TABLE OF CONTENTS The next example shows how to design a circuit with a mixture of analog and digital parts with multiple power planes and with a single ground plane split into analog and digital sections that have a common reference point.
Figure 9-59 shows the circuit design example. The circuit consists of a mixture of analog and digital parts (both heterogeneous and homogeneous), multiple power sources, and separate analog/digital grounds. It has an off-board connector, an analog signal conditioning circuit (the op-amp U1), an analog-to-digital converter (U2), a digital microcontroller (U3), and a digital serial-to-parallel shift register (U4). Note that the bypass capacitors have been omitted to accommodate the Demo part count limitations.
The circuit has five global power nets, which include analog power (V+, V ?) and analog ground (AGND) for U1 and U2, and +5 V digital power (VCC) and digital ground (GND) for U3 and U4. Analog and digital grounds are often kept separate to help prevent digital switching noise from affecting the analog ground. But in order for the circuit to work (especially when using analog-to-digital and digital-to-analog converters) the two grounds must have a common reference point. The two grounds are tied together through a low-impedance, single point connection at the connector.
The procedures and tools described in the first example are used to start a new project, place the parts onto the schematic...
