circuit-designpower supply
Took apart an Apple TV due to no power from the smps and its like 80% covered in a white rubbery adhesive.
I'm used to seeing this stuff (or similar) on bigger components like electrolytic caps to minimize vibration but I'm curious it's purpose on the bottom of the board where it's all smd? Other than to make it really annoying to chip away to test components
Let me start by saying that I have not done a complete review of your design. Doing that properly would take hours, and I just don't have that kind of time at the moment. Given that, below are my comments in no particular order.
You need much more capacitance on the power input and the +/-9v outputs of your converter. I would recommend something in the range of 22-100 uF. What you have there is a switching regulator and you need some caps to keep it fed, and to smooth out the output ripple. I would also increase the size of the caps on the "output" side of the inductors to at least 10 uF. 22 uF would be better.
The REF5025 needs a 22 uF cap on the output, according to the TI datasheet. Additionally, if you intend this to be low noise then the chip should be located on the same PCB, and ideally right next to, the chip that needs the 2.5v.
The LP2985, the +5v regulator, needs (you guessed it) more caps. At least 10 uF, 22+ uF would be better.
The filtering on your preamp will filter the noise out quite well.
Unfortunately, it will also filter out your signal. It has a cutoff
frequency near or below 1 KHz. You probably want something else.
Here is the frequency response from my own simulations.
You should put your preamps on the same PCB as the ADC's, for lowest noise.
Consider removing the 22 ohm + 4.7 nF filter on the output of your preamps-- but look at the app notes for the ADC and follow their recommendation for filtering. Depending on the ADC, this could increase your total noise.
That's all I have right now. Basically: More caps and less PCB's.
I was having the same problem. I tried a few grounding strategies to no avail. I then looked at the application notes for the Texas Instruments TPA3116 amp and noticed that they recommend decoupling capacitors of at least 100uF near the power pins on both ends of the chip. These caps are absent on this board.
Looking around eBay, I noticed one that doesn't have the big caps on the TPA3116 either, but it has a 10uF cap on the power pin of the op amp. I had one lying around so I soldered it between the resistor it sits next to and the ground plane and now I consider the amp usable.
I used a 10uF 16V cap because that is what I had. The board on ebay uses a 50V cap. Because the op amp runs at 5V, that seems like overkill.
I will probably add the recommended caps on the TPA3116 to see if I can reduce the little remaining noise and maybe help the low end a bit.
For clarity, here is a pic of the cap I added:
The wires are my failed attempt to solve the problem by directly connecting the op amp and the bluetooth module grounds to the TPA3116 ground.
Others have noted the same issue, even when powered by batteries.
Best Answer
Its a type of RTV silicone adhesive that helps prevent component vibration, larger components like capacitors are normally secured by it as any excess movement in them can add to the chances of broken solder joints. I can only imagine its been added here to help secure the transformer etc but has been applied in a really shoddy way