Analog Multiplexer. How to improve noise rejection
My project has two separate grounds: analog ground and digital ground (the separation is made with a ferrite bead).
I would like to know if there is any Application Note (i didn't find anyone) or document that explains which pin of the ADG709, among VSS and GND, must be connected to the digital gnd and which to the analog gnd to improve rejection to noise.
Clearly VDD and VSS are analog supplies and should be bypassed to AGND, respectively VSS connected to AGND in a single supply system. GND is the reference for the logic inputs and may be possibly connected to DGND, if you are sure to keep the VSS - GND maximum ratings. I would tend to connect GND to AGND as well.
In my opinion, separating AGNG and DGND is more a theoretical concept found in demonstration circuits and application notes than in real systems, that have more than one mixed signal device.
thank you FvM for your reply but now i'm more confused. Reading the datasheet at page 10 table 6 (ADG709 Pin Function Descriptions) they say:
pin 3 Vss is the most negative Power Supply Pin in Dual Supply applications. For single supply applications it should be tied to GND.
pin 14 Vdd Most positive Power Supply Pin
pin 15 Ground (0V) Reference
It seems to me that this mean that, if pin 15 Ground is the reference (reference for the analog signal i suppose), it should be tied to AGND not to the DGND! Where is my mistake?
The analog signal doesn't need a reference, it's for the logic input.
Ok thank you very much for the clarifications!