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## Page index

- Elec Reference - PCB Design Practices [elec-reference-pcb-design-practices]: https://wiki.cornellphysicalintelligence.com/llms-full.txt?page=elec-reference-pcb-design-practices

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# Elec Reference - PCB Design Practices

Page ID: elec-reference-pcb-design-practices
Source: https://wiki.cornellphysicalintelligence.com/llms-full.txt?page=elec-reference-pcb-design-practices
Section: electrical
Parent ID: None
Tags: 
Owner: Nathan Cunningham
Updated: 2026-09-14T20:52:22.710Z

::: tip How to Use this Document
Reference this document while working in the schematic design phase of PCB development.

Whenever unsure about when/how to implement a component, never guess. Assumptions can secretly kill the board's performance, so always check your work with this document and other tools.
:::

## Navigation

- Design Heuristics
- Common Pitfalls
- Key Components
 - Protection and Reliability
 - Signal Stability and Digital Logic
 - Power Supply
 - Transistor and Switching
 - Inductive Load and Motors
 - Analog and Sensor Interface
 - Communication Interface
 - Useful Learning Materials

## Design Heuristics
**Power Distribution & Power Integrity**
- Put decoupling capacitors close to IC power pins: Long traces add inductance and make the capacitor ineffective at high frequency
- Transmit power at higher voltage and lower current when practical: Reduces RI² losses, conductor size, and voltage drop
- Keep high-current paths short and wide: Reduces resistance, heating, voltage drop, and inductance
- Use LDOs when simplicity and low noise matter and the voltage drop/current are modest: They trade efficiency for simplicity and low noise
- Check effective capacitance of capacitors; DC bias can cause a much lower capacitance than expected, especially with low-capacitance X7R caps
- Put bulk capacitance near large transient loads: Supplies slower, larger bursts of current locally
- Add lots of test points for regulator I/O, important power rails, and ground

**Grounding, EMI, & Noise Control**
- Keep high-frequency current loops physically small: Reduces parasitic inductance and radiated EMI
- Give every high-speed signal a nearby return path: Current always travels in a loop; poor return paths increase EMI and signal-integrity problems
- Use a solid, continuous ground plane whenever practical: Creates low-impedance return paths and improves signal integrity. It is extremely highly recommended to avoid routing on or creating voids in the ground planes for> 2-layer boards
- Avoid unnecessary splits in ground planes: High-speed return current may otherwise be forced into a large loop. Keep analog and digital grounds as a common ground unless you have a reason to do otherwise.
- Place ground stitching vias where useful: Connects ground planes together to reduce impedance and improve EMI performance
- Surround RF/noisy circuitry with a via fence when appropriate: Reduces electromagnetic coupling
- Physically separate sensitive analog circuitry from noisy switching circuitry: Reduces conducted and radiated interference
- Keep switching nodes physically small: Fast dV/dt nodes capacitively couple noise into neighboring circuitry
- Locate filtering near the noise source: Prevents noise from propagating through the board

**Signal Integrity & Communication**
- Prefer differential signaling for high-speed or noisy environments: Improves rejection of common-mode noise and can reduce EMI
- Avoid routing high-speed signals near board edges: Reduces susceptibility to interference and unwanted radiation
- Keep differential pairs together and geometrically similar: Preserves differential impedance and minimizes skew
- For high-speed signals, ensure a gap of 3x the trace width between traces (generally, can vary) - reduces crosstalk between lines
- Minimize vias on critical high-speed signals: Vias introduce impedance discontinuities and parasitic inductance/capacitance
- Prefer serial over parallel communication for longer-distance data transmission when practical: Fewer conductors and fewer timing/skew issues; use multiplexing when appropriate
- Voltage-based systems generally want high input impedance and low output impedance; current-based interfaces generally follow the opposite principle
- Route high speed signals over ground planes, AC signals best return path is parallel to the forward path. Any blocks in that path will cause EMI
- Always check an IC pins idle states and signal direction to properly make use of pull up / down resistors. 


**Protection & Robustness**
- Put protection components close to the input/connector: Stop ESD and transients before they travel across the PCB
- Use pull-ups/pull-downs whenever a digital signal could otherwise float: Floating CMOS inputs can randomly switch and consume excess current
- Don't leave MOSFET gates floating: The gate stores charge and can turn the transistor on unpredictably

**Debugging, Testability, & Configuration**
- Implement test points for oscilloscope probes, multimeters, programming, and debugging
- Include status LEDs for verifying power rails, communication, boot state, or GPIO activity
- Use a 0 Ω resistor where you may want to disconnect or reroute a connection during debugging/testing
- Use a jumper/shunt header to manually connect selected nodes: Useful for selecting operating modes, addresses, power sources, etc.
- Use a solder jumper as a compact, inexpensive way to manually select PCB configurations



## Common Pitfalls

- Overcurrent: More current flows through a component or trace than intended
- Overvoltage: A node exceeds its allowable voltage
- Parasitics: Unwanted resistance, capacitance, and inductance from traces, vias, packages, etc.
- Undervoltage: Supply voltage falls below what the IC needs
- Voltage transients: Very brief voltage spikes/dips
- Reverse polarity: Power is connected backward
- ESD: Static discharge enters through connectors, buttons, exposed conductors, etc.
- Ground bounce: Current changes create voltage differences across supposedly common ground
- Poor return paths: Signal current cannot flow directly beneath or beside its outgoing trace
- Inadequate decoupling: IC cannot obtain transient current locally
- Crosstalk: One trace electromagnetically couples into another
- EMI: Board generates or receives electromagnetic interference
- Floating inputs: Digital or analog input has no defined voltage
- Bus contention: Two outputs try to drive the same line to opposite states
- Back-powering: Current enters an unpowered IC through an I/O pin



## Key Components

**Protection & Reliability**
| | |
| --- | --- |
| Flyback diode | Suppresses the large voltage spike produced when an inductive load like a relay, motor, or solenoid is switched off |
| Reverse-polarity protection | Prevents damage if the power supply is connected backward; commonly implemented with a diode or MOSFET |
| Schottky diode | Useful for low-voltage-drop protection, clamping, power OR-ing, and flyback suppression |
| TVS diode | Absorbs short, high-energy voltage transients from ESD, cables, automotive supplies, etc. |
| Zener diode clamp | Limits a node to approximately a chosen maximum voltage |
| ESD protection diode array | Protects USB, UART, CAN, buttons, connectors, and other exposed signals against static discharge |
| Fuse | Permanently disconnects power during excessive current |
| Resettable fuse/PTC | Limits current during a fault and automatically recovers after the fault is removed |
| Crowbar protection | Intentionally shorts the supply through an SCR/MOSFET when dangerous overvoltage occurs, usually blowing a fuse |
| Ideal-diode MOSFET | Performs diode-like reverse-current/reverse-polarity protection with much lower voltage loss |
| Current-limiting resistor | Protects LEDs, GPIOs, transistor bases, and other components from excessive current |
| Series protection resistor | Limits fault/ESD current entering an IC pin and can also reduce signal ringing |

**Signal Stability and Digital Logic**
| | |
| --- | --- |
| Pull-down resistor | Forces a signal LOW when nothing actively drives it |
| Pull-up resistor | Forces a signal HIGH when nothing actively drives it; essential for open-drain buses such as I²C |
| Series termination resistor | Reduces ringing and reflections on fast digital traces; often placed near the driving IC |
| RC debounce circuit | Filters mechanical switch bouncing before a button signal reaches digital logic |
| Schmitt-trigger buffer | Converts noisy or slowly changing signals into clean digital transitions |
| Voltage divider | Scales down voltages for ADCs, sensing circuits, reference generation, etc. |
| Logic-level shifter | Safely interfaces devices using different logic voltages, such as 5 V and 3.3 V |
| Open-drain / open-collector output | Allows multiple devices to safely share a line and is useful for wired-AND signaling |
| Weak pull-up/down + strong driver | Establishes a default state while still allowing an active device to override it easily |
| Unused-input biasing | Ties otherwise floating CMOS inputs HIGH or LOW so they do not randomly switch |

**Power Supply**
| | |
| --- | --- |
| Decoupling capacitor | Supplies very short bursts of current directly beside an IC and suppresses high-frequency supply noise |
| Bulk capacitor | Handles slower/larger current transients and stabilizes an entire power rail |
| Ferrite bead | Blocks high-frequency noise while allowing DC power through |
| LC / π filter | Provides stronger power-supply noise filtering than a capacitor alone |
| LDO regulator | Generates a cleaner lower-voltage rail with very little circuitry |
| Buck converter | Efficiently steps DC voltage down |
| Boost converter | Efficiently steps DC voltage up |
| Buck-boost converter | Maintains an output voltage when the input can be either above or below it |
| Load-switch MOSFET | Electronically turns power to an individual subsystem on/off |
| Soft-start circuit | Gradually powers a load to prevent large startup/inrush currents |
| Inrush-current limiter | Prevents large capacitors or loads from drawing a huge instantaneous current when plugged in |
| Power-good circuit | Tells a processor or other subsystem when a power rail has reached a safe voltage |
| Undervoltage Lockout (UVLO) | Prevents circuitry from operating when its supply voltage is too low |
| Power OR-ing | Allows a circuit to operate from either of two power sources without backfeeding one into the other |

**Transistor and Switching**
| | |
| --- | --- |
| H-bridge | Allows current through a motor in either direction, enabling forward/reverse control and braking |
| Low-side MOSFET switch | Lets a small MCU signal control a higher-current load connected to the positive supply |
| High-side MOSFET switch | Switches the positive supply rather than the ground connection |
| BJT transistor switch | Simple way for a low-current signal to control a larger current |
| MOSFET gate resistor | Controls MOSFET switching speed and reduces ringing/EMI |
| MOSFET gate pull-down | Keeps a MOSFET OFF while the MCU is booting or disconnected |
| Gate-driver IC | Provides the large instantaneous current needed to rapidly switch power MOSFETs |
| Bootstrap circuit | Produces the elevated gate voltage needed to drive an N-channel MOSFET on the high side |
| Dead-time circuit/control | Prevents both MOSFETs in a half-bridge from turning on simultaneously and shorting the supply |
| Relay | Allows a low-power electrical signal to switch a higher-voltage/current circuit with galvanic isolation |
| Optocoupler | Transfers a signal using light so two circuits can remain electrically isolated |

**Inductive Load and Motors**
| | |
| --- | --- |
| Flyback diode | Basic inductive-load protection for DC coils |
| TVS flyback clamp | Allows a relay/solenoid coil to discharge faster than with a normal flyback diode |
| RC snubber | Suppresses voltage spikes and ringing caused by switching inductive loads |
| RCD snubber/clamp | Dissipates switching transients in higher-power converters and inductive circuits |
| Motor suppression capacitors | Reduce high-frequency noise generated by brushed motors |
| Freewheeling diode | Provides a current path through an inductive load while switching it with PWM |

**Analog and Sensor Interface**
| | |
| --- | --- |
| RC low-pass filter | Removes high-frequency noise or provides simple anti-alias filtering |
| RC high-pass filter | Blocks DC while allowing higher-frequency AC signals through |
| Op-amp buffer/voltage follower | Prevents one circuit from loading another while preserving the same voltage
| Non-inverting amplifier | Amplifies a signal without reversing its polarity |
| Differential amplifier | Measures the difference between two signals while rejecting common voltage |
| Instrumentation amplifier | Precisely measures very small differential signals, especially from sensors |
| Current-sense resistor/shunt | Converts current into a small measurable voltage |
| Current-sense amplifier | Amplifies the voltage across a shunt resistor for an ADC or control system |
| Virtual ground / midrail reference | Creates an artificial midpoint so bipolar AC signals can be processed from a single supply |
| Precision voltage reference | Provides a more stable reference voltage than an ordinary regulator |
| ADC input RC filter | Reduces noise and provides a local charge reservoir for an ADC's sample-and-hold capacitor |

**Communication Interface**
| | |
| --- | --- |
| I²C pull-up resistors | Required because SDA and SCL use open-drain outputs |
| CAN termination resistor | Typically terminates both ends of a CAN bus to prevent reflections |
| Differential-pair termination | Matches the transmission-line impedance of high-speed differential signals |
| Common-mode choke | Filters common-mode noise on USB, Ethernet, CAN, and similar differential connections |
| USB ESD protection | Protects USB data pins from discharge entering through the connector |
| USB CC resistors | Tell USB-C devices about source/sink roles and connection state |
| RS-485 termination/biasing | Terminates the differential bus while establishing a known idle state |

**Useful Learning Materials**
- https://www.edn.com/category/blog/bogatins-rules-of-thumb/ - Eric Bogatin, Rules of Thumb for Signal Integrity 
- https://www.youtube.com/watch?v=ySuUZEjARPY - Rick Hartley, How to Achieve Proper Grounding
- https://www.youtube.com/watch?v=DIMIzKRmync - Eric Bogatin, Breaking Bad Habits in PCB Design
- https://www.youtube.com/watch?v=XumNc480qYo - Robert Feranec, Placement of Decoupling Capacitors
- https://www.youtube.com/watch?v=8i-ftULIjnM - Dr. Ridley, Voltage vs Current Mode for Converters
- https://www.youtube.com/watch?v=oBbTwxt7Sp4 - Dr. Ridley, Freq. Response Measurement Guide
- 






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