DAC digital supply decoupling (Guido?)

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Rotellian

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Joined 2003
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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Rotellian

Member
Joined 2003
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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tiroth

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Joined 2001
anidian.com
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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Guido Tent

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Joined 2001
TentLabs.com
Rotellian said:
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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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.

regards
 
Fin

Fin

Member
Joined 2003
Different approach by Philips

Hi Rotelian

In the CD624 (using SAA7321), the DAC shares the same regulator as the rest of the digital section - but the supply to the digital side of the dac IS decoupled before the inductor/resistor network with a 33uF/100nF and after the network with a 56pF.

I wonder how it is done in the QED Digit.
 
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