The circuit has two ground references: analog and digital. Which ground
should be connected to ADG706, pin 12(GND)? I think to digital ground
reference, is it right?
I would recommend connecting the GND terminal to Analog Ground, so that the
analogue signal remains as free from noise as possible. In this case the
digital control signal is generated (by the uP, logic) relative to the DGND
plane, but the switch receives the voltage relative to AGND. Therefore the
digital control signal may appear noisier due to the noise voltage existing
between DGND and AGND. This is usually not serious, since the digital signal
has a large noise immunity. If there is more than 100mV of noise between the
AGND and DGND plane then you need to use some special techniques to pass the
signal from one domain to the other (e.g. differential logic drive). This would
be an extreme case however.
The analogue supply or supplies should be decoupled to AGND plane which is
connected to the ADG.