I am looking for tips on how to layout the system grounds for the lowest noise floor. I am building a new DAC for personal use based on the PCM-1794A DAC's. I purchased some kits that use this dac but was dissapointed to see they used one large ground plane for the whole board, both digital and analog sides are all tied together. I am laying out a new PCB and was wondering if separate ground planes for the digital side and the analog side with a .01uf cap tied between them would yield better results. I have separate digital and analog power supplies.
It does not really matter whether you use separated ground planes for analog and digital areas on you pcb. The current always takes the way with the least impedance. Regarding digital (switching) supply and signal current, this way is the least inductive way which is the same way back as the supply or signal current, just on the ground plane. Hence, it's more a question about placing the parts and routing the wires.
Moreover I would recommend larger capacitance values (e.g. 100 nF and 1 uF, ceramic) and the use of ferrit beads to separate analog and digital supply voltages.
Taking cue's from what i have seen done in other DAC's I have separate power supplies for the Digital and analog sides. Each supply (Vdd, VccL, VccR & Vcc) has a massive amount of bypassing right at the dac chips .01, .1, 2uf, 47uf ceramic and tantalum caps each fed through a ferrite SMD chip.to try and create a wide bandwidth, low impedance supply.
I currently have a separate ground plane under the analog side and the digital side with separate power supply regulators for the Analog and +3.3v & +5v Digital supplies. The supplies are on a separate board with heavy gauge supply & ground wires from each supply to the DAC board. I thought to tie all the supplies to chassis ground at the power supply? maybe through 1ohm resistors to break up any ground loops?
I have .01uf ceramic caps very close to the DAC chips jumping between the digital to the analog ground planes. 3 total. Am i understanding correctly that i should change those caps to .1 and 1uf caps? or were you speaking of the supply bypassing caps?
Your help is greatly appreciated!
Oh, boy, ground plane split flame war about to commence!!!!
We have a joke around here, ask three engineers about their opinion on ground plane splits, and you'll get four different opinions! :)
Anyway, this part is pretty old, so we no longer stock EVMs for it, but I did find the attached users guide in our documentation system. Maybe it might be useful to you.
PS ok, ok, I'll admit I fall in the solid ground plane camp myself. Most days. :)
Audio Applications Engineering Manager
Dallas, TX USA
I honestly don't know which way is better or have any sort of opinion on it. Digital is new to me so that's why I am asking for help. As an audio amateur I have laid out plenty of analog audio boards But digital adds a whole lot of things i have never dealt with before into the mix and grounding is one area i have never been very strong with.
One product i looked at uses a solid ground plane. another has a quasi split plane. where there are large dividing areas that separate some of the analog sections from the digital sections even though the ground plane is tied together in several spots.I "thought" separate grounds would help keep the analog side clean? but again that is just my inexperience showing itself.
I guess i really need to find a digital guru i can send my Eagle files to and get feedback on the design as i work through it in sections.
if the 1794A is an old part now? maybe i should be looking at a newer part? Is there an upgrade so to speak? I picked the 1794A for the simplicity of running in hardware mode.
Yes, the PCM1794A is quite old, but it's still awesome!
If you don't need 132 dB of performance, you could look at our brand-new PCM5102, but it's "only" 112 dB.
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