Considerations for Placing and Routing General-Purpose Analog Circuit Boards
A "general-purpose" analog circuit board refers to a board utilizing conventional analog circuitry such as op amps, transistors, etc., which operates within a limited bandwidth, typically a few megahertz (MHz). This kind of circuit design is relatively tolerant of oversights in circuit board placement and routing, but some essential guidelines should be adhered to.
Ensuring proper ground and power distribution holds significant importance, along with the inclusion of decoupling capacitors. By following these recommendations, you can avoid most potential issues in your circuit board design.
Considerations for Placing and Routing General-Purpose Digital Circuit Boards
Digital circuit boards labeled as "general-purpose" house a range of digital circuitry, including gates, counters, and microcontrollers, capable of operating at speeds up to approximately 20 MHz. In the overall design, critical timing is typically not a major concern, making this board type more tolerant of placement and routing oversights. However, given that the circuitry is digital in nature, there are some additional considerations to bear in mind during the placement and routing process.
The majority of modern digital circuitry adopts the use of CMOS (complementary metal oxide semiconductor) technology, as opposed to the older TTL (transistor-transistor logic). CMOS logic offers advantages such as reduced power consumption and faster switching compared to TTL. Nonetheless, this heightened speed comes at a cost – an increase in noise. The rapid transitions exhibited by CMOS logic outputs, often at intervals of 1 nanosecond or less, result in more power supply noise and crosstalk compared to TTL logic.
To prevent potential power supply corruption in CMOS logic, it is crucial to employ suitable decoupling capacitors. For each CMOS chip, it is advisable to have a minimum of one capacitor, with a typical choice being a 0.1μF ceramic capacitor. These capacitors should be placed in close proximity to the power pin of the respective chip, as discussed in Chapter Two. For devices with multiple power pins, the rule of thumb is to use one decoupling capacitor per power pin.
Managing crosstalk on a general-purpose digital circuit board is usually not overly challenging. It is essential to avoid routing outputs near inputs, especially if the inputs are utilized when the outputs switch. This practice ensures that the abrupt output transitions do not couple onto the input lines at inopportune moments. While it may be impossible to entirely avoid close proximity between inputs and outputs, efforts should be made to minimize the length of such connections to mitigate potential coupling issues.
Crystal oscillators play a significant role in most microcontrollers, serving as the timing reference. To make the oscillator function correctly, it is essential to be cautious about its placement and routing on the board. Typically, crystal circuits require a small capacitor (around 15-pF ceramic) on each side of the crystal connected to ground. Additionally, some circuits may have a resistor in series with the feedback loop. To ensure proper functioning, the oscillator should be placed as close to the microcontroller as possible. The crystal and resistor can be positioned in a small loop directly next to the microcontroller pins, with the startup capacitors placed nearby. The board should be routed with the shortest possible paths, and the ground connections on the capacitors must be kept very short.
Refer to the PCB layout figure below for a visually correct example of a crystal oscillator placement and routing. Observe how X1 (the crystal) is enclosed within a small loop of traces starting and ending at U1 (the microcontroller). R2 (the series resistor) is also integrated into the loop, while C1 and C2 (startup capacitors) are connected from the two loop nodes to a ground plane (not shown). Notice how the components are placed in close proximity to each other, and the traces form a small, tight loop.
