Printed circuit boards (PCBs) are by far the most common method of assembling modern electronic circuits. They comprise a sandwich of one or more insulating layers and one or more copper layers which contain the signal traces and the powers and grounds; the design of the layout of PCBs can be as demanding as the design of the electrical circuit.
Most modern systems consist of multilayer boards of anywhere up to eight layers (or sometimes even more). Traditionally, components were mounted on the top layer in holes which extended through all layers. These are referred to as “through-hole” components. More recently, with the near universal adoption of surface mount components, you commonly find components mounted on both the top and the bottom layers.
The design of the PCB can be as important as the circuit design to the overall performance of the final system. We shall discuss in this chapter the partitioning of the circuitry, the problem of interconnecting traces, parasitic components, grounding schemes, and decoupling. All of these are important in the success of a total design.
PCB effects that are harmful to precision circuit performance include leakage resistances, IR voltage drops in trace foils, vias, and ground planes, the influence of stray capacitance, and dielectric absorption (DA). In addition, the tendency of PCBs to absorb atmospheric moisture (hygroscopicity) means that changes in humidity often cause the contributions of some parasitic effects to vary from day to day.
In general, PCB effects can be divided into two broad categories—those that most noticeably affect the static or DC operation of the circuit, and those that most noticeably affect dynamic or AC circuit operation, especially at high frequencies.
Another very broad area of PCB design is the topic of grounding. Grounding is a problem area in itself for all analog and mixed-signal designs, and it can be said that simply implementing a PCB-based circuit does not change the fact that proper techniques are required. Fortunately, certain principles of quality grounding, namely the use of ground planes, are intrinsic to the PCB environment. This factor is one of the more significant advantages to PCB-based analog designs, and appreciable discussion in this section is focused on this issue.
Some other aspects of grounding that must be managed include the control of spurious ground and signal return voltages that can degrade performance. These voltages can be due to external signal coupling, common currents, or simply excessive IR drops in ground conductors. Proper conductor routing and sizing, as well as differential signal-handling and ground isolation techniques enable control of such parasitic voltages.
One final area of grounding to be discussed is grounding appropriate for a mixed-signal, analog/digital environment. Indeed, the single issue of quality grounding can influence the entire layout philosophy of a high performance mixed-signal PCB design—as it well should.
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