---
title: "Routing factors and avoiding crosstalk"
subject: "Instrumentation II"
chapter: 8
key: "semester_5/instrumentation_ii:route-factors-crosstalk"
source: https://www.bctengineer.com/q/semester_5/instrumentation_ii/route-factors-crosstalk
site: BCT Engineering
---

# Routing factors and avoiding crosstalk

Instrumentation II · Chapter 8: Circuit Layout

- Asked in: 2078 Kartik, 2+4 marks
- Video: [How To Solve Capacitive and Inductive Crosstalk in Your Design - Altium Academy](https://www.youtube.com/watch?v=BX5KA3dKJMs) — Altium Academy
- Answer checked by Claude Sonnet 5.5 on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/instrumentation_ii/route-factors-crosstalk
- Whole chapter: https://www.bctengineer.com/md/semester_5/instrumentation_ii/chapter-8

## Question

What are the factors that need to be considered while routing the signal traces in circuit layout. How do you avoid crosstalk while making layout of the circuit?

## Short answer

Route traces short and direct with controlled impedance and continuous ground return, separating analog, digital, and power signals. To avoid crosstalk, use 3W spacing, reduce parallel run length, route adjacent layers orthogonally, and add ground guard traces.

## Answer

#### Factors for routing signal traces
- Keep traces **short and direct**, with matched length for buses and clocks to limit skew.
- Maintain **controlled impedance** (constant width and height above the plane) and terminate to avoid reflections.
- Provide a **continuous ground return** below the traces; avoid plane splits.
- Use **45-degree bends**, minimise vias and avoid stubs.
- Choose width for the **current and voltage** needs; follow the **manufacturer's design rules** (width, spacing, hole sizes).
- Separate analog, digital, power and high-speed signals.

#### Avoiding crosstalk in layout
1. Use the **3W rule** for spacing between traces.
2. Reduce **parallel run length**; route adjacent layers orthogonally.
3. Put traces close to the **ground plane**; use ground guard traces with vias.
4. Slow the edge rate where possible; use a series resistor at the driver.
5. Keep sensitive traces short and low impedance, and away from clock/power lines.
6. Use differential pairs for noisy or sensitive signals.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. Which is a routing factor for signal traces?
   A. Keep traces long
   B. Short direct traces with matched lengths and controlled impedance
   C. Use many vias
   D. Use 90-degree bends
   Answer: B. Short, matched, constant-impedance traces limit skew and reflections. (Key point: Routing factors)

2. Which pair of practices reduces crosstalk?
   A. 3W spacing and orthogonal routing on adjacent layers
   B. Parallel long runs and fast edges
   C. Removing ground plane
   D. Placing sensitive traces by clock
   Answer: A. Spacing and orthogonal routing reduce coupling and coupled length. (Key point: 3W rule and layers)

3. How does a series resistor at the driver help?
   A. It slows the edge rate, reducing crosstalk
   B. It speeds the edge
   C. It adds a stub
   D. It removes the return path
   Answer: A. Slower edges give lower dV/dt and dI/dt coupling. (Key point: Edge rate)

## Related in Instrumentation II

- [Factors while routing signal traces on PCB](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Aroute-factors) — Instrumentation II, chapter 8, asked 4×
- [Terminologies in routing signal traces](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Arouting-terminologies) — Instrumentation II, chapter 8
- [Reducing crosstalk in PCB routing](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Acrosstalk-routing) — Instrumentation II, chapter 8, asked 5×