CID CID Power Distribution & Grounding 1 — Questions and Answers
Question 1: What is the primary purpose of a power distribution network (PDN) in a PCB design?
- To route differential pairs efficiently
- To deliver stable voltage with minimal impedance to all active components (Correct answer)
- To isolate analog and digital ground planes
- To reduce layer count in the stackup
Correct answer: To deliver stable voltage with minimal impedance to all active components
A PDN is designed to supply stable, low-impedance power to all active devices across the board's operating frequency range.
Question 2: Which decoupling capacitor placement strategy best reduces power plane resonance in high-speed PCBs?
- Place capacitors at board edges
- Place capacitors as far from ICs as possible
- Place capacitors as close to IC power pins as possible with short vias (Correct answer)
- Use only bulk capacitors near the power connector
Correct answer: Place capacitors as close to IC power pins as possible with short vias
Decoupling capacitors placed close to IC power pins with short vias minimize parasitic inductance and most effectively suppress high-frequency noise.
Question 3: What phenomenon occurs when a power plane and its adjacent ground plane form a resonant cavity at certain frequencies?
- Skin effect
- Crosstalk
- Power plane resonance (Correct answer)
- Return current spreading
Correct answer: Power plane resonance
Power plane resonance occurs when the cavity formed between power and ground planes resonates at frequencies determined by board dimensions, causing impedance spikes.
Question 4: In a multi-layer PCB, what is the recommended maximum distance between a power plane and its reference ground plane to minimize PDN inductance?
- 10 mils
- 4 mils or less (Correct answer)
- 20 mils
- 50 mils
Correct answer: 4 mils or less
Tightly coupling the power and ground planes (4 mils or less) maximizes inter-plane capacitance and reduces PDN inductance significantly.
Question 5: What does the term 'target impedance' refer to in PDN design?
- The characteristic impedance of differential pairs
- The maximum allowable PDN impedance to keep voltage ripple within spec (Correct answer)
- The impedance of the power connector
- The termination impedance for transmission lines
Correct answer: The maximum allowable PDN impedance to keep voltage ripple within spec
Target impedance is the maximum PDN impedance level calculated to keep voltage ripple within acceptable limits for the IC's power budget.
Question 6: Which grounding strategy is most effective for mixed-signal boards with both analog and digital circuits?
- Completely separate ground planes with no connection
- Single unified ground plane with careful partition of analog and digital regions (Correct answer)
- Multiple floating ground islands
- Daisy-chain ground connections
Correct answer: Single unified ground plane with careful partition of analog and digital regions
A single unified ground plane with careful physical partitioning of analog and digital regions minimizes ground impedance while preventing digital noise from coupling into analog circuits.
CID CID Power Distribution & Grounding 1
What is the primary purpose of a power distribution network (PDN) in a PCB design?