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This training module introduces the effect of PCB layout for high speed performance

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- 1. High Speed Amplifiers Part 2 <ul><li>Source: Analog Devices </li></ul>
- 2. Introduction <ul><li>Purpose </li></ul><ul><ul><li>This training module introduces the effect of PCB layout for high speed performance. </li></ul></ul><ul><li>Outline </li></ul><ul><ul><li>Power Supply Bypassing </li></ul></ul><ul><ul><li>Parasitics </li></ul></ul><ul><ul><li>Ground and Power Planes </li></ul></ul><ul><li>Content </li></ul><ul><ul><li>42 pages </li></ul></ul>
- 3. Power Supply Bypassing <ul><li>Bypassing is essential to high speed circuit performance </li></ul><ul><li>Capacitors right at power supply pins </li></ul><ul><ul><li>Capacitors provide low AC impedance to ground </li></ul></ul><ul><ul><li>Provide local charge storage for fast rising/falling edges </li></ul></ul>
- 4. Power Supply Bypassing <ul><li>Keep trace lengths short </li></ul>EQUIVALENT DECOUPLED POWER LINE CIRCUIT RESONATES AT: f = 1 2 LC IC +V S C1 L1 0.1 µF 1 µH f = 500kHz
- 5. Power Supply Bypassing <ul><li>Close to load return </li></ul><ul><ul><li>Helps minimize transient currents in the ground plane </li></ul></ul>
- 6. Power Supply Bypassing <ul><li>Values </li></ul><ul><ul><li>Individual circuit performance </li></ul></ul><ul><ul><li>Maintains low AC impedance </li></ul></ul><ul><li>Ferrite beads </li></ul>
- 7. Parasitics <ul><li>Parasite – An organism that grows, feeds, and is sheltered on or in a different organism while contributing nothing to the survival of its host. </li></ul><ul><li>Parasitics in high-speed PCB’s, can degrade or destroy circuit performance! </li></ul>
- 8. Parasitics <ul><li>PCB parasites take the form of undesired capacitors, inductors and resistors embedded within the PCB </li></ul><ul><li>Parasitics are extremely difficult to remove from a PCB </li></ul><ul><li>Prevention is the best method to minimize parasitics </li></ul>
- 9. Trace/Pad Capacitance d A K = relative dielectric constant A = area in cm 2 d = spacing between plates in cm
- 10. Trace/Pad Capacitance d A K = relative dielectric constant A = area in cm 2 d = spacing between plates in cm Example: Pad of SOIC L = 0.2cm W = 0.063cm K= 4.7 A = 0.0126cm 2 d = 0.073cm C = 0.072pF
- 11. Trace/Pad Capacitance d A K = relative dielectric constant A = area in cm 2 d = spacing between plates in cm Example: Pad of SOIC L = 0.2cm W = 0.063cm K= 4.7 A = 0.0126cm 2 d = 0.073cm C = 0.072pF Reduce Capacitance 1) Increase board thickness or layers 2) Reduce trace/pad area 3) Remove ground plane
- 12. Approximate Trace Inductance All dimensions are in mm
- 13. Approximate Trace Inductance Example L= 25.4mm W = .25mm H = .035mm (1oz copper) Strip Inductance = 28.8nH At 10MHz Z L = 1.86 a 3.6% error in a 50 system All dimensions are in mm
- 14. Approximate Trace Inductance Example L= 2.54cm =25.4mm W = .25mm H = .035mm (1oz copper) Strip Inductance = 28.8nH At 10MHz Z L = 1.86 a 3.6% error in a 50 system All dimensions are in mm Minimize Inductance 1) Use Ground plane 2) Keep length short (halving the length reduces inductance by 44%) 3) Doubling width only reduces inductance by 11%
- 15. Via Parasitics L = inductance of the via, nH H = length of via, cm D = diameter of via, cm Given: H= 0.157 cm thick board, D= 0.041 cm Via Inductance Via Capacitance L ~ 1.2nh D 2 = diameter of clearance hole in the ground plane, cm D 1 = diameter of pad surrounding via, cm T = thickness of printed circuit board, cm = relative electric permeability of circuit board material C = parasitic via capacitance, pF Given: T = 0.157cm, D 1 =0.071cm D 2 = 0.127 nH C ~ 0.51pf pF
- 16. Via Cross Section T&H D D 2 D 1
- 17. Capacitor Parasitic Model C = Capacitor R P = insulation resistance R S = equivalent series resistance (ESR) L = series inductance of the leads and plates R DA = dielectric absorption C DA = dielectric absorption
- 18. Resistor Parasitic Model R = Resistor C P = Parallel capacitance L= equivalent series inductance (ESL)
- 19. Low Frequency Op Amp Schematic
- 20. High Speed Op Amp Schematic
- 21. High Speed Op Amp Schematic Parasitic Capacitance
- 22. Stray Capacitance Simulation Schematic
- 23. Frequency Response with 2pF Stray Capacitance 1.8dB peaking 1.8dB peaking
- 24. Stray Inductance Parasitic Inductance
- 25. Parasitic Inductance Simulation Schematic AD8055 24.5mm x .25mm” =29nH
- 26. Pulse Response With and Without Ground Plane 0.6dB overshoot
- 27. Transient Response AD8009 1GHz Current Feedback Amplifier R F 150Ω 402 Ω 402 Ω R G -5V +5V 10uF 0.1uF 0.1uF 10uF
- 28. Small Changes Can Make a Big Difference! Circuit B Circuit A
- 29. Improper Use of Scope Probe Ground Clip
- 30. Effect of Clip Lead Inductance
- 31. Proper Grounding for Scope Probe in High-Speed Measurments
- 32. Small Changes Make Big Differences Circuit B Circuit A 21ns 17ns 25% reduction in ringing duration and amplitude
- 33. Ground and Power Planes Provide <ul><li>A common reference point </li></ul><ul><li>Shielding </li></ul><ul><li>Lower noise </li></ul><ul><li>Lower resistance </li></ul><ul><li>Lower impedance </li></ul><ul><li>Reduces parasitics </li></ul><ul><li>Heat sink </li></ul><ul><li>Power distribution </li></ul>
- 34. Ground Plane Conductor I I Dielectric Ground Plane
- 35. Sensitive Analog Circuitry Disrupted by Digital Supply Noise Ground Plane and Trace Routing Wrong Way ANALOG CIRCUITS DIGITAL CIRCUITS V D V A + + I D I A I D I A + I D V IN GND REF INCORRECT Input Connector
- 36. Sensitive Analog Circuitry Disrupted by Digital Supply Noise Ground Plane and Trace Routing Wrong Way ANALOG CIRCUITS DIGITAL CIRCUITS V D V A + + I D I A I D I A + I D V IN GND REF INCORRECT
- 37. Sensitive Analog Circuitry Safe from Digital Supply Noise Ground Plane and Trace Routing Right Way ANALOG CIRCUITS DIGITAL CIRCUITS V D V A + + V IN I D I A I D I A GND REF CORRECT
- 38. <ul><li>Grounding Example: </li></ul><ul><ul><li>Top layer is solid ground. </li></ul></ul><ul><ul><li>Bottom has a trace/transmission line connecting the RF connector to the load. </li></ul></ul><ul><ul><li>Return current flows in the top layer ground plane directly above the trace on the opposite side. </li></ul></ul>Top Side Bottom side Ground Plane and Trace Routing Signal Input Termination Resistor
- 39. <ul><li>Grounding Example: DC Current vs. AC Current: </li></ul><ul><ul><li>In a split or broken ground, the return currents follow the path of least impedance </li></ul></ul><ul><ul><li>At DC, the current follows the path of least resistance </li></ul></ul><ul><ul><li>As the frequency increases, the current follows the path of least inductance </li></ul></ul><ul><ul><li>Since there is now a ‘loop’ the inductance can be quite high and the circuit can now propagate EMI/RFI </li></ul></ul>Ground Plane and Trace Routing DC current follows the path of least resistance AC current follows the path of least impedance
- 40. Grounding Mixed Signal ICs: Single PC Board ANALOG CIRCUITS DIGITAL CIRCUITS A A D D D V A V D ANALOG GROUND PLANE DIGITAL GROUND PLANE AGND DGND MIXED SIGNAL DEVICE A DIGITAL SUPPLY ANALOG SUPPLY SYSTEM STAR GROUND V A V D
- 41. Ground Plane Recommendations <ul><li>There is no single grounding method which is guaranteed to work 100% of the time! </li></ul><ul><li>Remove ground plane under op amps to reduce parasitic capacitance </li></ul><ul><li>At least one layer on each PC board MUST be dedicated to ground plane! </li></ul><ul><li>Provide as much ground plane as possible especially under traces that operate at high frequency </li></ul><ul><li>Use thickest metal as feasible (reduces resistance and provides improved thermal transfer) </li></ul><ul><li>Use multiple vias to connect same ground planes together </li></ul><ul><li>Do initial layout with split analog and digital ground planes </li></ul><ul><li>Follow recommendations on device data sheet (read datasheet) </li></ul><ul><li>Keep bypass capacitors and load returns close to reduce distortion </li></ul><ul><li>Connect analog, digital and RF grounds at one point </li></ul>
- 42. Additional Resource <ul><li>For ordering high speed operational amplifiers, please click the part list or </li></ul><ul><li>Call our sales hotline </li></ul><ul><li>For additional inquires contact our technical service hotline </li></ul><ul><li>For more product information go to </li></ul><ul><ul><li>http://www.analog.com/en/amplifiers-and-comparators/operational-amplifiers-op-amps/products/index.html </li></ul></ul>Newark Farnell

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