AK4490 for fun project (practical application) - diyAudio
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Old 24th October 2014, 09:03 PM   #1
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Join Date: Jan 2011
Location: Lower Mainland
Default AK4490 for fun project (practical application)

Hi All,

I'm interested in learning something about audio circuit design. The ultimate goal of this project is to discuss/learn about various supporting circuits for the design of a decent measuring/(hopefully good sounding) DAC. I'm hoping all you experts out there can give some/lots of pointers! The second goal of this design is to develop my own DAC to be used in my semi from scratch sound system if design proves at least somewhat successful.

Since using AKM device, might as well keep it in the family. Modules to be included in a custom enclosure:

1. Receiver Module, SPDIF/TOSLINK or other I2S.
2. By-passable SRC converter using AK4137EQ.
3. DAC module based on AK4490EQ.

I'd like to start with the AK4490EQ module:
Design goals:

- Stand-alone operation with I2S inputs fed from AK4137EQ/AK4115 or similar devices.

- On-board regulators. Willing to sacrifice a bit of board space for this. Allow pads for use of external regulator if I muck something up.

- All power supply and output stage buffering to be included on-board. Allow pads for use of external buffers if I muck something up.

- The rest keep as simple as possible. I think running in stereo mode is easiest. Could run 2 modules if MONO mode is desired.

- Keep board as small as possible but do not sacrifice design for this.

- IC cost is not a prime issue.

I'm a little fuzzy on I2S protocol workings so I will have to do some reading. As a starting point, I wish to use linear regulators which drop voltage from a reasonable power supply. Targeting toroidal type source but willing to entertain other options if there is a good case for lower measured noise from this source and/or the filtering demands do not get out of hand.

Lets start the discussion off with the AK4490 power supply and VRef circuits, AVDD, DVDD and VREF. Linear regulator choices......I've used "TPS7A4700" and "TPS7A3301" before and know they work. They are small and have a wide input voltage range. Separate regulators will be used for AVDD, DVDD and VREF. Other suggestions?

For VREFR/L is there any benefit to using a low noise op-amp vs the adjustable low noise linear regulators suggested above? The AK4490 evaluation design uses AD817 for these supplies. Perhaps absolute level regulation by feedback resistor tuning is a pain? Perhaps a voltage reference and low noise op-amp inverter will work just as well? I guess this has output voltage implications as too.

Lastly, I apologise if these concepts have been covered elsewhere. I've done a quick search but it is a large board. Links to the correct threads are welcome

Last edited by mikealanlee; 24th October 2014 at 09:05 PM.
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Old 27th October 2014, 11:34 PM   #2
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Ok Linear Regulator Circuit is straight forward. Attached. Decided to use 4 separate regulators for AVDD, DVDD, VREFR and VREFL as per AK4490 reference design. SMT parts and layout close will be close to TPS7A4700 reference board.

Any suggestions for VREFR/L supplies are welcome.

+VIN will come from a regulated supply as per Rod Elliot low power "super regulator" topology. Input supply around 7-10V filtered DC.

Suggestions for I2S header is also welcome. Probably go with 0.1" header unless there are valid technical reasons to change.
Attached Images
File Type: jpg PS_1.jpg (84.6 KB, 298 views)

Last edited by mikealanlee; 27th October 2014 at 11:37 PM.
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Old 28th October 2014, 08:03 AM   #3
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Hi, interesting thread, subbed!
You may consider using inductors in your project. I have found lately a great document about decoupling and design of PCB, PM me your mail and I'll send it to you! I'm not entirely sure if independent supplies for VREF are really needed, they might not be worth the money and work, probably, just feeding them from AVDD through pi filter is going to be enought.
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Old 28th October 2014, 06:44 PM   #4
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Thanks for the suggestions SomekPoland. Looks like the ADM7150 is pretty good for this application. Attached is the new PS schematic. Also it is starting to look like a passive filter between VDDR and VREF will do the job (also your suggestion).

So now thinking is separate Regulators for DVDD, AVDD, VDDR and VDDL. Passive filter from VDDR to VREFR, passive filter from VDDL to VREFL. Eval kit mentions a 2200uF cap between VREF pins can reduce low frequency distortion.
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Old 28th October 2014, 07:05 PM   #5
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I'm interested in a slight variation of your proposed project - one that has spdif input, ASRC, and spdif output. Is there any chance that you could add an (optional) DIT to provide the spdif out?
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Old 28th October 2014, 07:24 PM   #6
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Hi Charlie,

I hadn't planned on that route but the boards may be more flexible that way. I can see the application, good suggestion. I'll look into it.

Mike
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Old 28th October 2014, 07:27 PM   #7
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Awesome! I'm subscribed.
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Old 28th October 2014, 11:50 PM   #8
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Barely legible but below is first crack at AK4490EQ main workings (output filters excluded). Any comments regarding de-coupling and overall workings of the IC are welcome.

Regarding connectors, current thinking is ribbon cable for the digital lines. For PCM this should be ok, but does anyone have any experience running the DSD lines via ribbon cable? Anyone have alternative transmission/connector suggestions for the DSD high-speed clock/data lines?

Thanks,
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Old 29th October 2014, 01:02 AM   #9
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If you plan on "high" speed digital with DSD for instance ribbon cable is likely fine if you alternate ground and signal lines on the ribbon. For "low" speed digital I2S I can get away with only one ground per group.

The image you posted is barely legible on account of the resolution and the fact that the placement grid dots are showing. Can you remove the grid and upload at a little higher res?
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Old 29th October 2014, 10:57 AM   #10
marce is offline marce  United Kingdom
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Join Date: Jun 2007
Location: Blackburn, Lancs
With a 32 bit convertor I would think of your task as a high precision digital/analogue design.
IMHO you would get better signal integrity and avoid a lot of problems if you could have one board
instead of two connected by ribbon cable. All those headers and connectors are going to add impedance mismatches to some quite critical signals,
this could lead to SI problems especially with clocks. The bare minimum if you must use a connecting cable is
GND/SIG/GND/SIG etc, some newer 0.5 FFC/FPC type cables might be better.
4th picture down shows these for reference...
6moons audio reviews: Lindemann Audio musicbook:15 and musicbook:50


I would recommend a 4 layer board for this design, data sheet looks a bit confusing regarding the ground plane
I would use one contiguous ground with careful component placement then have some strategic slits near the convertor.



http://www.hottconsultants.com/pdf_f...ixedsignal.pdf

http://www.x2y.com/filters/TechDay09...%20_JohnWu.pdf

http://www.ti.com/lit/an/slyt499/slyt499.pdf

http://www.ti.com/lit/an/slyt512/slyt512.pdf

http://www.analog.com/static/importe...als/MT-031.pdf

http://www.ti.com/lit/an/slyt498/slyt498.pdf

The attached text file has a series of generic PCB design related links...
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File Type: txt PCB related links.txt (5.1 KB, 25 views)
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