New AK4396 DAC board design support needed

[rsdio]

I'm coming into this thread late, but it appears that you're using Eagle. If so, make sure you are using the ERC (electrical rule check) and DRC (design rule check) to check your schematic and layout for obvious errors.

I also wanted to suggest that you try out www.FreeDFM.com, who will test your PCB design for manufacturing and send a report on any errors found. This goes beyond the DRC in Eagle. 4PCB will also give you a quote, and might even phone you, but you are not obligated to buy your PCB from them just because you use their FreeDFM service.

[SunRa]

Thanks for that piece of info. I was already using ERC and DRC for debugging, although some rules in the DRC are not helping me very much. For example it was showing me that I don't have enough clearance between the pads of a ssop28 footprint...

I will try freeDFM, sounds like an interesting options. I wasn't aware of it.

The project took an interesting turn in the meantime. I have an almost complete DAC made on a breadboard with crude voltage regulators and grounding. I will use it for making sure I understand how the DAC works and what control pins I need.

After this the board will be reviewed and I will include some other features:

1. I did some research and it appears that the VREF input needs a very high quality source. I plan to feed it through a buffer around a low noise op-amp, a la Buffalo dac.

2. The same holds for VCOM pin. This one needs DVDD/2 so I need another voltage reg. and I plan to also feed it through an op-amp buffer.

3. I plan to add some slots for experimenting with other sources and regulators.

All in all this should be quite a reference board once finished!

[rsdio]

Originally Posted by SunRa

Thanks for that piece of info. I was already using ERC and DRC for debugging, although some rules in the DRC are not helping me very much. For example it was showing me that I don't have enough clearance between the pads of a ssop28 footprint...

That is not uncommon when using the default DRC. Keep in mind that DRC cannot be a universal test, like ERC. You must create a set of Design Rules that match your PCB fab house parameters. Some places, like Sunstone Circuits, will actually provide an Eagle .dru file that you can use.

Basically, when the SSOP28 footprint fails, it means that your Design Rules specify spacing that is actually larger. It's actually quite complex to translate PCB maker specs to .dru, but you can get boards to pass usually. Otherwise, you might have to modify the footprint to match the capabilities of your fab, or modify your .dru to truly reflect the capabilities of your fab.

I have several .dru files in my database, one for each product pricing level available from the PCB houses that I work with. Then, when working on a given circuit board, I can load the corresponding .dru according to the project budget, and run DRC to make sure that the board design will work. There are even reasons to change the .dru on the same board. For example, you might use a cheap PCB service for prototyping, and then change to a higher quality for production once you get the volume discount.

[5th element]

Originally Posted by SunRa

2. The same holds for VCOM pin. This one needs DVDD/2 so I need another voltage reg. and I plan to also feed it through an op-amp buffer.

Vcom is an output pin that you provide decoupling for.

[SunRa]

Thanks for the additional tips rsdio. Unfourtunately the fabs around don't seem too much into eagle, I'll ask for .dru files though. I've also used the one available from sparkfun.

5th element, yes, you are right, I mixed things up. I guess the best decoupling tehnique remains a good cap. Is there any "active" solution for decoupling?

By the way today I've started the ak4396 with exaU2I. It sounded great although the implementation is crude comparing to the one I am planning here. Still, it was a good idea to do this as the PCB will suffer plenty modification. By the way, not all of the control pins are having internal pull-up as I thought. Check below the schematic I arrived for starting this DAC. Maybe will be useful for others too.

I'll come back with the revised board also.

[rsdio]

Originally Posted by SunRa

Thanks for the additional tips rsdio. Unfourtunately the fabs around don't seem too much into eagle, I'll ask for .dru files though. I've also used the one available from sparkfun.

It's always worth it to ask, but I must admit that few fabs will provide the .dru files for you. You have to learn how to create them yourself.

It might be worth comparing the default Eagle DRU with the one from Sparkfun, just to learn how they are different.

I have also noticed that certain assembly shops want a CAD file, which is easily produced within Eagle using their GenCad.ulp file (from the CadSoft website).

Basically, the more you work with a certain fab and/or assembly house, the more you will learn about how to create the files they need. Sometimes you'll get lucky and there will be specific Eagle versions of the files available, but mostly you have to create them yourself.

[prairiemystic]

Originally Posted by 5th element

Vcom is an output pin that you provide decoupling for.

I would use an op-amp buffer to drive Vcom. It's a 1/2 VA divider (from internal 750-975 ohm resistors) and there would be sonic benefits. Otherwise, at least leave a pad or something to try this.

[rsdio]

Originally Posted by prairiemystic

I would use an op-amp buffer to drive Vcom. It's a 1/2 VA divider (from internal 750-975 ohm resistors) and there would be sonic benefits. Otherwise, at least leave a pad or something to try this.

The AK4396 defines VCOM as an Output only. Nowhere in the data sheet does it allow driving VCOM from an external source. Recommendation is to attach parallel capacitors for filtering, and there is also mention that a Time Constant is set by the RC network formed by the internal resistance and the external capacitance. It seems highly likely to me that driving this pin from an external op-amp, which is a very low impedance source, would defeat the intended function of that pin and might even cause damage.

From what source did you decide that using an op-amp buffer would be beneficial, much less even allowed in the first place?

[SunRa]

The functioning of that pin is not clear to me also...

I've seen one app for a ucontroller, or PIC, where the VCOM was also driven with a buffer instead being decoupled with a capacitor.

I can't find that page though..

[5th element]

In the evaluation module AK only decouple this pin. If the DAC required a voltage to be present on this pin I would expect it to cease operation should that voltage not be provided. Either that or there would be gross distortion generated as a result.