I don’t know what schematic YOU’RE looking at, but there’s only one ground on this one, that’s why the OP’s original question. There’s no separate grounds. There’s no caps “connected to the wrong ground”. There’s no “long field wiring”. You’ve answered some totally different question.The ferrite bead prevents RF from the digital circuitry from getting coupled to the analog side.
OP's SAMD21G18 will generate up to 48MHz hash+harmonics on DGND which you do not want on your AGND.
A ferrite bead joining two grounds (analog and digital) is common practice and discussed in Analog Devices app notes. Usually used above 20-bit resolution for ADC and DAC use.
If you don't believe in it, then just put a 0R in place. This will put you in the "ground everything together" camp which is another approach to grounding mixed-signal circuitry.
The need for it shows up with long field wiring to analog sensors and such, conducted emissions are terrible as the cables are essentially antennas radiating the MCU's emissions from DGND.
Also note the ferrite bead approach fails if your decoupling caps are connected the wrong ground, i.e. when the ADC is powered from DVDD so the cap can essentially bypass the ferrite bead and defeat its purpose.
The Adafruit board is not a good design.
It is filtering the supply going into the Adafruit board, the OP's schematic is wrong and he has interpreted the circuit diagram of theAdafruit board wrong, it is nothing to do with the on-board GND...
!!!
Both schematic as well as PCB layout is given in the eval board user guide.I cannot find the schematics/docs for the latest AD1955 EVAL board in the public domain
Any indications that a newer version than Rev.0 linked in my previous post exists at all?I cannot find the schematics/docs for the latest AD1955 EVAL board in the public domain.
With another DAC IC CS4398 I get no continuity, only a substrate diode between AGND and DGND.
So with these IC's grounding is done differently it seems.
The AGND and DGND pins should be tied together with solid ground plane fill underneath the converter extending out
to the GND side of the decoupling caps for VA, VD, VREF, and FILT+.
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