Impedance is something every engineer works with, but few can explain it clearly. It seems simple yet is hard to put into words.
Impedance is something every engineer works with, but few can explain it clearly. It seems simple yet is hard to put into words.
We’ll use a quick question-and-answer format to help you fully understand impedance.
Q1: What is impedance?
A: In a circuit containing resistance, inductance, and capacitance, the opposition to alternating current is called
impedance, denoted by
Z.
Impedance consists of resistance, inductive reactance, and capacitive reactance, but is
not a simple sum of the three. Its unit is the
ohm (Ω).
PCB impedance refers to the electrical impedance between traces, power planes, loads, and other components on a circuit board.
PCB impedance control is critical in PCB design to ensure performance and stability. In high-speed circuit design, proper impedance is essential; improper impedance causes severe noise interference.
Q2: What is single-ended impedance?
A: Single-ended impedance is a type of characteristic impedance. It refers to the impedance of
one single signal line in a circuit.
In single-ended transmission, signals travel through a single signal line.
Q3: What is differential impedance?
A: Differential impedance applies to differential signal structures for impedance control.
The driver inputs two signals with opposite polarities, transmitted over two differential traces; the receiver processes them by subtraction.
This method improves signal integrity and noise immunity in high-speed analog and digital circuits.
Q4: What is coplanar impedance?
A: Coplanar impedance is the measured impedance of a signal line traveling between GND/VCC planes.
Coplanar waveguide is a common planar conductor structure in high-frequency and microwave circuits.
Coplanar impedance depends on waveguide geometry, conductor width, dielectric parameters, etc. Adjusting these controls electromagnetic wave propagation.
Q5: Why is single-ended impedance controlled at 50Ω?
A: 50Ω is the industry default, offering easy manufacturing and low loss. It is not mandatory.
Standards vary by interface: 75Ω is standard for long-distance communications, and cables/antennas require matching PCB impedance.
Special chips can lower impedance by improving drive capability to enhance EMI and crosstalk suppression.
For example, Intel specifies impedances of 37Ω, 42Ω, or even lower.
Q6: Why is differential impedance controlled at 100Ω?
A: Differential and single-ended impedance values are set based on history and application needs, mainly to match specific interfaces and chips for stable signal transmission.
Common values:
100Ω, 90Ω, 85Ω.
These reduce signal reflection, interference, and distortion, improving transmission efficiency.
Q7: Why are differential traces routed in parallel?
A: Parallel routing improves noise immunity and maintains impedance continuity by keeping coupling between the two lines consistent.
Differential signals do not strictly require parallelism. If not parallel:
- Ensure spacing > 5W
- Single-ended impedance is consistent
- External interference is low
In test fixtures, differential lines are often single-ended 50Ω traces. Differential transmission works as long as they connect to the receiver, even with flying leads or coaxial cables.
Q8: Can differential traces be unequal in length?
A: No. Equal length is the most important physical rule for differential pairs.
The receiver performs difference calculation on the two signals. Unequal lengths cause
phase error, which can lead to transmission errors.
Even a small phase shift (e.g., 30°) degrades the eye diagram, converts differential-mode components to common-mode, and reduces immunity.
Typical tolerance: ≤5mil; maximum allowed: 10mil.
Q9: Can differential pairs be routed on different layers?
A: Yes, and theoretically better than same-layer routing:
- Easier fanout from BGAs
- Greatly reduces the glass weave effect
In practice, manufacturing limits cause random misalignment between layers, severely degrading performance. This method has practical drawbacks.
Q10: What affects impedance in high-speed PCB design?
A:
- Trace width: inversely proportional — narrower → higher impedance; wider → lower impedance
- Copper thickness: inversely proportional — thicker → lower impedance; thinner → higher impedance
- Trace spacing: proportional — wider spacing → higher impedance; narrower → lower
- Coplanar spacing: proportional — larger distance to adjacent conductors → higher impedance
Q11: What PCB material choices affect impedance?
A:
- Dielectric thickness: proportional — thicker dielectric → higher impedance
- Dielectric constant (Dk/Er): inversely proportional — higher Dk → lower impedance
- Solder mask thickness: inversely proportional — thicker solder mask → lower impedance
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