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Old 22nd June 2011, 03:12 AM   #1
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Wink Urgen question about de-coupling capacitor for PCM1704

hi all
The decoupling capactiors for PCM1704 in datasheet:
"Aluminum electrolytic capacitors are recommended for larger values."
REF DC = 47uf
SERVO DC = 47uf
BPO DC = 100uf
Is there ESR critical ? May I use the ultra-low ESR capacitors (e.g. X5R MLCC) to replace? (Sound quality is not matter, will it be unstable?)
If anyone knows the answer, please let me know, very thanks.
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Old 23rd June 2011, 03:04 AM   #2
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OSCON caps are recommended for larger value decoupling caps for digital circuitry.
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Old 23rd June 2011, 03:44 AM   #3
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Thks a lot. OSCON of which manufactuerer and what type and rating would be prefered? Matsushita? I only hear that Matsushita is producting quality OSCONs.
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Old 23rd June 2011, 04:28 AM   #4
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not know which manufacturer might be best. Mouser has Vishay OSCONs here in both SMD and thru hole varieties. If you can get Matsushitas I would expect them to be quality as well.
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Old 23rd June 2011, 05:19 AM   #5
gootee is offline gootee  United States
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Quote:
Originally Posted by Kenneth Zhu View Post
hi all
The decoupling capactiors for PCM1704 in datasheet:
"Aluminum electrolytic capacitors are recommended for larger values."
REF DC = 47uf
SERVO DC = 47uf
BPO DC = 100uf
Is there ESR critical ? May I use the ultra-low ESR capacitors (e.g. X5R MLCC) to replace? (Sound quality is not matter, will it be unstable?)
If anyone knows the answer, please let me know, very thanks.
Why do you want to use ultra-low ESR caps for decoupling? Typically you wouldn't want the ESR too low, for decoupling, since the ESR might help to add damping for resonances involving the decoupling capacitance and the inductance of the supply traces or wires.
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Old 23rd June 2011, 07:02 AM   #6
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Thks Gootee. Your rationale is quite reasonable.
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Old 23rd June 2011, 04:50 PM   #7
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PCM1704 is a digital circuit building analog waveform. Global sound of your DAC will depends of sonic signature of those capacitors. The choice is not free.
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Old 23rd June 2011, 08:35 PM   #8
Lautar is offline Lautar  Ukraine
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im using spatial for decoupling caps


Oscon its bad idea
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Old 23rd June 2011, 11:38 PM   #9
gootee is offline gootee  United States
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Originally Posted by Eric Juaneda View Post
PCM1704 is a digital circuit building analog waveform. Global sound of your DAC will depends of sonic signature of those capacitors. The choice is not free.
What makes you think that?

You have not presented any evidence linking the quality of the bypass capacitors to the sound quality of the output.

I am NOT saying that you are wrong. I am saying that you have not yet given enough information to support your position. But maybe it's my fault, since I am not familiar with that IC.

I could still postulate that if the digital part of the chip is what depends on the bypass capacitors, then the analog output would almost certainly be unaffected by their quality, unless they were so bad that they caused a whole bit to be misconstrued, which seems extremely unlikely. That's the true beauty of going digital.

Bypass capacitors act as small point-of-load power supplies so that when the IC has sudden/high-rate demands for current, then the caps can supply it much more easily and better than the inductive power supply rail traces or wires, which, if made to try, would then also induce voltage spikes on the rails (v = L dI/dt; ouch).

Are there analog active filters or analog output amplifiers or impedance converters (buffer amplifiers) built into that IC's output stages? In that case, you could easily be correct.

You might already know all of this, but, the "standard" way to bypass, if you don't want to analyze thoroughly and it isn't too critical, is a non-"low ESR" electrolytic in parallel with a small-value lower-quality ceramic cap (i.e. not NPO or C0G type). But if I were bypassing something where it might matter for the sound quality, I might use some combination of high-quality film capacitors, and probably even also some electrolytic. BUT, because of the low ESRs, then, UNLESS I was doing a full massive analysis regimen that took into account ALL of the parasitic effects in both the actual components and in the traces and layout, etc etc etc, I would almost-certainly add a small-value (typically 3 to 30 Ohms) high-quality series resistance just upstream in series with each set of bypass capacitors. Otherwise, it's just like ASKING for problems, especially with digital stuff involved. It's even possible that you might think that you're hearing the effects of different capacitors' "sonic signatures" when it might actually be the different resonances that different caps allow to be excited, causing different-sounding problems on the power supply rails. Maybe you should even try putting a larger electrolytic capacitor on the upstream side of that small-value resistor, to better-supply the film caps. You could even split the resistor and put one on each side of the electrolytic, to provide low-pass filtering in both directions. Also, you would typically want to make sure that you have at least large-ish value and very small value bypass caps, with values depending on what the chip might try to pull, which depends on the amplitudes and the time-rates-of-change and the time-durations of the possible current requirements. You might also want to try fine-tuning the individual bypass cap values that you use. But always put the smallest ones as close to the IC pins as possible.

By the way, you should be able to use an oscilloscope and SEE whether or not anything like audio signals are being handled by the bypass capacitors.

Cheers,

Tom

Last edited by gootee; 23rd June 2011 at 11:45 PM.
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Old 24th June 2011, 06:58 AM   #10
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Default Inside PCM1704

First of all, it might be useful to look inside PCM1704 to understand the utility of each capacitors.
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