label: Printed circuit board(PCB),PCB designs
Printed circuit board(PCB) are by far the most common method of assembling modern electronic circuits. Composed of a sandwich of insulating layer (or layers) and one or more copper conductor patterns, they can introduce various forms of PCB errors into a circuit, particularly if the circuit is operating at either high precision or high speed. PCBs, then, act as “unseen” components wherever they are used in precision circuit designs.
Since embedded designers don’t always consider the PCB electrical characteristics as additional components of their circuit, overall performance can easily end up worse than predicted. This general topic, manifested in many forms, is the focus of this series of articles. PCB effects that are harmful to precision circuit performance include leakage resistances; spurious voltage drops in trace foils, vias, and ground planes; the influence of stray capacitance, PCB dielectric absorption (DA), and the related “hook.” 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.Another very broad area of PCB design is the topic of grounding. PCB Grounding is a problem area in itself for all analog designs, and it can be said that implementing a PCB-based circuit doesn’t change that fact. 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 signialcant advantages to PCB-based analog designs, and an appreciable amount of this appendix 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, PCB 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, ana-log/digital environment. This topic is the subject of many application calls, and it is certainly true that interfacing with ADCs (or DACs) is a major part of the system PCB design, and thus it shouldn’t be overlooked. Indeed, the single issue of quality grounding can drive the entire PCB layout philosophy of a high-performance mixed-signal PCB design—as well it should.PCB Resistance of Conductors PCB Every engineer is familiar with resistors, although perhaps fewer are aware of their idiosyncrasies. But too few engineers consider that all the wires and PCB traces with which their systems and circuits are assembled are also resistors. In higher-precision systems, even these trace resistances and simple wire interconnections can have degrading effects. Copper is not a superconductor—and too many engineers appear to think it is! Illustrates a method of calculating the sheet resistance R of a copper square, given the length Z, the width X, and the thickness Y. At 25ºC the resistivity of pure copper is 1.724 10–6 Ωcm. The thickness of standard 1-ounce PCB copper foil is 0.036 mm (0.0014). Using the relations shown, the resistance of such a standard copper element is therefore 0.48 Ω/square. One can readily calculate the resistance of a linear trace by effectively “stacking” a series of such squares end to end, PCB to make up the line’s length. The line length is Z and the width is X, so the line resistance R is simply a product of Z/X and the resistance of a single square, as noted in the figure. For a given copper weight and trace width, a resistance/length calculation can be made. For example, the 0.25 mm (10 mil) wide traces frequently used in PCB designs equates to a resist-ance/length of about 19 mΩ/cm (48 mΩ/inch), which is quite large. Moreover, the temperature coefcient of resistance for copper is about 0.4%/ºC around room temperature.
This is a factor that shouldn’t be ignored, in particular within low-impedance precision circuits, where the TC can shift the net impedance over temperature. PCB trace resistance can be a serious error when conditions aren’t favorable. Consider a 16-bit ADC with a 5 kΩ input resistance, driven through 5 cm of 0.25 mm wide 1 oz PCB track between it and its signal source. PCB The track resistance of nearly 0.1 Ω forms a divider with the 5 kΩ load, creating an error. The resulting voltage drop is a gain error of 0.1/5000 (0.0019%), well over 1 LSB (0.0015% for 16 bits). So, when dealing with precision circuits, the point is made that even simple design items such as PCB trace resistance cannot be dealt with casually.
There are various solutions to address this issue, such as wider traces (which may take up excessive space), the use of heavier copper (which may be too expensive), or simply choosing a high-impedance converter. But the most important thing is to think it all through, avoiding any tendency to overlook items that appear innocuous on the surface. The gain error resulting from resistive voltage drop in PCB signal leads is important only with high precision and/or at high resolutions or where large signal currents flow. Where load impedance is constant and resistive, adjusting overall system gain can compensate for the error. In other circumstances PCB, it may often be removed by the use of “Kelvin” or “voltage sensing”, The gain error resulting from resistive voltage drop in PCB signal leads is important only with high precision and/or at high resolutions or where large signal currents flow. Where load impedance is constant and resistive, PCB adjusting overall system gain can compensate for the error. In other circumstances, it may often be removed by the use of “Kelvin” or “voltage sensing”.
