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Old 10th July 2010, 01:50 AM   #21
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Quote:
Originally Posted by abraxalito View Post
Post it up by all means. If you have measurement data which shows input stage crossover distortion that would be really something.
Unfortunately we didn't take photos of the spectrum analyzer's display. But what were getting was the noise floor stayed constant at the spec an's -90dB limit and the harmonics rose relative to the signal. Side by side comparisons at similar power levels against a 49811 were at the spec an floor of 0.003%. So I'm reasonably confident the 3886's the distortion source and it's not a measurement artifact.

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Originally Posted by abraxalito View Post
Were you only referring to the 20kHz plots when you claimed the input stage was causing 0.5% distortion?
Unless my memory's failing me, we measured at 1kHz; the 0.5% THD's at 100uW. Applying the 6x rule of thumb you prefer to the 10mW value in the 8 ohm, 28V, 1kHz chart in the datasheet---which is the one closest to the test conditions we were using---gives about 0.2% at 100uW. As far as I'm concerned that's close enough to be in reasonable agreement; the curve's wiggly, it's a rule of thumb that Self's evidently changed from time to time or uses somewhat differently in different conditions, etc.

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Originally Posted by twest820 View Post
I should go back and check the PSRR measurement conditions on the 3886; it's possible the supply induced output error's attenuated only by PSRR and not excess loop gain, in which case I've overestimated the part's performance.
Yeah, the feedback loop's in place in figure 2 of the datasheet and has the typical 26dB gain. So the PSRR measurements are reasonably inclusive of the excess gain.

Last edited by twest820; 10th July 2010 at 01:53 AM.
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Old 10th July 2010, 02:09 AM   #22
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Originally Posted by twest820 View Post
So I'm reasonably confident the 3886's the distortion source and it's not a measurement artifact.
I'm confident enough too that the 3886 is the distortion source. But you went further - saying that the distortion came from the input stage. I'm still unclear how you'd know that it was that stage causing the distortion, given that you only measured the output.

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Unless my memory's failing me, we measured at 1kHz; the 0.5% THD's at 100uW. Applying the 6x rule of thumb you prefer to the 10mW value in the 8 ohm, 28V, 1kHz chart in the datasheet---which is the one closest to the test conditions we were using---gives about 0.2% at 100uW.
Fair enough that you're saying 0.2% THD is at 0.1mW output power. Just bear in mind that from the datasheet, the 3886 gives 92.5dBA SNR at 1W. So the SNR at 0.1mW will be 40dB below this, i.e. 52.5dBA. 0.2% THD is -54dB, not A-weighted. Noise still dominates the THD+N figure.
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Old 10th July 2010, 02:29 AM   #23
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Originally Posted by abraxalito View Post
I'm still unclear how you'd know that it was that stage causing the distortion, given that you only measured the output.
As I alluded to earlier, that's based on the equivalent circuit in the datasheet and comparing measurements of class AB and XD operation.

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Originally Posted by abraxalito View Post
Noise still dominates the THD+N figure.
I think this is the third time I've pointed out the assumption the 3886 noise floor's invariant with power doesn't agree with measurement. I'd also point out 92.5 dB is a typ figure, 1W is four orders of magnitude away from the power level in question, and probably neither of us has measured enough parts to know how the SNR probability distribution varies as a function of output power. I know I haven't.
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Old 10th July 2010, 03:02 AM   #24
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Originally Posted by twest820 View Post
As I alluded to earlier, that's based on the equivalent circuit in the datasheet and comparing measurements of class AB and XD operation.
Yeah, you alluded to that but how about unpacking it and put some flesh on the bones? We could all learn something about distortion in chipamps if you set forth your methodology and posted up some results.

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I think this is the third time I've pointed out the assumption the 3886 noise floor's invariant with power doesn't agree with measurement.
You can point out that what I say doesn't agree with measurement all you like but I'll humour you until you post up the measurements which show it varies. So far, no cigar.

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I'd also point out 92.5 dB is a typ figure, 1W is four orders of magnitude away from the power level in question, and probably neither of us has measured enough parts to know how the SNR probability distribution varies as a function of output power. I know I haven't.
Its reasonable to assume the noise floor doesn't vary with power until measurements overturn the assumption. National's own measurements do not, but they're marketing materials mostly so its quite possible other measurements might. The inner workings show 1k LTP degeneration resistors to be the most likely cause of the noise. Over to you to show that the assumption's flawed - but remember, measurements trump claims every time.
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Old 10th July 2010, 03:43 AM   #25
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Originally Posted by abraxalito View Post
but remember, measurements trump claims every time.
Then, as you do not accept mine, I do believe you'll have to conduct your own.
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Old 10th July 2010, 03:54 AM   #26
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Originally Posted by twest820 View Post
Then, as you do not accept mine...
Its not a matter of acceptance its a matter of them so far being invisible.
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Old 10th July 2010, 04:17 AM   #27
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Yep, the only person who can make your data become visible is you.
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Old 10th July 2010, 04:36 AM   #28
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You seem to have become inordinately confused. Equally though you might not be being serious You originally said :

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Then, as you do not accept mine
The invisible data in question is yours. You believe you have it, but you haven't shown it, so to me its still invisible. I'm not claiming any data so far. But if and when I do gather some, I'll be sure to show it to any interested party who asks for it.
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