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Reducing PCB design costs: From schematic capture to PCB layout
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Short Description: circuit boards. If an external design house provides your. printed circuit board layouts, ... Except where mandated by government requirements, testing of ...
Content Inside: Amplifiers: Op Amps Texas Instruments Incorporated Reducing PCB design costs: From schematic capture to PCB layout By Tom Hendrick Engineering Technician, Advanced Analog Products Modern EDA software packages provide a host of Figure 1. Schematic library symbols tools that allow designers to draw schematics in order to produce printed circuit boards. Understand- ing the basic requirements of these tools and the interactions between them can help to reduce the cost of PCB designs. + The basics Schematic capture packages contain various tools and features that make the process of entering a Op Amp Capacitor Resistor Diode schematic and documenting a circuit easy on the designer. One common feature among popular schematic capture packages is an array of libraries. Another is the ability to output netlists that are compatible with various simulation and PCB layout Figure 2. Component electrical pin numbers packages. The schematic capture process creates a database of symbolized parts and a netlist describing the connections between the symbols. PCB layout packages have their own suite of tools A C 5 and features designed to streamline the creation, 7 verification, and documentation of a physical printed 6 1 circuit board. All the board designer needs to do is 1 + define the outline of the board, add footprints from a or decal library, import the netlist, and route the con- 2 1 2 2 nections. Netlist comparisons will verify that the 3 2 board matches the schematic. Online error checking 1 warns of open- or short-circuit conditions. Design rules can be set up to check things such as matched net lengths, routing stubs, and parallelism. Polarized Dual Op Amp Capacitor Resistor Diode Sounds simple, doesn't it? Draw a schematic, output the netlist, and sit back to wait for your prototype assemblies. Simple, that is, until the call from the assembly shop tells you that a part does not match its footprint, or you spend hours tracking down a diode or capacitor that was laid out backwards. An understand- a schematic, but information required to create a printed ing of the process that takes place when exporting a circuit board is missing. As with the symbol library, the schematic netlist is necessary to ensure your board layout vendor-supplied part type libraries are intended as a start- will be correct. ing point for a customized user library. Customizing a vendor-supplied library requires that the Schematic libraries user assign attributes detailing the specific part to be used Schematic libraries are based on familiar symbols such as in the design. A generic vendor-supplied part type "resistor," those shown in Figure 1. for example, could be copied to a customized user library, The symbol libraries are generic by nature. They are be given a footprint attribute "1206," and then be renamed intended as building blocks for various part types. to part type "RES_1206_SMT." When this new part is Schematically speaking, the same resistor symbol can be called from the user library, it would always require that a used for a 1/8-watt surface mount or a 100-watt chassis- 1206-size surface-mount footprint be used to represent mount device. The op amp symbol could be a single device this device on the printed circuit board. in a 5-pin SOT package or a dual device in an 8-pin DIP. To create a valid schematic part, these symbols must, at a PCB libraries minimum, have electrical pin numbers assigned and be The PCB libraries are based on the footprints designated saved in a part type library. by industry standards. They usually include common Quite often, this is the extent of the part type libraries surface-mount and through-hole devices. To create a supplied with the schematic capture tool (see Figure 2). footprint, or decal, pads are arranged in a specific pattern Enough information is given to allow the designer to create and designated with an electrical pin number. Figure 3 is 48 Analog and Mixed-Signal Products August 2000 Analog Applications Journal
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