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WM8816 Volume/Balance Controller: Request for comments
WM8816 Volume/Balance Controller: Request for comments
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Old 11th October 2019, 10:18 AM   #1
zgtc is offline zgtc  Europe
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WM8816 Volume/Balance Controller: Request for comments
Default WM8816 Volume/Balance Controller: Request for comments

Hi all,

I am designing a Volume/Balance controller based on the WM8816 chip. I know there are millions of ready to use designs, with this or similar chips, in this forum (ie Balanced Volume Controller / Line Stage, My new preamp design: Arduino, 6 input selector, MDAC attenuator, IR etc or Building a complete Preamp with an Arduino, remote, volume and input control). But I'm aiming at the simplest possible implementation based on the Typical Application (Figure 5) in the WM8816 datasheet, because I just need SE-SE.

I'm looking for comments and ideas to improve both the design and my (lack of) EasyEDA skills, since this is my first ever PCB "design". So please bear with me.

Digital Volume Balance WM8816 v0.1.png

Thank you so much
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Old 11th October 2019, 10:24 AM   #2
zgtc is offline zgtc  Europe
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WM8816 Volume/Balance Controller: Request for comments
Attaching a new image since from the previous rendering it's not possible to see all the wirings, so you don't have to open the gerbers

Digital Volume Balance WM8816 v0.1 wiring.png
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Old 12th October 2019, 02:16 AM   #3
Mark Tillotson is offline Mark Tillotson
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Using wide short traces from power and ground pins to ceramic decoupling caps is customary to provide less stray inductance on those routes.


Ground-planes are normally used, separate ones for digital and analog joined at single star-grounding point.
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Old 12th October 2019, 11:43 AM   #4
zgtc is offline zgtc  Europe
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WM8816 Volume/Balance Controller: Request for comments
Quote:
Originally Posted by Mark Tillotson View Post
Using wide short traces from power and ground pins to ceramic decoupling caps is customary to provide less stray inductance on those routes.


Ground-planes are normally used, separate ones for digital and analog joined at single star-grounding point.
Hi

I thought about it yesterday and even tried to add two ground planes, but being my first PCB attempt I couldn’t find the space. Maybe if I put the WM chip on the bottom side I can probably reposition the caps closer to the chip pins. Will try later this weekend.

Thank you!
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Old 12th October 2019, 09:25 PM   #5
zgtc is offline zgtc  Europe
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WM8816 Volume/Balance Controller: Request for comments
OK, tried my best.

I moved the WM8816 chip to the bottom layer and repositioned all elements. I think caps are much closer to the pins now.

Also created two ground planes, but I was not able to join DGND and AGND planes, auto-router was destroying the join trace all the times. So I created a 0R resistor to function as a GSTAR. It's possibly the worst of the idea

Here's the updated files:

Photo view:
DIGITAL VOLUME_BALANCE v0.2.png

Wiring top view:
DIGITAL VOLUME_BALANCE v0.2 wiring.png

Wiring bottom view:
DIGITAL VOLUME_BALANCE v0.2 wiring bottom.png

Thank you again for the help.
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Old 15th October 2019, 10:36 AM   #6
zgtc is offline zgtc  Europe
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WM8816 Volume/Balance Controller: Request for comments
Is using a 0R resistor (or short piece of wire) to join digital and analog ground planes a bad thing?
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Old 15th October 2019, 01:16 PM   #7
BrianGasberg is offline BrianGasberg  Denmark
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Quote:
Originally Posted by zgtc View Post
Is using a 0R resistor (or short piece of wire) to join digital and analog ground planes a bad thing?

Using a 0R resistor will work, otherwise try to look into using a "Net tie" instead.
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Old 15th October 2019, 01:52 PM   #8
sgrossklass is offline sgrossklass  Germany
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I am more concerned by how you split the groundplane tbh. This makes no sense whatsoever as shown. You want the analog groundplane under the WM8816 analog section, too. I think you'd best rotate the chip by 180°, so that you get a chance of giving the digital stuff a little corner of its own.

Also, the connection from LMO/RMO to the opamp's inverting inputs should be as short as possible to avoid undue capacitive coupling; perhaps even consider sparing out the groundplane underneath.

Splitting the groundplane under the chip used to be a common recommendation for mixed-signal ICs (like DACs) in the 2000s. Eventually people found that they could actually get better performance out of a non-split groundplane after all, but for something like a WM8816 this should work perfectly fine.

If you have a spot for a 0R resistor already, I would reserve another for a ceramic capacitor next to it.
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Old 15th October 2019, 02:28 PM   #9
Mark Tillotson is offline Mark Tillotson
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You only need to discriminate in the layout between analog and digital ground, in the schematic they can be the same signal - then all you have to do is provide a narrow neck in the ground-plane as the join point.


Remember the whole point is to keep digital current flows from going near any analog routing. Digital currents are very noisy even if a few mA because they slew in a few nanoseconds.
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Old 15th October 2019, 03:34 PM   #10
zgtc is offline zgtc  Europe
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WM8816 Volume/Balance Controller: Request for comments
Hi everyone, and thank you again for hand-holding me through the learning process.

Quote:
Originally Posted by BrianGasberg View Post
Using a 0R resistor will work, otherwise try to look into using a "Net tie" instead.
Quote:
Originally Posted by Mark Tillotson View Post
You only need to discriminate in the layout between analog and digital ground, in the schematic they can be the same signal - then all you have to do is provide a narrow neck in the ground-plane as the join point.
Quote:
Originally Posted by sgrossklass View Post
I am more concerned by how you split the groundplane tbh. This makes no sense whatsoever as shown. You want the analog groundplane under the WM8816 analog section, too. I think you'd best rotate the chip by 180°, so that you get a chance of giving the digital stuff a little corner of its own.
OK, I drew what I understand from your words in the attached image.

The WM8816 chip was already in the bottom layer in previous design, because I think it makes positioning the caps a bit easier, so it's actually rotated to make the analog side closer to the opamp. I just drew the ground layer with the small join just in the middle of the chip, but the chip is on the other side, so should I create another ground layer in the bottom side?

Quote:
Originally Posted by sgrossklass View Post
Splitting the groundplane under the chip used to be a common recommendation for mixed-signal ICs (like DACs) in the 2000s. Eventually people found that they could actually get better performance out of a non-split groundplane after all, but for something like a WM8816 this should work perfectly fine.
So the alternative would be to have a digital ground in bottom layer (chip layer) and the analog ground in the top layer (opamp)?


Quote:
Originally Posted by Mark Tillotson View Post
Remember the whole point is to keep digital current flows from going near any analog routing. Digital currents are very noisy even if a few mA because they slew in a few nanoseconds.
Noted, thank you

Quote:
Originally Posted by sgrossklass View Post
Also, the connection from LMO/RMO to the opamp's inverting inputs should be as short as possible to avoid undue capacitive coupling; perhaps even consider sparing out the groundplane underneath.
I will make sure to position the opamp closer to the chip, moving the data header to the other side.

Quote:
Originally Posted by sgrossklass View Post
If you have a spot for a 0R resistor already, I would reserve another for a ceramic capacitor next to it.
Do you mean the join of the planes should also have a capacitor?

Thank you again, sorry for the stupid questions
Attached Images
File Type: jpg wm8816-opamp-planes.jpg (52.5 KB, 105 views)
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