   >>>  <<< Artwork Digital Ckts General Analog Ckts 1 PCB DESIGN

   >>>  <<< Artwork Digital Ckts General Analog Ckts 2 Topics 1.General Considerations in Layout Design 2.Layout Design for Analog Circuits 3.Layout Design for Digital Circuits 4. Artwork Considerations References W.C. Bosshart, Printed Circuit Boards: Design and Technology, TMH, 1992 C.F. Coombs : Printed Circuits Handbook, McGraw-Hill, 2001 R.S. Khandpur : Printed Circuit Boards : Design, Fabrication, and Assembly, McGraw-Hill, 2005.

   >>>  <<< Artwork Digital Ckts General Analog Ckts 3 1.GENERAL CONSIDERATIONS IN LAYOUT DESIGN 1.GENERAL CONSIDERATIONS IN LAYOUT DESIGN Main issues Component interconnections Effects of parasitics Physical accessibility of components Power dissipation Subtopics 1.1 Parasitic effects 1.2 Supply conductors 1.3 Component placement

   >>>  <<< Artwork Digital Ckts General Analog Ckts Parasitic Effects 1.1 Parasitic Effects R & L of conductor tracks C between conductor tracks Resistance Resistance of 35 μm thickness, 1 mm wide conductor = 5 mΩ/cm Change in Cu resistance with temperature = 0.4% / °C Current carrying capacity of 35 μm thickness Cu conductor (for 10 °C temperature rise): Width (mm)1410 Ic (A)2411

   >>>  <<< Artwork Digital Ckts General Analog Ckts 5 Capacitance Tracks opposite each other - Run supply lines above each other - Don’t let signal line tracks overlap for any significant distance Tracks next to each other - Increase the spacing between critical conductors - Run ground between signal lines Inductance To be considered in High frequency analog circuits Fast switching logic circuits

   >>>  <<< Artwork Digital Ckts General Analog Ckts Supply Conductors 1.2 Supply Conductors Unstable supply & ground due to Resistive voltage drop Voltage drop caused by track L and high freq. current Current spikes during logic switching  local rise in ground potential & fall in Vcc potential  possibility of false logic triggering. Solutions Conductor widths : W (ground) > W (supply) > W(signal) Ground plane Track configuration for distributed C between Vcc & ground Analog & digital ground (&supply) connected at the most stable point

   >>>  <<< Artwork Digital Ckts General Analog Ckts Component Placement 1.3 Component Placement Minimize critical conductor lengths & overall conductor length Component grouping according to connectivity Same direction & orientation for similar components Space around heat sinks Packing density Uniform Accessibility for adjustments component replacement test points Separation of heat sensitive and heat producing components Mechanical fixing of heavy components

   >>>  <<< Artwork Digital Ckts General Analog Ckts 8 2. LAYOUT DESIGN FOR ANALOG CIRCUITS 2. LAYOUT DESIGN FOR ANALOG CIRCUITS Supply and ground conductors Signal conductors for reducing the inductive and capacitive coupling Special considerations for Power output stage circuits High gain direct coupled circuits HF oscillator /amplifier Low level signal circuits

   >>>  <<< Artwork Digital Ckts General Analog Ckts Ground & Supply Lines 2.1 Ground & Supply Lines Separate GND (& V cc ) lines for analog & digital circuits Independent ground for reference voltage circuits Connect different ground conductors at most stable reference point Supply lines with sufficient width and high capacitive coupling to GND (use decoupling capacitors) Supply line should first connect to high current drain ckt blocks Supply line independent for voltage references

   >>>  <<< Artwork Digital Ckts General Analog Ckts HF Oscillator / Amplifier Decoupling capacitor between Vcc & GND  Capacitive load on o/p Reduce capacitive coupling between output & input lines Vcc decoupling for large BW ckts. (even for LF operation) Separation between signal & GND to reduce capacitive loading

   >>>  <<< Artwork Digital Ckts General Analog Ckts Circuits with High Power O/P Stage 2.3 Circuits with High Power O/P Stage Resistance due to track length & solder joints  modulation of Vcc & GND and low freq. oscillations Large decoupling capacitors Separate Vcc & GND for power & pre- amp stages

   >>>  <<< Artwork Digital Ckts General Analog Ckts High Gain DC Amplifier 2.4 High Gain DC Amplifier Solder joints  thermocouple jn Temp gradients  diff. noisy voltages Temp.gradients to be avoided Enclosure for stopping free movement of surrounding air

   >>>  <<< Artwork Digital Ckts General Analog Ckts Low Level Signal Circuits A) High impedance circuits - Capacitive coupling B) Low impedance circuits - Inductive coupling

   >>>  <<< Artwork Digital Ckts General Analog Ckts 14 High -Z circuits High -Z circuits If R » 1∕ jw(Cxy+Cy) then coupled Vy = Va × [Cxy/(Cy+Cxy)] Increase separation between low level high Z line and high level line (decrease Cxy) Put a ground line between the two (guard line) Example: Guard for signal leakage from FET output to input

   >>>  <<< Artwork Digital Ckts General Analog Ckts 15 Low – Z Circuits Low – Z Circuits Voltage induced in ground loops due to external magnetic fields Current caused in the low- Z circuit loop due to strong AC currents in nearby circuits Vm = - (d/dt)  B dA Avoid ground loops Keep high current ac lines away from low level,low Z circuit loops Keep circuit loop areas small

   >>>  <<< Artwork Digital Ckts General Analog Ckts LAYOUT DESIGN FOR DIGITAL CIRCUITS Main problems Ground & supply line noise Cross-talk between neighboring signal lines Reflections : signal delays, double pulsing

   >>>  <<< Artwork Digital Ckts General Analog Ckts Ground & Supply Line Noise Noise generated due to current spikes during logic level switching, drawn from Vcc and returned to ground Internal spike: charging & discharging of transistor junction capacitances in IC ( 20 mA, 5ns in TTL) External spike: charging & discharging of output load capacitance Ground potential increases, Vcc decreases: improper logic triggering. Problem more severe for synchronous circuits. Severity of problem (increasing): CMOS, ECL, TTL.

   >>>  <<< Artwork Digital Ckts General Analog Ckts 18 Solution for ground & supply noise Decoupling C between Vcc & ground for every 2 to 3 IC’s : ceramic, low L cap. of 10 nf for TTL & 0.5 nF for ECL & CMOS Stabilizes Vcc-GND (helps against internal spikes Not much help for external spikes Low wave impedance between supply lines (20 ohms): 5 to 10 mm wide lines opposite each other as power tracks Ground plane : large Cu area for ground to stabilize it against external spikes Closely knit grid of ground conductors (will form ground loops, not to be used for analog circuits) Twist Vcc & GND line between PCBs

   >>>  <<< Artwork Digital Ckts General Analog Ckts Cross-talk 3.2 Cross-talk Occurs due to parallel running signal lines (ECL: 10cm,TTL: 20 cm, CMOS: 50 cm) Problem more severe for logic signals flowing in opposite directions Solutions Reduce long parallel paths Increase separation betw. signal lines Decrease impedance betw. signal & ground lines Run a ground track between signal lines

   >>>  <<< Artwork Digital Ckts General Analog Ckts Reflections Caused by mismatch between the logic output impedance & the wave impedance of signal tracks. Signal delay (low wave imp.) Double pulses (high wave imp.) TTL (Z:  ) 0.5 mm signal line with GND plane, 1 mm without GND plane. Signal lines between PCBs twisted with GND lines. ECL (Z: 50  ) mm signal line with GND plane, or nearby gnd conductor. CMOS (Z: 150 – 300  ) 0.5 mm signal line without GND plane. Gnd not close to signal lines.

   >>>  <<< Artwork Digital Ckts General Analog Ckts 21 Logic Family:TTLECL CMOS Signal–GND Zw (  ) Signal line width (mm) 0.5 with gnd 1, no gnd with gnd0.5, no gnd Vcc -GND Zw (  ) < 5< 10< 20 Vcc line (mm)52 to 32 GND line (mm)Very broad (plane /grid) Broad (plane/grid) 5 Summary of Layout Design Considerations (for 1.6 mm thickness, double sided boards)

   >>>  <<< Artwork Digital Ckts General Analog Ckts ARTWORK RULES Conductor orientation Orientation for shortest interconnection length. Conductor tracks on opposite sides in x-direction & y- direction to minimize via holes. 45° or 30° / 60° orientation for turns. Conductor Routing Begin and end at solder pads, join conductors for reducing interconnection length. Avoid interconnections with internal angle <60°. Distribute spacing between conductors.

   >>>  <<< Artwork Digital Ckts General Analog Ckts 23 Conductor   routing examples

   >>>  <<< Artwork Digital Ckts General Analog Ckts 24 Solder Pads Hole dia Reduce the number of different sizes mm clearance for lead dia. Solder pad Annular ring width ≥ 0.5 mm with PTH ≈ 3 × hole dia without PTH Uniformity of ring around the hole. Conductor width d > w > d/3.

   >>>  <<< Artwork Digital Ckts General Analog Ckts 25