How to differentiate digital and analog ground?

If you are just starting in the great big world of electronics designs, you will probably be faced of how to manage grounds and how to differentiate digital and analog grounds to ace your electronics designs.

What the difference between analog and digital ground?

In an ideal world, there is no difference between analog and digital grounds.  In the real word it’s different, the noise is more important in the digital ground.
Why digital ground? Principally because in saturating logic, such as CMOS and TTL, they draw large and fast current spikes from its supply during switching. For exemple the sub or well are locally connected to the sources of P-N Mos and the mos are switching from rail to rail (Vcc-Gnd), this will create noise.
Usually, digital circuits  are more robust to noise, which have only two values 0/1 contrary to analog circuits that are working on specific working points depending on requirements.
Contrary to digital ground, analog ground will have slower Dv/Dt that will definitely create less noise.

Does it really matter if my signals are noisy? Of course it does! It matters when using analog signals where you don’t want erroneous voltage offsets and noise superimposed on the signals.  The classic example would be an audio signal where a few mV of noise makes the difference between acceptable and unacceptable.

 

So, how can you avoid noise? A good practice dictates that the layout is arranged such that ground currents from the digital circuits do not flow through the ground path for the analog circuits.  Both analog and digital ground should not be mixed up to avoid coupling from digital noise to analog.

Ground plane layout

The layout will be crucial to reach the best performances. The goal is to separate the analog and the digital grounds. The best way to do it consist of created two different ground planes (AGND and DGND) and tie them in a single-point ground (see the image below).


This method is the fundamental concept of a “star” or single-point ground system.
In practice, the current returns must consist of large area ground planes to obtain low impedance to high-frequency currents.

Decoupling capacitor in IC

Power supply pins should be decoupled directly to the ground plane using low-inductance, generally ceramic surface-mount capacitors or through-hole mounted ceramic capacitors (limit their lead length less than 1 mm to reduce impedance). The decoupling capacitors should be as close as possible to the IC power pins.

Why IC designers don’t include decoupling capacitors directly inside the chip?

Well, this is a good question, and the answer is quite simple. The capacitance of a capacitor is given by the equation:


Where A represents the surface of the electrode and d the distance between the plates. As you may notice, these days chips are  getting smaller and smaller so adding capacitors inside the chip would considerably increase the size of the package of chip.

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