At very high frequencies, the performance limits of high speed circuitry are often determined by printed circuit board layout. So attention has to be paid for grounding, power supply distribution, and signal paths. Before starting layout give enough thought about the signal flow on printed circuit board. Following are few tips which need to be considered while doing PCB layout.
- Typically used dielectric materials for PCB fabrication are FR4, Rogers 04003, Rogers RT/Duriod, etc.,
- While doing layout for high speed circuits pay attention for grounding, power supply bypassing and signal path.
- Maintaining low impedance from VCC to GND over a band of interested frequencies ensures filtering of noise from power supply rails, and keep the noise away from finding the path through the components.
- The important function of a bypass capacitor is to provide a very low impedance path between VCC and GND at frequencies of interest.
- Tantalum capacitors filters low frequency noise and ceramic capacitors filters high frequency noise.
- Paralleling of tantalum and ceramic chip capacitors offers low impedance across wide band of frequencies.
- Place the bypass ceramic chip capacitor on the same side of the component on the board, and as close as possible to supply pins.
- Reduces parasitic inductance : Ground plane reduces trace inductance by magnetic field cancellation between the conductor and ground plane.
- Minimize resistive losses due to skin effect.
- Stray capacitance to ground, helps in filtering.
- Concept of Analog and Digital ground.
- Ground plane on both side of PCB, particularly covering all unused space on the component side offer low impedance ground connection for return path currents.
- Stich the top and bottom ground planes with vias.
- Remove the ground plane in the area of amplifier inputs to reduce parasitic capacitance and coupling ground plane noise into amplifier input.
- Keep signal lines as short and straight as possible.
- Place the components as close as possible to reduce the trace length.
- Sometimes the parasitic capacitance as low as 1pF at the input terminals, provides a feedback and lead to oscillations.
- For differential circuits the signal paths should be as symmetrical as possible.
- microstrip vs. stripline for signal path?
Thermal land and Thermal vias
- In some ICs the heat dissipated in it is transferred to its surroundings through the thermal pad on the bottom of IC. To transfer this heat energy, a thermal land has to provided on PCB.
- Thermal land/pad on PCB transfer the heat from IC to PCB ground plane through thermal vias stitched on it. Typical diameter of vias are 13-15mil with 40mil pitch. Used solid vias instead of web or spoke to improve thermal conductivity of vias.
- Thermal conductive materials such as thermal grease or thermal paste between thermal pad of IC and thermal pad of PCB improves thermal conductivity.
- Terminate the inputs and outputs properly. Also place the termination as close as possible to IC pins. If there are unused sections of opamp/logic gates terminate them properly.
- Reducing noise in supply voltage place less constraints on PSRR of opamps.
- Debugging : Keep test points on the topside of PCB so that they can be accessed comfortably for testing.
- Choose the same dimension for all the passive, so that it is easy to prepare BOM and get them.
- Using surface mount components help in
- reducing the board size
- reducing the stray inductance and capacitance.
- In general the typical thickness of 2-layer PCB is 62mil/1.6mm.