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Old 3rd September 2003, 01:14 PM   #1
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Default DAC digital supply decoupling (Guido?)

Close examination of the dac section of my cd player (Rotel using bitstream dac) reveals that the digital supply has no decoupling - only a choke to limit the noise. All the analogue supplies are nicely done using blackgates. Now obviously they know what they're doing. But would it be beneficial to decouple the digital supply close to the chip (after the choke)? Cheers
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Old 3rd September 2003, 07:07 PM   #2
tiroth is offline tiroth  United States
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Yes. Are you sure there aren't caps hiding under the board? Choke and no decoupling cap is a recipe for disaister. I find it hard to believe a manufacturer would include a (relatively) expensive item like a choke but not even a 100nF ceramic cap.
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Old 3rd September 2003, 07:59 PM   #3
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Definitely no decouple caps. Dac supplied by 5v reg just for dac which is something. The digital section vdd1/2 (digital section and oscillator) is supplied from the 5v trace from a choke and a resistor arrangement. I too find this weird. Maybe they found that the biggest difference was to use good caps for the analogue part of the dac. In the application notes on the datasheet it has this arrangement for feeding these 2 pins, but before the choke/resistor there appears to be some decoupling (hence quite far from the chip - perhaps there is a reason why its this way round) The designer appears to have relied upon the fact that the 5v supply for the dac is seperate and nearby to not bother with the decoupling before the choke/resitor as specified. I wonder if decoupling after the choke would be ok?
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Old 3rd September 2003, 08:08 PM   #4
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Quote:
Originally posted by tiroth
Yes. Are you sure there aren't caps hiding under the board? Choke and no decoupling cap is a recipe for disaister. I find it hard to believe a manufacturer would include a (relatively) expensive item like a choke but not even a 100nF ceramic cap.
Hi,
yeah probably SMD ceramics at thesolderside......
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Old 3rd September 2003, 09:52 PM   #5
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Just to clarify, can guarantee that there are no hidden caps underneath. I know, ive looked! In fact no smd at all. Weird because on an older rotel there is.
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Old 4th September 2003, 11:59 AM   #6
hifi is offline hifi  Sweden
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Is it on a specific ic (if so what modell) or on all? there are some modern chip with integrated decoupling caps..

if so no further decoupling is needed or desired.


/micke
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Old 4th September 2003, 12:39 PM   #7
tiroth is offline tiroth  United States
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Well, at any rate due to the presence of the choke as a damping element I think you can feel free to slap whatever decoupling caps you want after it (don't overload your regulator though if it can't take a large capacitance). That choke is presenting high impedance at high frequencies where you want it to be as low as possible, and needs a cap after it to work properly.

If you can measure (or see) the noise on the rails I'd compare before and after. I'd bet there is a significant improvement.
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Old 4th September 2003, 01:17 PM   #8
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hmm interesting. No capacitance on board (10 year old philps dac design - saa7323 bitstream) Well im sure ill find out pretty quickly if its not a good idea to bung on the cap. Cheers - anymore thoughts obviously welcomed!
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Old 5th September 2003, 08:08 PM   #9
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Quote:
Originally posted by Rotellian
hmm interesting. No capacitance on board (10 year old philps dac design - saa7323 bitstream) Well im sure ill find out pretty quickly if its not a good idea to bung on the cap. Cheers - anymore thoughts obviously welcomed!

Hi Rotelian,

At first I thought: On chip decoupling

Now the thing is 10 years old, so forget about that (I started at Philips Semiconductors by then, and no on chip decoup available by then, too expensive)

I'd suggest to apply some decoupling, but leave the chokes in place.

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Old 5th September 2003, 08:18 PM   #10
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Hi Guido,

OK ill try that - thanks! Quick question though - im assuming be ok to put the caps after the choke close to the chip as pos?
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