EXAMPLE 4: HIGH-SPEED DIGITAL DESIGN

This example demonstrates how to stack up layers and design transmission lines for a high-speed digital PCB. The example also demonstrates how to create a moated ground area with a bridge around a high-frequency crystal oscillator, how to perform pin/gate swapping, and how to create a heat spreader using vias to the Ground plane. The example circuit is shown in Figure 9-156.

Figure 9-156: High-speed digital circuit schematic

The BOM for this example is shown in Table 9-8. The circuit consists of a (fictional) high-speed, low-pin-count microcontroller/digital signal processor (uP-EXD10) driven by a 66-MHz clock (X1), a digital to fiber optic interface IC (FO-TX, which mimics an ADN2530 but with fewer pins), a fiber-optic laser diode (LD1), and a couple of 54ALS00 NAND gates used for I/O decoding. The digital signals have rise and fall times from 200 ps to 1.9 ns and require controlled impedance traces (see the Analog Devices ADN2530 data sheet for an example application). In a real design, more bypass capacitors would be used on the circuit, but the design is scaled down to keep the design simple. The parts and footprints are located on the Web site for this book.