I. Analog Circuits and Digital Circuits
Analog circuits are circuits used to transmit, transform, process, amplify, measure, and display analog signals. Analog signals are continuously changing electrical signals. Analog circuits are the foundation of electronic circuits, and they mainly include amplifier circuits, signal processing circuits, oscillator circuits, modulation and demodulation circuits, and power supplies.
Digital circuits, or digital integrated circuits, are complex circuits composed of numerous logic gates. Compared to analog circuits, they primarily process digital signals (i.e., signals are represented by two states, 0 and 1), thus exhibiting stronger anti-interference capabilities. Digital integrated circuits include various gate circuits, flip-flops, and various combinational logic circuits and sequential logic circuits constructed from them. A digital system generally consists of control and arithmetic units. Driven by a clock, the control unit controls the arithmetic unit to perform the desired actions. Digital circuits can be interconnected with analog circuits through analog-to-digital converters and digital-to-analog converters.
II. Understanding the Role of Ground and Power Layers in Digital and Analog Circuits
Low-impedance, large-area ground planes are crucial for both analog and digital circuits. Ground planes not only provide a low-impedance return path for high-frequency currents (generated by high-speed digital logic) but also minimize EMI/RFI radiation. The shielding effect of the ground plane also reduces the circuit's sensitivity to external EMI/RFI. Ground planes also allow the use of transmission line technologies (microstrip or stripline) that require controllable impedance to transmit high-speed digital or analog signals. Using a "bus wire" as "ground" is completely unacceptable because it has impedance at most logic transition equivalent frequencies. For example, a #22 standard conductor has an inductance of approximately 20 nH/inch. A transient current with a slew rate of 10 mA/ns generated by a logic signal flowing through 1 inch of this conductor at this frequency would create a unwanted voltage drop of 200 mV.
For signals with a peak-to-peak range of 2 V, this voltage drop translates to approximately 10% error (about 3.5 bits of precision). Even in fully digital circuits, this error can lead to a significant decrease in the noise margin of the logic circuit.
Figure 1 shows a typical example of digital return current interfering with analog return current (top diagram). The conductor inductance and resistance of the grounding path are shared by the analog and digital circuits, causing mutual interference and ultimately introducing errors. One possible solution is to have the digital circuit current return path flow directly to GND REF, as shown in the bottom diagram. This is the basic principle of "star grounding," or single-point grounding. Achieving true single-point grounding in systems with multiple high-frequency return paths is difficult because the physical length of the individual current return path conductors introduces parasitic resistance and inductance, which does not conform to the low-impedance grounding principle for high-frequency currents. In practice, the current loop must consist of a large-area ground plane to achieve low-impedance grounding under high-frequency currents. Without a low-impedance ground plane, it is almost impossible to avoid the aforementioned shared impedance, especially at high frequencies.
All integrated circuit ground pins should be directly connected to a low-impedance ground plane to minimize series inductance and resistance (meaning avoid using IC sockets or similar devices). Traditional IC slots are not recommended for high-speed devices. Even in "small-size" slots, additional inductance and capacitance can introduce unwanted shared paths, degrading device performance. If the slot must be used with a DIP package, such as during prototyping, individual "pin slots" or "cage sockets" are acceptable. The above pin slots are available in both covered and uncovered versions (AMP product models 5-330808-3 and 5-330808-6). The use of spring-loaded metal contacts ensures good electrical and mechanical connections for the IC pins. However, repeated insertion and removal may degrade performance.
