PCB Design Feedback?

[elliot42]

Would anyone like to give me some feedback for my PCB layout?
I'm going to build a bunch of the synthesizer modules from http://www.electricdruid.net/ and instead of doing a PCB for each different circuit I thought I'd design PCBs for the common modules in the circuits, them being:

• PIC "brain"
• Bessel filter
• Offset cancellation and gain (5Vpp around 2.5V -> 10Vpp around 0V)
• Potentiometer CV input

I've done the layout for a Bessel filter that uses a TL072 and can be configured to be a low-, high-, or band-pass filter. I've used resistors in the schematic for all the filter components, but some will be capacitors depending on the configuration.

Some questions, statements:

• I realise the "+/G" above the output should be reversed (I just noticed).
• What would be a better reference letter for the resistor/capacitor parts?
• General layout feedback?

[amspire]

A great idea. I would love to have a pile of boards on hand to make an instant filter. The thing I would suggest it to make filters with Hi-Q stages, you need accurate and stable capacitors - like polystyrene and silver mica caps. These can often require larger packages. I would try and add extra holes as much as possible so different sized components can be used.

[elliot42]

That's a good idea, thanks.
I'll add in options for 5 or 7.5mm lead spacing for the components that can be either resistors or capacitors. 5mm package width seems reasonable too, except for Wima caps, but they're pretty wide compared to other suppliers.
That should cover poly cap options up to about 63V 1uF from Multicomp, Epcos and Panasonic (only up to .56uF from Panasonic, 1uF is 6.9mm wide).

Would caps that large or larger normally be required for this type of filter? The values used in the schematics I've looked at so far range from a few pico farads to 10nF, which would all be available in packages with 5mm lead spacing. I don't know that much about filter design.

I also haven't looked at mica caps, but a quick search on e14 shows a range of strange lead spacings...

[elliot42]

I guess I should include 2.5mm spacing for ceramic capacitors too.

Odd spacing for Micas: http://www.cde.com/products/mica-capacitors/
Are they normally just bent to fit spaces that are multiples of 2.5mm/100mil?

[elliot42]

In response to some of my own ramblings...

I've calculated a few filter designs using TI Filter Pro and can't see that I'd realistically need cap values greater than .5uF, and values that size and smaller in polyester and from different suppliers will fit within a 5mm wide footprint with 5mm lead spacing. 7.5mm lead spacing increased the size of the board too much for my liking, I'm trying to keep it compact and low-cost.

Small value mica caps should fit OK too, leads bent slightly to fit in the 2.5mm or 5mm lead spacing. Larger values seem to make the package of mica caps very big very quickly

I've renamed the board to what it really is, a Sallen-Key Filter, that can be configured as low-, high-, or band-pass with different filter responses including Bessel, Butterworth and Chebyshev.

Attached are some updated layouts. Can anyone give me some feedback, negative or positive about the layout, if I could be doing something better, etc.? I haven't done much PCB layout before.

[sacherjj]

It seems like you are breaking up a lot of your ground plane for no real reason.  The + and - power lines do not need to be on the bottom.  There is no trace running past them on the top.  This would eliminate large sections in your ground plane.  It probably won't really matter on an audio project (although it can), but it is bad practice to have if you do it with higher speed stuff.

[elliot42]

Thanks for the tips sacherjj. I've got an updated version as well as a few more boards layed out already, I've just been a bit slack getting it posted up here.

[elliot42]

I updated the filter board a bit; moved the power traces, shifted the IC and rotated C1 and C2 so they're not so squished.
Also attached is a layout for a level shifter and a tiny board for a potentiometer CV (control voltage) input. The level shifter schematic is from this: http://www.musicfromouterspace.com/analogsynth_new/HOT_TIPS/PDF/DCCoupledLevelShifter_assembly.pdf

I was also considering increasing the dimensions of the two larger boards to the same size so I can line them up neatly or even stack them together. Maybe 2x1.4 inches, even though that will increase the price a little, it might be useful later on.