From Complete PCB Design Using OrCAD Capture and PCB Editor


This example shows how to design a circuit with a mixture of analog and digital parts, multiple power planes, and a single Ground plane split into analog and digital sections that have a common reference point.

Mixed-Signal Circuit Design in Capture

Figure 9-90 shows the circuit design example. The circuit consists of a mixture of analog and digital parts, multiple power sources, and separate analog/digital Ground planes. It has an off-board connector, an analog signal conditioning circuit (the op amp U1), an analog-to-digital converter (U2), a digital micro-controller (U3), and a digital serial-to-parallel shift register (U4). Note that the bypass capacitors have been omitted to accommodate the part count limitations of the demo version of the software.

Figure 9-90: Mixed-signal schematic design in Capture

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, 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 page, connect the...

Products & Services
Digital-to-Analog Converters
Digital-to-analog converters (DAC) transform a digital number into a corresponding analog voltage or current.
Signal Conditioners
Signal conditioners provide amplification, filtering, converting, and other processes required to make sensor output suitable for reading by computer boards.
Analog-to-Digital Converters
Analog-to-digital converters (ADC) sample an analog signal and convert it to a series of digital values to represent the signal to a computer processor.
Signal Converters
Signal converters contain inputs for one type of signal and outputs of another. Features can also include filtering and amplification or attenuation.
Signal Isolators
Signal isolators provide electrical (galvanic) isolation between the input and output circuits. They couple the signal to the output through a transformer or optical isolator. Signal isolators also break the direct electrical (galvanic) path between two or more loop points. They are used to protect against dangerous measured-variable voltages and provide increased protection from surges and spikes.

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