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In many cases it is a good idea to keep these two grounds seperate or at least loosely tied together. Some applications can have them tied firmly without consequences. However sometimes the digital signals can be disturbed by a higher current analog signal or some analog can be influenced by a high frequency digital circuit. In most of my designes it is important to keep thes grounds seperate except for a single point of reference which I can control with either a single jumper or a low ohm resistor. It helps eliminate ground loops of current and gives you an oportunity to minimize ground noise between the two.
Logic gates that placed on digital ground generate unsufficient switched noise for analog part of design.That is why analog ground and digital ground usualy is separated. A. & D. grounds can be connected together 3 ways (trade-off): 1) before pad 2) 2 pad to one pin 3) 2 separated pins, pcb connection.
For IC design there are some issues. This issues is related with Substrate coupling. If u are interested i can explain or u can read about in "Design of Analog CMOS Integrated Circuits - RAZAVI" book part 18.3.
Sometimes it's also a good idea to have separate analog supplies along with a separate ground (+5Vdigital, gnd_digital and +5Vanalog and gnd_analog).
The common tie point can be by connecting the two powers and two grounds together with ferrite beads to help filter high frequency noise from the analog circuitry
Basically in a high spped design this issue comes up separating both analog and digital ground.To seperate the DGND and AGND we use a precesion INDUCTOR and in the CAD design we group all the ANALOG GROUND and we put a ISLAND opening to seperate the DGND.But finally we need to short both the GNDS.
digital signals are characterized by rail-to-rail voltages, whereas analog signals between them. so in circuits which have both digital and analog parts if the are sharing same ground then switching digital signals will inject noise. this might affect analog signals and hence malfunctioning of the circuit occurs.
Friend, a good book to this topic with clear explaination is "High-Speed Digital Design: A Handbook of Black Magic", Johnson and Graham. Generally, digital ground is the gound point for digital comppnents and analogue ground is the ground point for analogue comppnents. Isolation of these two ground points is very importance when design of mixed-signal system (eg system with digital circuit and analogue interfacing). The book has suggested some ways to make the grounding better from noise induction.
Hi
Analog circuits are senstive to supply bouncing and supply noise. While digital circuits are nooise because of the continous switiching nature of the digital circuits.
That's why designers always prefers to separate analog ground from digital ground to avoid coupling of noise coming from digital circuits to analouge ones
Basically in a high spped design this issue comes up separating both analog and digital ground.To seperate the DGND and AGND we use a precesion INDUCTOR and in the CAD design we group all the ANALOG GROUND and we put a ISLAND opening to seperate the DGND.But finally we need to short both the GNDS.
I theory ground should be the same, otherwise there wont be a reference point for the analog to digital conversion. Noise in the signal is anavoidable, it is inherent from the temperature in semiconductors. There is another source of noise which comes from the Vdc source though. it must be elliminated otherwise the readins will be wrong. some technique consist in placing a condenser as closer as possible to the ADC CI between +Vcc and GND terminals.
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