I have some problems with noise from the ADCs at my ADAU1701.
We started making products (e.g. active speakers) with this DSP several years ago, but we always had problems with input noise. With adjusting the input and output levels we managed to make this acceptable, but now we want wo make a new product line and of course we want to improve this.
Because we make most of our electronics in THT we designed a small PCB, containing most of the necessary peripherals. It is a 4-layer PCB and in the past we even used THT-parts on it, except the adau of course. Because I thought, the subotimal layout and dimensions of the parts would be responsible for the noise, I designed a pin-compatible full-SMD-version of it and tried to fulfil all the requirements concerning the layout. We got 4 samples of the PCB now and the noise is still the same.
Some things about my design:
For the input of the system we always use an additional buffer stage, shown below.
This one does definately not produce the noise I am talking about, If I just cut the connection between this and the input-ADCs there is no change.
The ADC-resistors are 18kOhms and they are all on the DSP-PCB. Here is the layout of the old and the new version:
And the inner layers:
Other things about the project:
- The DSP is running at 96kHz with a 12.288MHz crystal, PLL Mode
- We use the Input-ADCs as balanced input in this project and as stereo-input in others with the same noise problem
- I made a simle program with an adjustable volume block between inputs and output to verify that the noise is coming from the ADCs
What I already tried:
- replace the resistor in the PLL by a capacitor, like seen here in this board somewhere, no effect
- replace with other values and even disconnect the resistors on the ADCs, no effect or worse noise
- make a completely new layout (see above)
- put an inductor in the 3v3-supply, right in front of the DSP-PCB, no effect
- Add a small offset to the ADCs, like seen here in this board somewhere, no effect
- mute or power down the ADCs, -> silence, of course
Now, I do not have any ideas anymore, what I can try. From the evaluation boards (we have the old, large one and the mini-version) I know that the noise does not need to be this much.
If anyone has some ideas about that, it would be very helpful. We want to start the production of the new product line very soon and I want to fix this issue.
Thanks in advance,