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How to select PCB substrate materials?
PCB substrate selection requires a balance among design requirements, mass producibility, and cost. Design requirements include electrical and mechanical aspects. This material issue is critical for ultra-high-speed PCBs (operating frequencies above GHz). For example, commonly used FR-4 material has significant dielectric loss at GHz frequencies, which severely attenuates signals and may be unsuitable. Electrically, verify that the dielectric constant and dielectric loss are compatible with the design frequency.
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How to avoid high-frequency interference?
The basic principle to avoid high-frequency interference is to minimize electromagnetic field interference from high-frequency signals (crosstalk). Increase the spacing between high-speed signals and analog signals, or add ground guard/shunt traces next to analog signals. Also, prevent noise coupling from digital ground to analog ground.
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How to resolve signal integrity issues in high-speed design?
Signal integrity is essentially an impedance matching problem. Factors affecting impedance matching include the signal source structure, output impedance, trace characteristic impedance, load characteristics, and trace topology. Solutions include termination and adjusting the trace topology.
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How is differential routing implemented?
Two rules govern differential pair routing: ① Keep the two traces at equal length; ② Maintain a constant spacing (determined by differential impedance) to ensure parallelism. Parallel routing has two forms: side-by-side (same layer) and over-under (adjacent layers). Side-by-side routing is more commonly used.
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How to implement differential routing for a clock signal with only one output?
Differential routing is only meaningful when both the signal source and receiver are differential. Thus, differential routing cannot be used for single-ended clock signals.
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Can a matching resistor be added between a differential pair at the receiver?
A matching resistor is typically added between a differential pair at the receiver, with a value equal to the differential impedance to improve signal quality.
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Why should differential pairs be routed close and parallel?
Differential pairs require appropriate close and parallel routing. The spacing defines the differential impedance (a key design parameter), and parallelism ensures consistent differential impedance. Uneven spacing causes impedance discontinuities, degrading signal integrity and introducing timing delays.
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How to resolve theoretical conflicts in practical routing?
Separating analog and digital grounds is fundamentally correct. Ensure signal traces do not cross ground splits (moats), and avoid enlarging return current paths for power and signals.
Crystal oscillators are analog positive-feedback oscillation circuits requiring stable loop gain and phase for reliable oscillation. They are highly susceptible to noise; even guard traces cannot fully isolate interference, and distant placement introduces ground plane noise. Thus, place crystals as close to the chip as possible.
High-speed routing and EMI requirements often conflict. The basic rule is that resistors, capacitors, or ferrite beads added for EMI must not violate signal electrical specifications. Prioritize routing and stack-up strategies (e.g., route high-speed signals on inner layers) to reduce EMI before using discrete components to minimize signal degradation.
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How to resolve conflicts between manual and automatic routing for high-speed signals?
Modern advanced routing tools support constraint settings to control routing patterns and via counts. EDA vendors vary significantly in constraint capabilities (e.g., serpentine trace control, differential pair spacing). Manual routing flexibility also depends on the routing engine (e.g., pushing traces, vias, or copper pours). Select a router with a robust routing engine to resolve this conflict.
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About test coupons.
Test coupons verify the characteristic impedance of fabricated PCBs using a Time Domain Reflectometer (TDR). Impedance control covers single-ended and differential signals. Trace widths and spacing on the coupon must match the design. Critical is the ground point location: to reduce ground lead inductance, the TDR probe ground is near the probe tip, so the test signal and ground points on the coupon must match the probe specifications.
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In high-speed PCB design, copper can be poured in blank areas of signal layers. How to assign ground and power connections for copper pours across multiple signal layers?
Copper pours in blank areas are mostly grounded. For copper pours adjacent to high-speed signals, maintain sufficient spacing to avoid reducing trace characteristic impedance. Avoid impacting impedance of other layers (e.g., in dual strip-line structures).
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Can signal traces above a power plane use the microstrip model for impedance calculation? Can signals between power and ground planes use the stripline model?
Yes. Both power and ground planes serve as reference planes for impedance calculation. For a 4-layer board (top → power → ground → bottom), top-layer traces use the power plane as the reference in the microstrip model.
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Can software-generated test points on high-density PCBs meet mass-production test requirements?
Software-generated test points comply with test requirements only if they meet test equipment specifications. Tight routing or strict test point rules may prevent automatic test point addition, requiring manual supplementation.
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Does adding test points affect high-speed signal quality?
Test point impact depends on the installation method and signal frequency. External test points (not using existing vias/pins) add a small capacitor or stub to the trace, slightly affecting high-speed signals. The impact correlates with signal frequency and edge rate, verifiable via simulation. Minimize test point size and stub length (per test equipment limits).
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How to connect grounds between multiple PCBs in a system?
When power or signals transfer between PCBs (e.g., from Board A to B), equal current returns through the ground plane (Kirchhoff’s Current Law), taking the lowest-impedance path. Assign sufficient ground pins at interfaces to reduce impedance and ground noise. Analyze current loops (especially high-current paths) and adjust ground connections to direct return currents and minimize interference to sensitive signals.
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Can you recommend foreign technical books and documents on high-speed PCB design?
High-speed digital circuits are applied in communications, networking, and computing. Communication PCBs operate at GHz frequencies with up to 40 layers. Computing platforms (PCs/servers) reach 400MHz (e.g., Rambus). High-density routing demands blind/buried vias, microvias, and build-up processes, supported by mass-production manufacturers.
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Two commonly used characteristic impedance formulas:
Microstrip: Z₀ = {87/√(Er+1.41)} × ln[5.98H/(0.8W+T)]
W = trace width, T = copper thickness, H = distance to reference plane, Er = dielectric constant. Valid for 0.1 < W/H < 2.0 and 1 < Er < 15.
Stripline: Z₀ = [60/√Er] × ln{4H/[0.67π(T+0.8W)]}
H = distance between two reference planes, trace centered. Valid for W/H < 0.35 and T/H < 0.25.
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Can ground traces be inserted between differential signal lines?
No. Differential signaling relies on coupling (flux cancellation, noise immunity). Inserting a ground trace destroys the coupling effect.
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Do rigid-flex PCBs require special design software and specifications? Where can I have such boards fabricated in China?
Standard PCB design software generates Gerber files for FPC fabrication. Manufacturers impose design rules (trace width, spacing, via size) based on process capabilities. Add copper reinforcements at flex bends. Search for “FPC manufacturers” online for fabrication services.
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What are the principles for selecting PCB-to-chassis ground points?
Select ground points to provide low-impedance return paths and control current flow. For example, connect the PCB ground plane to chassis ground near high-frequency devices or clock generators via mounting screws to minimize loop area and electromagnetic radiation.
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Where to start with PCB debugging?
For digital circuits: ① Verify all power voltages meet specifications (including power sequencing for multi-rail systems); ② Confirm clock frequencies are normal and signals have monotonic edges; ③ Verify reset signals comply with specifications. The chip should generate the first clock cycle if these checks pass. Debug per system operation principles and bus protocols.
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For a fixed-size PCB requiring more functions, higher routing density increases crosstalk, and narrow traces raise impedance. What techniques apply to high-speed (>100MHz) high-density PCBs?
Crosstalk severely impacts timing and signal integrity. Key guidelines:
- Control continuous, matched characteristic impedance.
- Set trace spacing (typically 2× width); verify via simulation for timing/signal integrity.
- Select proper termination.
- Avoid overlapping parallel traces on adjacent layers (higher crosstalk than same-layer adjacent traces).
- Use blind/buried vias to increase routing area (higher cost).
- Maintain differential pair parallelism and equal length as much as possible.
- Reserve differential and common-mode termination to mitigate timing/signal integrity issues.
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LC filters are commonly used for analog power supplies, but why do they sometimes underperform RC filters?
Filter performance depends on the target noise frequency and inductor value. Inductive reactance correlates with inductance and frequency. LC filters underperform at low frequencies with small inductances. RC filters waste power (resistor losses) and require power-rating checks.
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How to select inductor and capacitor values for filtering?
Inductor value depends on noise frequency and transient current response. Large inductances slow transient current delivery, increasing ripple. Capacitor value correlates with allowable ripple (smaller ripple needs larger capacitance), affected by ESR/ESL. For switching power supply outputs, consider pole/zero effects on feedback loop stability.
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How to meet EMC requirements without excessive cost?
EMC-related costs include additional ground layers, ferrite beads, and chokes. Combine with mechanical shielding for compliance. PCB design techniques to reduce radiation:
- Use slow-slew-rate components to minimize high-frequency content.
- Place high-frequency devices far from external connectors.
- Ensure high-speed signal impedance matching, correct layer assignment, and short return paths to reduce reflection/radiation.
- Place adequate decoupling capacitors at power pins (verify frequency response and temperature characteristics).
- Split ground near external connectors and connect connector ground to chassis ground.
- Add guard traces for high-speed signals (check impedance impact).
- Retract power planes by 20H (H = distance between power and ground planes).
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Why separate digital and analog grounds in PCBs with mixed analog/digital blocks?
Digital circuits generate power/ground noise during level switching, proportional to signal speed and current. Without ground splitting, high digital noise contaminates sensitive analog circuits even if signals do not cross. Ground splitting is only unnecessary if analog circuits are far from high-noise digital circuits.
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Why can digital and analog grounds share a single plane if blocks are separated and signals do not cross?
Digital return currents flow under their traces. Crossing analog/digital traces injects digital noise into the analog ground. Separating blocks and preventing signal cross-over eliminates this coupling, allowing a unified ground plane.
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How to consider impedance matching in schematic design for high-speed PCBs?
Impedance matching is critical for high-speed designs. Characteristic impedance depends on routing layer (microstrip/stripline), reference plane distance, trace width, and substrate material—determined only after routing. Simulation tools may miss impedance discontinuities; reserve terminations (e.g., series resistors) in schematics. The root solution is to avoid impedance discontinuities during routing.
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Where to obtain accurate IBIS model libraries?
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What EMC/EMI rules should designers follow in high-speed PCB design?
EMI/EMC covers radiated emission (>30MHz) and conducted emission (<30MHz); address both. Optimize early: component placement, stack-up, critical routing, and component selection. For example:
- Place clock generators far from external connectors.
- Route high-speed signals on inner layers with matched impedance to reduce reflection.
- Use slow-slew-rate components.
- Select decoupling capacitors with suitable frequency response.
- Minimize high-speed current loop area to reduce radiation.
- Split ground planes to contain high-frequency noise.
- Optimize PCB-to-chassis ground points.
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How to select EDA tools?
Current PCB tools lack strong thermal analysis. PADS or Cadence offer good cost-performance for other functions. PLD beginners can use vendor IDEs; single-point tools suit million-gate designs.
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Recommend EDA software for high-speed signal processing and transmission.
Innoveda PADS works well for general design with supporting simulation (70% of applications). Cadence is cost-effective for high-speed/mixed-signal designs; Mentor excels (especially design flow management).
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Explain PCB layer definitions.
Topoverlay (top silkscreen/top component legend): Top-layer component designators (e.g., R1, C5, IC10).
Bottomoverlay: Bottom-layer component designators.
Multilayer: Pads/vias appearing on all layers (e.g., 4-layer board). Defining as Top Layer restricts the pad to the top layer.
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What are key considerations for 2G+ high-frequency PCB layout and routing?
2G+ PCBs are RF circuits (beyond high-speed digital). RF layout/routing must integrate with schematics due to distributed effects. Passive RF components use parameterized definitions and custom copper shapes, requiring EDA tools with parameterized component and copper editing (e.g., Mentor Boardstation RF module). Agilent EEsof is a standard RF analysis tool with Mentor interface support.
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What rules govern microstrip design for 2G+ high-frequency PCBs?
RF microstrip design requires 3D field solvers to extract transmission line parameters; all rules derive from this field extraction.
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For an all-digital PCB with an 80MHz clock, what protection circuits ensure sufficient drive strength besides ground guards?
Clock drive strength requires clock driver chips (not protection circuits) to split one clock into multiple point-to-point signals. Match driver impedance to loads, ensure valid signal edges, and include driver propagation delay in timing calculations.
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What interface minimizes interference for clock transmission from a dedicated clock board?
Long clock traces increase transmission line effects. Use differential signaling (e.g., LVDS) for long distances; unnecessary for low-speed clocks.
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27MHz and SDRAM clock lines (80–90MHz) have 2nd/3rd harmonics in the VHF band causing strong interference. How to suppress harmonics besides shortening traces?
Odd harmonics dominate with 50% duty cycle (no even harmonics); adjust the duty cycle. For unidirectional clocks, use source-series termination to suppress secondary reflections without degrading edge rate. Calculate termination resistance via the provided formula.
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What is trace topology?
Topology (routing order) defines the routing sequence for multi-port net connections.
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How to adjust trace topology to improve signal integrity?
Topology impact varies by signal direction (uni/bi-directional), level type, and simulation requires deep circuit knowledge. No universal optimal topology exists.
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How to reduce EMI via stack-up design?
EMI requires system-level solutions. Stack-up provides short return paths, reduces coupling area, and suppresses differential-mode interference. Tightly coupling ground and power planes (with ground plane extension) suppresses common-mode interference.
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Why pour copper on PCBs?
Copper pouring serves three purposes: ① EMC: Large ground/power copper acts as shielding (e.g., PGND for protection); ② Process: Ensures plating uniformity and lamination stability for low-routing layers; ③ Signal integrity: Provides continuous return paths for high-speed signals and reduces DC power routing. Additional uses: heat dissipation and special component mounting.
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What routing considerations apply to systems with DSPs and PLDs?
Check the ratio of signal rate to trace length; address signal integrity if propagation delay matches edge time. For multiple DSPs, clock and data trace topology impacts signal quality and timing.
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What routing tools are alternatives to Protel?
Alternatives include Mentor WG2000/EN2000/PowerPCB, Cadence Allegro, and Zuken Cadstar/CR5000, each with unique strengths.
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What is a signal return path?
Return current: High-speed digital signals flow from driver to load via transmission lines, returning via the shortest ground/power path. High-speed signaling charges the dielectric capacitance between the transmission line and reference planes; signal integrity (SI) analyzes electromagnetic field characteristics and coupling.
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How to perform SI analysis for connectors?
IBIS 3.2 defines connector models (EBD models). SPICE models suit special boards (e.g., backplanes). Multi-board simulators (HyperLynx, IS_multiboard) use connector distributed parameters from datasheets for acceptable accuracy.
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What termination methods are available?
Termination (matching) splits into source and parallel termination. Source termination uses series resistors. Parallel termination includes pull-up, pull-down, Thevenin, AC, and Schottky diode matching.
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What factors determine termination method selection?
Termination depends on buffer characteristics, topology, signal level, detection method, duty cycle, and system power consumption.
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What rules govern termination selection?
Digital design prioritizes timing. Termination improves signal stability for valid sampling (level/edge validity). Refer to Mentor ICX materials or
High Speed Digital Design: A Handbook of Black Magic for electromagnetic principles.
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Can IBIS models simulate component logic functions? How to perform board/system-level simulation?
IBIS models are behavioral and cannot simulate logic functions. Use SPICE or structural models for functional simulation.
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In mixed analog/digital systems, two methods exist: ① Split analog/digital grounds (connected by copper/ferrite beads, unified power); ② Split analog/digital power (connected by ferrite beads, unified ground). Are they equivalent?
Yes, theoretically equivalent—power and ground are identical for high-frequency signals. Ground/power splitting prevents digital interference to analog circuits but may disrupt return paths. Modern mixed designs avoid splitting and use physical separation of analog/digital blocks.
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What are FCC and EMC?
FCC: Federal Communications Commission (US regulatory body).
EMC: Electromagnetic Compatibility (standard for electromagnetic emission/immunity).
FCC issues standards with defined test methods.
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What is differential routing?
Differential signaling transmits data via two identical, polarity-opposed signals, sampled by voltage difference. Route parallel with constant width and spacing to maintain consistency.
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What PCB simulation tools are available?
High-speed digital SI tools include ICX, SignalVision, HyperLynx, XTK, SpectraQuest; HSPice is also used.
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How do PCB simulators perform layout simulation?
High-speed designs use multi-layer boards with dedicated power/ground planes to improve signal quality and simplify routing.
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How to ensure stability for signals above 50MHz in layout/routing?
Minimize transmission line effects for high-speed digital signals. Route signals above 100MHz as short as possible. Signal speed is defined by rise time; stabilization methods vary by signal type (TTL, GTL, LVTTL).
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Outdoor units integrate RF, IF, and low-frequency monitoring circuits on one PCB. What substrate requirements apply? How to prevent cross-interference?
Mixed-circuit design is challenging. Isolate RF circuits as independent boards with shielding cavities; use single/double-sided RF PCBs for minimal distributed effects. RF boards prefer high-Q substrates (low Er, small distributed capacitance, high impedance, low delay). Split PCB into RF/digital sections with ground via fences and shielding boxes.
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What solutions does Mentor offer for PCBs integrating RF, IF, and low-frequency circuits?
Mentor’s board-level tools include dedicated RF modules: ① RF Schematic: Parameterized models and bidirectional interface with Agilent EEsof; ② RF Layout: Custom RF pattern editing and EEsof integration (simulation results back-annotate to schematics/layout). Design management enables reuse and collaboration, widely used for mobile phone designs with Agilent EEsof.
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A 12-layer PCB has three power planes (2.2V, 3.3V, 5V). How to handle grounding?
Dedicated power planes improve signal quality by avoiding cross-plane splits (a major signal integrity factor). Power/ground planes are equivalent for high-frequency signals. Additional considerations: power-plane coupling (adjacent ground reduces AC impedance) and symmetric stack-up.
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How to verify PCB compliance with design specifications before delivery?
Manufacturers perform electrical continuity tests and X-ray inspection (etching/lamination defects). Post-SMT boards use ICT testing (requires design-in test points). Special X-ray equipment isolates fabrication defects.
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Should ESD characteristics be considered during chip selection?
Maximize ground area for 2-layer and multi-layer boards. Review chip ESD specifications (performance varies by manufacturer). Mechanical protection is also critical for ESD immunity.
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Should ground traces form closed loops to reduce interference?
Minimize loop area to reduce interference. Route grounds in a tree (non-closed) structure and maximize ground area.
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Should the emulator and PCB share a common ground if powered separately?
Separate grounds are preferred to avoid interference, but most systems require common grounding per specifications.
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Should multiple PCBs in one system share a common ground?
Yes, practical systems require unified grounding. Separate power supplies reduce interference but are rarely feasible.
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A handheld metal-enclosure product with an LCD fails ESD testing (contact 1100V, air 6000V; horizontal coupling 3000V, vertical 4000V). CPU clock: 33MHz. How to pass ESD testing?
Metal enclosures intensify ESD issues (LCD vulnerability). Add internal insulating materials, strengthen PCB grounding, and ground the LCD (implementation depends on mechanical design).
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What ESD considerations apply to systems with DSPs and PLDs?
Protect human-contact areas via circuit and mechanical design. ESD severity increases in dry environments and for sensitive systems. Prioritize ESD prevention in all designs.
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