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Old 28th July 2013, 08:46 PM   #1
Dr_EM is offline Dr_EM  United Kingdom
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Default CS3318 PCB Layout

Hi all, I've been hearing about the CS3318 a fair bit recently, and how it's supposed to exhibit lower noise than the PGA2310 in use. Well, I've found the PGA2310 does have some noise and I'm not sure why that should be, but I may well have an application for a 6-8 channel volume coming up and am learning PCB layout, which the CS3318 certainly requires!

So, here's my attempt, it's my first real design in Eagle other than a practise board so it's probably far from optimal. The IC package is from another device in the database and wired up according to the datasheet. Voltage regulators are adjustable since a -9V fixed couldn't be found, they should produce a little under +/-9V.

A couple of things:
  • The coupling capacitors are perhaps a bit small but I wanted film and not to make the board too massive, they can by bypassed with wire links if preferred but maybe a double-hole mounting which could accept electros would be useful?
  • Will the ground fill under the IC suck too much heat while soldering? Perhaps it should be more isolated but I'd need to draw a seperate polygon for this I think, and if I did that, perhaps a hatched fill would make sense.

Otherwise, it's pretty much a standard application with on-board regulators. Any input appreciated!
Attached Images
File Type: png CS3318 Board Composite.png (100.8 KB, 658 views)
File Type: png CS3318 Board Top Layer.png (77.3 KB, 646 views)
File Type: png CS3318 Board Bottom layer.png (71.6 KB, 638 views)
File Type: png CS3318 Schematic.png (28.4 KB, 648 views)

Last edited by Dr_EM; 28th July 2013 at 08:49 PM.
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Old 28th July 2013, 09:00 PM   #2
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Hi some tips:

- Please use 10 F bypass caps on R8 and R10. Explanation would be an insult
- Use a better 3.3 V regulator. Won't cost more than the current one.
- Place the 100 nF decoupling caps much closer to the chips pins.
- maybe the input pins should be referenced to GND by means of 100 kOhm resistors (so before the caps) ? It will avoid clicks/plops when plugging cables in and out ...
- don't you need the pull up resistors on pin 5 and 6 (check data sheet) ? Adding the pads won't cost money and maybe you will need them in your application later on.
- I would use standard electrolytic caps or SMD tantalums for C11, C14 and C16. I am not so sure if LM317/LM337 like MLCC ceramic caps with very low ESR at their outputs.
- what about output caps ? The CS3318 has max. 5 mV offset so maybe I am overcautious. I would definitely use 100 Ohm stopper resistors on each output
- mmmm, just googled this and I think some of my comments are justified : http://www.cirrus.com/jp/pubs/rdData...DB3318_DB2.pdf

Keep the film caps, you won't regret it. Electrolytic caps won't bring better performance, on the contrary.
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Last edited by jean-paul; 28th July 2013 at 09:17 PM.
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Old 29th July 2013, 04:30 PM   #3
Dr_EM is offline Dr_EM  United Kingdom
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Great suggestions, thanks!

I've implemented most of them. To be specific, I plan that this might get used in a multi-channel audio interface to create a system which can use PC based DSP crossovers. The signal chain would be:

USB-I2S - Multi-channel DAC - CS3318 Volume - Line drivers

With a microcntroller board too naturally. For this reason, the the input resistors and output coupling capacitors can be unused since the line driver would remove DC offset and the DAC will be permanently linked. Power on/off muting would occur on the line driver board, as far upstream as possible. The 100 Ohm output stoppers are a good call regardless of whether you directly drive a coax or not so I included them.

I'm not sure if the new 3.3V regulator represents much of an upgrade but it should do the job of providing the 1mA or so the logic interface needs. The 100uF electro easily satisfies the ESR requirements and at 10V rating is quite compact. I had planned to use SPI interfacing but you're absolutely right, why not include pads for the 2k pull-ups in case, so they're added.

I moved onto the PCB design quite quickly as this was mostly an exercise in learning PCB design in Eagle and so I missed out tricks like the feedback filter for the LM317/337 but they've now been added. I managed to add these without increasing board area at all, though the silk-screen might benefit from some adjustment. +/-9V Regulator output caps are now just 10uF electros.

I would hope the CS3318 100nF bypass are all close enough now, I've certainly done far worse and never had stability issues with op-amps.

Thanks again and any further comments are welcome!
Attached Images
File Type: png CS3318 Rev1 Composite.png (118.6 KB, 614 views)
File Type: png CS3318 Rev1 Top.png (92.6 KB, 117 views)
File Type: png CS3318 Rev1 Bottom.png (86.4 KB, 86 views)
File Type: png CS3318 Rev1 Schematic.png (32.2 KB, 145 views)

Last edited by Dr_EM; 29th July 2013 at 04:33 PM.
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Old 29th July 2013, 08:15 PM   #4
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Much better but at least one of the 100 nF can be closer.

Quote:
Originally Posted by Dr_EM View Post
I would hope the CS3318 100nF bypass are all close enough now, I've certainly done far worse and never had stability issues with op-amps.
Getting away with it = not going for optimal performance.... Better do it right at once and chances are likely that you will indeed never have stability issues

If you add GND reference resistors and a control board I would not be surprised if some members will call for a Group Buy ....
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Last edited by jean-paul; 29th July 2013 at 08:17 PM.
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Old 30th July 2013, 06:16 PM   #5
Nisbeth is offline Nisbeth  Denmark
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Looks very good. Might I suggest that you move the screw holes closer to the sides of the board - especially the top ones look like they might be difficult to access if the caps are populated.

/U.
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Old 30th July 2013, 07:48 PM   #6
Dr_EM is offline Dr_EM  United Kingdom
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You're right, it's a high performance chip and if I'm going to the trouble/expense to get boards made I may as well get it as refined as possible at the design stage. I will move those screw holes outward too Nisbeth, thanks! Easy to forget how tiny this is really!

I'm up for producing a group buy though I will need a completed CS3318 board in my hand to test the software with, couldn't be confident of that working without it, but that's not to say a control board couldn't be laid out prior to completing the software. A PIC18F series will most likely be employed and support for rotary encoder, remote control and LCD is planned.

I'm unsure what you mean by ground reference resistors? Are these the ones for referencing the CS3318 internal gain stages, in which case the design would need substantial alterations.
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Old 31st July 2013, 05:58 AM   #7
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Just resistors from each input to GND (before the caps) to avoid plops when plugging in devices. You don't need them but members of a GB probably will. Maybe it a good idea to add pads for healthy input cabling instead of ribbon cable with all its crosstalk problems. The reference design is a very good starting point. Cirrus used PCB mount RCA connectors which are a relief considering wiring. I would keep tedious wiring to an absolute minimum as it is prone to problems, looks terrible and it costs money and a lot of time especially when custom cables have to made. Optimally you could throw the board in a case, just connect relatively simple power supply and control wiring and you're ready to use it. Adding RCA connectors will be a large benefit afterwards ! They are omitted often because of their initial cost but then one tends to forget cabling costs time and money too. Just imagine your self wiring the future PCB in a case and now think again: why do I make it like this ?

Same could count for output caps. Adding the pads + GND reference resistors there too won't cost much and one can DC couple amps with this board. On the other hand: 90 % of amps already have an input caps so you could skip these. It would make the board more versatile but also larger. Drawback is that one needs to use jumpers when the caps are not needed.

You could also think about adding rectifiers and filter caps so that the board will be self powered. And while you are at it you could add the control PCB at the same time This non modular approach will be a hit in 2014 ! It is called "Complete Designing". Please join it as you can be a CDer too Our friend Rudi, an avant-garde CDer, was converted long ago:

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Quote:
Originally Posted by Nisbeth View Post
Looks very good. Might I suggest that you move the screw holes closer to the sides of the board - especially the top ones look like they might be difficult to access if the caps are populated.

/U.

As a mounting hole fundamentalist I can only agree in astonishment (that I hadn't noticed it)
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Last edited by jean-paul; 31st July 2013 at 06:25 AM.
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Old 31st July 2013, 06:22 AM   #8
ds23man is offline ds23man  Netherlands
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I have been playing with the CS3318 and had some issues with it:

1. Do NOT let it run without the digital powersupply, the analog circuits start to oscillate and burn..........
2. The mute input must be terminated to ground or VD, otherwise crosstalk from the control lines will cause a severe distortion in the analog outputs.
3. Analog powersupply decoupling as mentioned in the datasheet is done wrong, should be caps from rail to ground... and not between rails.
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Old 31st July 2013, 06:32 AM   #9
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Quote:
Originally Posted by ds23man View Post
3. Analog powersupply decoupling as mentioned in the datasheet is done wrong, should be caps from rail to ground... and not between rails.
On the contrary: this is correct. Cirrus decouples rails and between V+ and V- which is both old fashioned and good but you don't see it much in reality as it is a forgotten technique it seems. Data sheet also mentions decoupling from Va+ to Va- in text. Please check their reference board where they did it like that on a real PCB. Their engineers would not make the same "mistake" twice I hope:

http://www.cirrus.com/jp/pubs/rdData...DB3318_DB2.pdf

People using OPA627 and OPA637 know the caps from + to -. It is the only way to keep those opamps doing what they have to do optimally...
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Last edited by jean-paul; 31st July 2013 at 06:40 AM.
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Old 31st July 2013, 06:36 AM   #10
ds23man is offline ds23man  Netherlands
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Small addition:

The reset pin is also linked to the power up and down of the analog circuits.

The chip must be kept reset if the digital supply is low and the analog supply is present. This can be done with a small one dollar watchdog chip.
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