to be giving the correct results. The question "Why?" is still unanswered....
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Old 14th December 2007, 02:35 PM   #10
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Maybe dithered 1 kHz is better.
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