PCB for PCM2902 (digital out)

[paologatto]

For the USB B-type connector I can suggest the Digikey part WM17109-ND, that suits perfectly the corresponding "Berg" component in Eagle

And.. yes, I recommend too the 2 metal mounting lugs of the connector being soldered in the bottom part of the PCB (and connected to digital ground), otherwise the connector isn't stable at all.

Bye
Paolo

 

[guzzler]

dsavitsk; lists looks ok apart from the 560uF 10V. It's, unfortunately, too big to fit in the allocated space. I'll see if I can squeeze a little more space into there, I think there's room to move the logic and TTL conversion network back a few mill, hopefully get it up to 10mm diameter.

Nisbeth; not on yet; but I have a plan

What usually happens is the holes on the USB libary part cut out a big keep out zone, which prevents a pad being added. So; I'm going to edit the libary a little to add the pads in, should be straight forward, and yes, very useful!

g

 

[paologatto]

I haven't seen the schematic but.. is the 560 uF capacitor directly connected to +V from USB port? If yes, this could be a too high value for some USB cards or hubs, that see it as a short circuit when the device is connected.

Usually there is resettable fuse inside USB card and hubs and you will have to reboot the pc or disconnect and reconnect the USB hub to restore, so not a big problem. The solution could be a soft-start circuit or the use of a smaller value capacitor.

The capacitor value can be modified later anyway and an high value could work with a good USB card..

 

[guzzler]

No, it's through an inductor; so that should provide enough DC resistance to prevent a short circuit. A soft start circuit could be implemented; but it would be a bit of a cram. I'll run through some numbers tonight and see what I come with

g

 

[halman]

here is yet another pcm2902 spdif out layout.

 

[dsavitsk]

Quote:

Originally Posted by guzzler dsavitsk; lists looks ok apart from the 560uF 10V. It's, unfortunately, too big to fit in the allocated space. I'll see if I can squeeze a little more space into there, I think there's room to move the logic and TTL conversion network back a few mill, hopefully get it up to 10mm diameter.

Digikey lists P12705-ND which is Panasonic FC 560uF 6.3V and appears to be 8mil wide. Unfortunatly, they are out of them. Are there better options?

-d

 

[Glassman]

wow whwre did you get the library for the trafo? is that scientific conversion one? I've recently created my own library part for it.. very nice layout btw.

 

[guzzler]

Here's my latest, got the mounting holes sorted out, added more decoupling. I've put it just in board style so it's easier for people to see. If you'd like a 3D rendering, let me know and I'll put it up. Unfortunately, the 10mm capacitor isn't going to squeeze in there, just not possible without either overlapping the screw or enlarging the board. It's 3.5mm pitch, which means you essentially HAVE to use the 560uF I've specced at 6.3V. The alternative is 470uF or 390uF, both of which fit. To be honest though, by the time I can get these printed, they should be back in stock I should think. Anyway, enjoy...

Halman, nice alternative, especially PCM2902 on the solder side, but increases height and vulnerability IMO (I've got a couple of 0805 on solder side, optional though). Still not convinced by the transformer output. The way I see it, if people want to use a transformer, they can just run a couple of wires or to an optical chip or whatever... Think you forgot the the 1uF capacitors as well (4 in the top right), unless you're going surface mount ceramics instead of polyester..

g

 

[paologatto]

Very nice layout guzzler. Bravo!

The idea of an inductor before the big capacitor is good, I suppose it works well also for filtering the digital noise from the USB line.

 

[dsavitsk]

Quote:

Originally Posted by guzzler Unfortunately, the 10mm capacitor isn't going to squeeze in there, just not possible without either overlapping the screw or enlarging the board. It's 3.5mm pitch, which means you essentially HAVE to use the 560uF I've specced at 6.3V. The alternative is 470uF or 390uF, both of which fit. To be honest though, by the time I can get these printed, they should be back in stock I should think.

Expected ship date from DigiKey is March 31, so perhaps right on time.

 

[Nisbeth]

Gus, one more thing I think you should consider adding to the board, if you can fit it in, is an indicator LED for the USB power. There's no need to put the LED on board, because the LED will most likely be panelmounted anyway, but a single pad in the V+ line would actually be enough (a second pad in the ground plane would be good but not essential)

/U.

 

[dsavitsk]

Quote:

Originally Posted by guzzler means you essentially HAVE to use the 560uF I've specced at 6.3V. The alternative is 470uF or 390uF, both of which fit.

How about using a 820UF 6.3V? It appears that it will fit.

-d

 

[guzzler]

Yep, 820uF would fit too, but as paulo said it might be a little hard on the USB supply.

g

PS, not had time to look at putting a pad on for an LED, should be okay though

 

[00940]

A simple question.

- Why do you bother with an external regulator when it doesn't seem to affect purely digital performances ? TI only publishes a difference only for the ADC. (thd going from 0.1% to 0.01%). I understand from the function block that it could be useful for the DAC too but... you don't have pads to get the analog out, so I don't see the need to put it onboard.

 

[guzzler]

00940; you may have a point there. If you look at Figure 39 in the datasheet, they don't have a separate regulator. And from the block diagram it appears that the Vbus is regulated to 3.3V internally. I've not really had time to look in detail at the PCM2902 datasheet, just did a layout for it. Figure 39 certainly implies we could get rid of it. It would leave more space around the bottom left to reorientate the inductor and move the capacitor a little which would probably allow space to put an RCA connector directly onto the board. I'll look into it within the next week or so, got some project work to do but when that's out the way it should be okay!

g (idiot)