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If I followed the datasheet recommendations, wouldn't that make your point moot? I don't know, I'm just wondering. TI seem to have compensated for that in the REG101 because they say in the first couple paragraphs of the datasheet that an output cap is unnecessary for stability.
Absolutely... But I don't read every word of datasheets and I don't know anyone who does :d
While an output cap may not be necessary for stability it still helps for load regulation and an unwise choice can *cause* instability.
Try your layout only on the topside first. It might not be that hard to route since there are few signals that cross stages and their path is very linear. I think you can probably fit it all on one side.
For analog, especially if you're driving low impedance phones, I'd keep the voltage relatively low. This will make it easier to ensure that the necessary transient current is available on a small board. 9V seems a good compromise to me. So I'd boost to about 10V and use a good linear reg to get 9V. A ~4.75V linear reg for AVDD (directly from USB power) and 3.3V for DVDD.
Use REG101 for both analog supplies if you can get it. I've had a hard time finding anything with similar specs. LP2985 is about as good, but it's fixed voltage only. If you really wanted to use a 'quality' reg for the digital supply you could use one. Otherwise I'd choose the TI from the ones you linked, it's specs are a lot better and it's not much pricier. Plus I like TI more than Fairchild :P
Read the datasheets carefully. Jambo is right, LDO regs can be finicky, so make sure you're loading them properly. If they recommend particular part numbers for capacitors, use those parts.
Since the Wolfson is spec'd only at 5V, I'd hazard a guess that performance does degrade with AVDD at <5V. Whether it's worth wasting the current regulating your high voltage rail down to 5V is up to you. Personally I'd probably go with the 4.8V solution.
The DC boosting would be for the amp section, though your point is well taken. I'm just assuming that as input voltage increases for the DAC, the better the efficiency and performance. Does my assumption correspond to reality? Then I could just run both supplies off of a REG101's 4.75V output, could I not? It'll run a little hotter and draw more power, but if the USB can take it, I'm down. If it doesn't improve performance in any significant way, going all the way down to 3.3V is cool too. Thanks for that.
According to the datasheet the performance indeed increases with voltage but efficiency decreases. At 5V (both AVDD and DVDD), you have a 32ma current draw. At 3.3V, a 24ma draw. It's worth noting that the digital section is accounting for most of that change. So, having a first regulator at 3.3V for the DVDD and a second at 4.75V for the AVDD seems the best compromise between efficiency and performance. And it can run directly from USB power. I thus echo error401 on this.
I don't think one should do without LDO in such a project. The 2 or 3V of headroom a conventionnal regulator needs are a luxury. Just follow the layout and part indications of the datasheet. To avoid problems with the reg101, it is often good practice to have after the regulator: a "not too low esr" electrolytic cap (like 10uF), followed by a ferrite and a ceramic cap (10-100nF) right at the supply pin of the powered device.
To layout your digital section (the whole wm8740), as well as the associated regulators, the best way to proceed on a two sided board is to have your chips sitting on a groundplane, with all (if possible) the signal traces running on the other side. It usually provides for the shortest ground loops. For the amp, scrap the groundplane and go for star ground.
Good luck
edit: and don't forget the thermal relief option on the groundplane.
ok guys, how could iMod get better than squeezebox, transporter or olive music station? could something like this really worths a shot? squeezebox3 is around the same price of iPod.
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Headphone isn't a toy.....use for music listening only!
There're people who can learn and people who can't...
ok guys, how could iMod get better than squeezebox, transporter or olive music station? could something like this really worths a shot? squeezebox3 is around the same price of iPod.
The Squeezebox uses the BB PCM1748, iPod/iMod (best case scenario - 4G iPod Photo) uses the WM8975. Just compare the specs. It looks like the Squeezebos DAC performs better than the iMod one, but that's DAC specs alone. Best tests would be to actually listen to both and see which one sounds better using the same files. There's an idea, how about listening Not trying to be a jerk, but way too many times people recommend things they never heard themselves that it's just a good idea to get your hands on both of the things you're trying to compare and have a listen yourself.
No offense but I own these sources.....
-iMod (G3 DIY, Photo for both DIY and RWA)
-Heavily modded Linn Genki (can beat up Ayre CX-7e's ass)
-DAC1
I have listened to.....
-cheap $400 Sony CD player
-squeezebox v3 (no mods)
Too bad, iMod couldn't beat up anything I have tried.....maybe just for my impressions.
We're talking about a digital out so you can use whatever DAC implementation you want. Not sure why all this talk about Squeezebox vs. iMod is relevant...
While lying in bed and enjoying some good tunes, I realized that the Pico is pretty much what I'm trying to design. Portable DAC/amp, rechargeable battery, tiny, great chips that presumably give good sound. It's a pricey unit, but it's pretty much what I was shooting for. The only difference is that there is no portable iPod external DAC feature; the DAC is only for USB. Nevertheless, I'm sure some minor drilling or a one-off request would enable that, and all my problems would be solved. It's been fun trying to design this DAC/amp, but it seems that the niche has already been filled. I don't plan on buying a Pico anytime soon, but it seems it's on my horizons. Maybe I'll send Justin a quick email after he's done all the Pico pre-orders.
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- they're - they are
While lying in bed and enjoying some good tunes, I realized that the Pico is pretty much what I'm trying to design. Portable DAC/amp, rechargeable battery, tiny, great chips that presumably give good sound. It's a pricey unit, but it's pretty much what I was shooting for. The only difference is that there is no portable iPod external DAC feature; the DAC is only for USB. Nevertheless, I'm sure some minor drilling or a one-off request would enable that, and all my problems would be solved. It's been fun trying to design this DAC/amp, but it seems that the niche has already been filled. I don't plan on buying a Pico anytime soon, but it seems it's on my horizons. Maybe I'll send Justin a quick email after he's done all the Pico pre-orders.
Or alternatively, is there a solution for all Pico, Predator, and other USB DAC owners out there to somehow extract the digital signal from an iPod (or other player) to deliver to the DAC in USB? I've got my fingers crossed that one of you brilliant DIY guys can come up with some custom soldering/cable job that will make this a possibility
P.S. I would get on the list now for the Pico (wait time is about 10 weeks, and payment isn't due until shortly before shipping, plenty of time to save up).
The fun way of doing this is to open up whichever iPod you have, identify the Wolfson DAC you have, steal the I2S signal from those pins and send them to unused pins on the dock, make a dock connector to steal those signals, open up your Pico and solder to the I2S input of the Wolfson WM8740. That's a pretty tall order cause you could destroy both your iPod and Pico in one afternoon. Either that or you could pay MSB for their iPod transport. Non-video iPods have more unused pins than video-enabled iPods, but I think you could still work something out with the existing pins. Check out Apple iPod dock interface pinout and signals @ pinouts.ru.
__________________
"Ears that hear and eyes that see - the LORD has made them both." Proverbs 20:12
Team University-fi | Team Edmonton-and-Surrounding-Urbanities(1)(2)
Rants: - there - in or at that place; to or into that place; at that point or stage; in that matter, respect, or relation; used interjectionally to express satisfaction, approval, encouragement or sympathy, or defiance
- their - of or relating to them or themselves especially as possessors, agents, or objects of an action; his or her
- they're - they are
Not trying to be a jerk, but way too many times people recommend things they never heard themselves that it's just a good idea to get your hands on both of the things you're trying to compare and have a listen yourself.
Linear Mode
