Thanks for the great advices! Most of them make a lot of sense to me.
- Check the ADC biasing scheme
- Consider driving the ADC differentially (via fully differential amplifier, something like TI THS4130) as this usually yields to best dynamic range
Do you mean the way reference is generated? I am using the internally generated Vref (1.25V).
I just read up on differential amplifiers, and they do seem to be more suitable. For the THS4130, the datasheet says it has both differential input and output. Does that mean for the output, I can just tie the negative to ground, and positive to input of the PGA?
- PGA will need the ground thermal pad vias, they are missing? Also, traces under the PGA create risk of short circuit for same reason, solder mask is not reliable insulator!
Hmm I totally ignored the thermal pads. Will look into that. I am not sure if I can completely eliminate traces under the PGA, but I'll see what I can do. Maybe I can make sure there is only one voltage, and make the thermal pad float (the datasheet says it's electrically insulated).
The FPGA also has a similar problem, but I can't think of a way to do the power distribution without using the space under the FPGA (2 voltages with pins on all 4 sides). Maybe I can move them to the solder layer, but I'm not sure how much that will help (since the vias still need to be there). Or maybe I can just push them away from the center to clear the thermal pad area.
- Fill the top side with copper too and place stitching vias between top and bottom grounds in something like 10x10 mm grid
- Remove the thermals on non-through-hole component vias, they just make the layout worse
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- Add more global decoupling capacitor everywhere on your VCC nets
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- Do not share ground vias, use one via per ground connection
That make sense. Will make those changes.
- Power distribution net seems somewhat flimsy considering what you are trying to do here
Can you please elaborate on that?
- Or even better, consider seriously using multilayer PCB for mixed signal project like this, SI and analog noise issues can drive you crazy otherwise
- If you can't afford that, then use as thin PCB substrate as you possibly can, 0.8 mm thick or even less.
Multilayer PCB would be great, but since this is my first board, I'm not sure if going multilayer would be a good idea. An oscilloscope is probably not a good idea for a first board, too, but...
![Tongue :P](https://www.eevblog.com/forum/Smileys/default/tongue.gif)
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Will use a thin PCB.
- Try to use more SMD components, especially in the input stage
Is there other advantages to SMD components beside board size? I used through hole components there because I thought I may have to change components there later (eg, using variable capacitor for better matching), and it's easier with through hole.
Oh and this is the first project I'm using SMD components, too
![Smiley :)](https://www.eevblog.com/forum/Smileys/default/smiley.gif)
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Thanks again for your time.
Will make changes and post updated board tonight.