Chip designers often internally partition the ground-reference net (or substrate) for an ADC into isolated analog and digital regions. The sensitive analog circuits and analog reference generators connect to the analog ground region. The chip's high-powered I/O drivers connect to the internal digital ground region. Physically distinct AGND and DGND pins (or balls) then individually connect the internal analog and digital regions, respectively, to the outside world. The analog section also gets its own, separately filtered power-input pins.
When the digital outputs of the ADC switch are low, the DGND pin carries large, high-speed currents. Chip designers recognize that these currents induce transient voltages across the inductance of the DGND pin. This ground-bounce noise, also called simultaneous-switching noise, makes the voltage on the chip's internal digital ground net (or substrate) different from the voltage on the pc-board ground. The ground-bounce noise may not bother the digital logic, which has a high tolerance to noise, but it renders the chip's internal digital ground net useless as an analog reference voltage.
The AGND pin solves this problem. As long as the internal analog circuits don't pump much current through the AGND pin, there is not much of a voltage drop across the inductance of this pin. The internal analog circuits connected to the AGND pin therefore see a true, quiet indication of the actual pc-board ground voltage as sensed at the AGND via.
On your pc board, you should tie together the AGND and DGND pins. Otherwise, any voltage differences between the points at which you connect these pins may interfere with the chip's ability to communicate across its own internal analog-to-digital boundary. ADCs, of course, have a certain tolerance to such noise, but you don't know what that tolerance is. Furthermore, the chip's internal tolerance to noise may already be fully allocated to handle the expected worst-case amount of ground bounce on the DGND pin.
In a low-resolution, 8-bit system, which needs only about 60 dB of noise isolation, you can use one big, solid ground plane for everything (analog and digital), physically separating the analog stuff from the digital stuff to control crosstalk. This architecture satisfies the requirement of tying together AGND and DGND.
In higher resolution systems requiring more noise isolation, you begin to worry about noise induced by stray digital currents flowing across the analog ground region of your pc board. To prevent these currents, you may choose to divide your pc ground into two regions—an analog region and a digital region—connected at one common point directly under the ADC. The common connection must be short and fat enough so that little voltage difference appears between the DGND and AGND pins of the ADC.
You must also arrange the common connection so that it does not encourage ground currents circulating between the analog and digital regions (or else you defeat the whole point of isolating the grounds). That's the tricky part about having separate DGND and AGND plane regions: It doesn't help to separate them unless you also prevent high-speed currents from passing from one to the other.
If you have floating inputs (differential, transformer-coupled, or optically coupled) you are in luck. Floating inputs don't require that you tie your analog ground region to any particular reference. The special analog ground region for a floating-input system therefore ties to other grounds at one point, underneath the ADC, and nowhere else. Because the analog ground region has only one connection, stray digital currents aren't tempted to pass through it.
Chassis-referenced inputs introduce new complications, because your analog ground region will then need to touch the common connection underneath your ADC and the chassis ground reference for the input signal. But an analog ground region touching two other grounds attracts stray currents, defeating the purpose of isolation.
The architectural concept in Figure 1 circumvents the two-connection problem by connecting the ADC, the analog ground region, and the digital ground region all at one point. The analog circuitry goes in the left ear, and the digital circuitry goes in the right. No active circuitry goes in the middle except the ADC. Because the analog ear connects only at one point to the other grounds, the system sees it as a dead end for stray digital currents; they won't go in. You might call this a "Mickey Mouse" approach, but it works.