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Old 16th March 2013, 01:43 PM   #2131
cviller is offline cviller  Denmark
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I am currently working on a AB based on pcm1792 (slow progress). The plan is to expose the current out, so that I can do the I/V conversion on a separate board (I'll be using Pass Labs D1 stage and the newer JFET version).

I really like the clock setup on the original AB-1.2, so I have basically copied the whole thing.

However, I have a couple of things around the SN74HC163D counters which I am bit unsure about.
1. It halves the clock frequency from the crystals, right? Does it provide any other benefits, or could it be omitted (adjusting the circuit to either the high freq or find another crystal)?
2. Why is the VDD_+3V3 used, wouldn't it be better to use the regulated VDD_XO1 or 2?
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File Type: png ab_cv_clock.png (131.1 KB, 197 views)
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Old 16th March 2013, 01:56 PM   #2132
borges is offline borges  Norway
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Hi,

The clock dividers must divide MCLK down to 16MHz or less for the AVR32 serial interface. It is clocked from the digital domain because it only feeds signals to the digital module. So no power and minimal ground paths are shared with analog 3.3V.

A while ago we briefly considered synchronous charge pumps or switch mode power. That's not used, though. One chip is needed, the other is a nice-to-have.

I know you asked me about isolators. For now I suggest you add that to your board. I have another DAC design with module, isolators, FPGA and PCM1704. MCLK is sent to the module and I2S from it in a 3+1 device. Remind me if I haven't sent you the sch by tonight.

BÝrge

Last edited by borges; 16th March 2013 at 01:58 PM.
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Old 16th March 2013, 04:22 PM   #2133
cviller is offline cviller  Denmark
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Sorry, I just realised I have mis-read the schematics. So I understand - clever stuff.

Any thoughts on the best chip for isolation?
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Old 16th March 2013, 05:05 PM   #2134
borges is offline borges  Norway
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I'm glad you like the idea. I believe it was right to place the dividers in the digital domain. If you X-ray an AB-1.2 you'll see that the routing around the dividers also split the ground plane.

I have attached a schematic of the way I use dividers in my CD player. (The other two pages are just boring special connectors, so I omitted them.)

It uses the SiLabs SI8641EC-B-IS1 is nice because it defaults to '1' (i.e. UART TX or SPI CSN are not activated.) It fits in a narrow SOIC16, and the price isn't off the planet. Other than that I have no reason to choose Si's technology over any other isolator.

This board plays music and seems to work very well. MCLK is passed to the module, and I2S is passed from it through Iso3. Iso1 isn't 100% done yet. In this prototype it transfers the MCLK selector to the clock in the analog domain). For the future I might add UART for HID commands and ignore Resete/Prog.

The RC filters which work in paralell with S1 is something I'm not using. It was a way to experiment with the module setting its own Prog and Reset through GPIO pins.

The way I'm using it not the module is Host powered. The optocoupler would add a device-controlled power gate. The isolators would consume power from USB, so I'm using the coupler to save current.

BÝrge
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Old 16th March 2013, 09:35 PM   #2135
cviller is offline cviller  Denmark
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Thanks for the schematics and suggestions!

Now you mention x-ray, it sounds like you are using 4 layer pcb. I was actually planning on using 2 layer with gnd on component side and vdd on solder side (where possible). Not perfect for decoupling but so much cheaper...
Is it worth it to upgrade to 4 layer to get better decoupling and easier routing?
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Old 17th March 2013, 09:29 AM   #2136
borges is offline borges  Norway
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Artists use 2 layers :-)

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

BÝrge
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Old 17th March 2013, 01:44 PM   #2137
cviller is offline cviller  Denmark
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Haha, then I'll continue to see if I can create art - hopefully with the side effect of being functionally usable.

Thanks for the the link - highly interesting. Since university, I have forgotten more about this topic than I dare to admit.
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Old 18th March 2013, 09:36 PM   #2138
borges is offline borges  Norway
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I draw circuit boards all day for a living these days. A massively tight 8-layer board is quite a challenge, but so is definitely keeping track of your return currents in a 2-L audio setup, too.

BÝrge
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Old 19th March 2013, 01:38 AM   #2139
davada is offline davada  Canada
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I was going to post pictures of the boards I just got back from China.
But now I'm scared to knowing there is pro around.

I often point people to Henry Ott's web page.

Cheers,
__________________
David.
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Old 19th March 2013, 02:19 AM   #2140
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Quote:
Originally Posted by davada View Post
I was going to post pictures of the boards I just got back from China.
But now I'm scared to knowing there is pro around.

I often point people to Henry Ott's web page.

Cheers,
Don't be shy mate, some may comment and give tips or see things that help you learn


Chris
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