Q
Where should I tie GND connection, to digital or analogue ground?
A
As long as there is some interconnection somewhere in the system between the
Analog GND and Digital GND, the bipolar switches can function with VL connected
to +5V digital and GND connected to digital ground.
Usually considerations about DGND or AGND relate to the additional noise that
will be added to the analog signal. In the case of an analogue switch there are
really only 2 ways that noise can be added to the signal
1- Changes to the value of switch Ron
2- Capacitive coupling of high frequency signals onto the signal path
1. Ron changes according to the analog power supplies of the switch and their
relation to the signal voltage (Ron is higher as the signal gets closer to the
supplies). The digital control signals are saturated inputs which are then
level shifted inside the switch to control the switching FETs. Therefore the
Ron is not strongly dependent on the precise value of the digital control
voltage, or the values of VL and Ground.
2. If the digital ground and digital power supply are extremely noisy then in
any case you should keep them far away from the sensitive analog signals, and
this also means you should not connect them to the analog switch.
