Hi!
I'm making myself a DAC, just for learning purposes (and have some fun) based on the PCM2912A. While checking the datasheet for the evaluation module I noticed that the ground traces, found here in blue, are much wider, in some sections, than what they need to be. What's the purpose of that? Having a pseudo ground plane? And why the copper ground pour under the IC?
Thanks in advance!
PS: I'm not sure if this is the correct section for the post.
I'm making myself a DAC, just for learning purposes (and have some fun) based on the PCM2912A. While checking the datasheet for the evaluation module I noticed that the ground traces, found here in blue, are much wider, in some sections, than what they need to be. What's the purpose of that? Having a pseudo ground plane? And why the copper ground pour under the IC?
Thanks in advance!
PS: I'm not sure if this is the correct section for the post.
The trace inductance is proportional to the ratio of length/width. The lower, the better.
Under the IC is a low inductance system common.
A proper ground plane with careful parts placement should be much better than star grounding.
Use smt decoupling with short connections to keep the parasitics low.
But I'll have to use vias to connect to the ground plane. Is it still preferable?
Make 2 separate pours for AGND and DGND and connect them together somewhere at the power input's capacitors .
To me it's an open question whether or not it's a good idea to split ground planes at all; two former colleagues of mine were very much in favour of having one and only one ground plane because they never managed to get split-ground-plane PCBs to work properly.
If you do split them, connecting them at the DAC seems to be the most logical solution. It results in the smallest detour for the return currents of signals that cross the boundary between digital and analogue.
If you do split them, connecting them at the DAC seems to be the most logical solution. It results in the smallest detour for the return currents of signals that cross the boundary between digital and analogue.
Agreed, one ground plane can be fine. However, its still probably a good idea to think about where ground currents are flowing. For example, if most of the digital circuitry is in one area of the circuit board, the ground return connection to its power supply probably should be located around there somewhere and not necessarily next to the analog ground return connection at the other end of the circuit board.
This is the must for the RF circuits only
[Markw4=rayma;]Agreed, one ground plane can be fine.[/Markw4]
There are may be occur additional interferences from DGND to AGND in this case
Why it (the low GND impedance) is a must for the RF circuits only? We are talking here about 22.xxxx - 49.xxxx MHz frequencies or even higher for ESS chips.
That is why the star ground under the DAC chip is the best solution. The different currents will flow on the shortest way and the interferences between the different currents will be extremely low.
The star ground "ideea"should also be respected even there is a dedicated ground plane on a 4 layers PCB project.
I have got very good results with 2 layers PCB also, but I had to use 2 ground planes carefully inter-connected, one on each PCB side and a mirror placement of the DAC chips (it is a dual mono DAC project).
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