Hi Avishai,
[quote user="Avishai Henley"]1. What is the coupling the chip between PD, AGNDF/S and GND? Are they electrically isolated? Are they shorted? Is there resistance/capacitance between them? Can you send me the equivalent circuit between the 4 nodes (PD, AGNDF/S and GND)?[/quote]
The AGNDF/S pins are shorted internally. The thermal pad is connected to the die substrate through a thermally conductive material that is not intended to be electrically conductive. There is some small conductive property of the thermal material, though, so it is often intentionally connected to a net – typically the negative most potential of the die. In this case is AGND is connected to the thermal pad via a high impedance (megaohms). DGND and AGND are internally connected by back-to-back diodes to ensure that they are kept within 0.3V of one another. The leakage current is minimal.
[quote user="Avishai Henley"]2. I seem to have made a mistake and connect P3V3_D (3.3V, digital voltage) to the VDD input pin - instead of analog voltage. What is the coupling between VDD, and the other 4 ground nodes (PD, AGNDF/S and GND)?[/quote]
The coupling of any digital noise associated with the supply provided for the VDD pin is likely dominated by other design related factors such as bulk and local decoupling capacitors or other PCB parasitics.
[quote user="Avishai Henley"]Please note that the input voltages into the VCA (that is controlled by the output voltage of this DAC) is in the range of 50uV. Noisy ground and supply of the VCA damage the input signal. I am afraid that the DAC causes leakage from the digital domain to the analog domain.[/quote]
The datasheet specifies a parameter called digital feed-through that describes how much of the digital signals can couple into the analog output. The digital-feed-through of this device is specified 0.2 nV-s.