High Speed Schematic Design

2. Presentation Contents
• Types of Terminators
• Terminator Resistor Selection and Cross-Talk
• Power System Distribution
• Selection Criteria of Bypass Capacitor
• Clock distribution fundamentals

3. Introduction to Terminators
A cable needs to be terminated when
• It’s long (its length exceeds 1/6 the electrical length of the rising edge)
and reflections occur
• It’s short (its has large inductance and drives a large capacitive load)
and ringing occurs

4. Types of Terminators
• End Terminations
• Series Terminations
• Middle Terminators

5. End Terminations
• The driving wave propagates at full intensity all the way down the
cable
• All reflections are damped by the terminating resistor
• The received voltage is equal to the transmitted voltage

6. End Terminations
• The termination arrangement just discussed rarely appears in TTL or
CMOS circuits because of the large drive current to maintain a high
state.
• The driver must supply VCC/R1 to the terminating resistor With Z0
equal to 65ohm, a 5-V signal requires 5/65 = 76mA Current.
• For High Current Requirement split termination used.

7. End Terminations
• The parallel combination of R1 and R2 must equal Z0.
• We must not exceed loh max (maximum high-level output current).
• We must not exceed Iol max (maximum low-level output current).
• TTL and CMOS sinks current in low state and sources current in high state
• ECL sources current in both states.
• Y1=1/R1 and Y2=1/R2

8. End Terminations

9. Other Topologies Used with End Terminations

10. Series Terminators

11. Middle Terminators

12. AC Biasing for End Terminators

13. End Terminations for Differential Lines

14. Terminator Resistor Selection

15. Terminator Resistor Selection and Cross-Talk
• To compute the worst-case terminating mismatch, the uncertainty in Zo is
added to the uncertainty in the terminating resistor.
• Power handing capacity of many resistors declines at elevated
temperatures.
• Resistor bodies have thermal resistance rating (Degree Celsius rise per
Watt)
• The vertical mount has a lower thermal resistance in still air than the
horizontal mount.
• The horizontal mount has a lower inductance because the leads stay low.
Along with resistance value, a tolerance, and power rating, the next most
important factor is the parasitic series inductance.
• Every 1% of reactance causes 1/2% of reflection.

16. Terminator Resistor Selection and Cross-Talk

17. Cross-Talk

18. Cross-Talk

19. Cross-Talk

20. Power Regulators
 LDO
 Switching Regulators
• Buck
• Boost
• Buck-Boost
 Power Modules

21. LDO
Pros:
Linear Operation
Easy to implement
No noise addition
Cost Effective
Less space on board
Cons:
Heats More
Less efficient

22. Switching Regulator
Pros:
 Most efficient
 Can boost from Low input to higher output
Cons:
 Switching noise addition
 Ripple noise addition
 Need more supporting components
 Costly compare to LDO

23. Power System Distribution
• Power Rule 1. Use low-impedance ground connections between
gates.
• Power Rule 2. The impedance between power pins on any two gates
should be just as low as the impedance between ground pins.
• Power Rule 3. There must be a low-impedance path between power
and ground.

24. Voltage reference used with single-ended logic

25. Common-path noise caused by a ground connection

26. Common-path inductance in power wiring

27. Single-plane power system

28. Differential signal transmission between gates

29. Power System Design Take Care
• Sense wires correct for resistance in power distribution wiring.
• Inductance in power wiring presents a much harder problem than resistance.
• Use lower-inductance wiring.
• Use logic immune to power supply noise.
• Reduce the size of changing power supply currents.
• It is almost impossible to reduce wiring inductance by simply using a bigger wire.

30. Power System Design Take Care
• A power supply provides low impedance at low frequencies. Local bypass
capacitors provide low impedance at higher frequencies.
• The best way to get very low inductance is to parallel a lot of small capacitors.
• Power and ground planes separated by 0.01 in. of FR-4 have a capacitance of 100
pF/in.
• Wide, flat parallel structures work much better as distribution wiring than round
wires.

31. Selection Criteria of Bypass Capacitor
• Lead inductance acts like an inductor in series with a capacitor. ESR acts like
a resistor in series with a capacitor.
• Together they degrade a capacitor's effectiveness as a bypass element.
• For large-valued capacitors, smaller packages have higher series inductance
and ESR than larger packages.
• Capacitor performance varies widely.

32. Selection Criteria of Bypass Capacitor

33. Selection Criteria of Bypass Capacitor
• When mounting components on the back side of any printed circuit board,
determine whether your manufacturing shop will use the reflow or wave solder
assembly method.
• Aluminum electrolytics are the workhorse capacitors most often used for board-
level bypass. Their characteristics are similar to those of tantalum, which has an
even higher dielectric constant at a slightly higher cost.
• The Z5U dielectric material has a higher dielectric constant than X7R but worse
temperature and aging properties. Below 10°C, Z5U is not recommended.

34. Selection Criteria of Bypass Capacitor
• The X7R dielectric material has a lower dielectric constant than Z5U, but better
temperature and aging properties.
• Higher-dielectric-constant materials pack more capacitance into a smaller space
but have poor temperature coefficients and aging instability.
• Aluminum electrolytics do not work well in cold applications.

35. Clock distribution fundamentals
• Timing margin measures the slack, or excess time, remaining in each
clock cycle.
• Timing margin protects your circuit against signal crosstalk,
miscalculation of logic delays, and later minor changes in the layout.
• Clock skew has as much of an impact on overall operating speed as
any other propagation delay.

36. Clock Tree

37. Clock distribution fundamentals
• Slow the rise time of the driver.
• Lower the capacitance of each tap.
• Lower the characteristic impedance of the clock distribution line, (Zo).
• A 20 Ὠ clock line is 2.5 times less sensitive to the capacitance of clock taps
than a 50 Ὠ line.
• A single driver can service two or more source-terminated lines under
restricted circumstances.

38. Single clock driver feeding two source-terminated lines

39. Fixed Delay Adjustments

40. Adjustable Delays

41. Automatically Programmable Delays

42. Delays for Timing Requirement
• Delay elements are built from three basic building blocks transmission
lines, logic gates, and passive lumped circuits.
• A fixed delay cannot cancel variations in board fabrication or active
component delay.
• An adjustable delay compensates for actual delays, not just nominal delays,
elsewhere in the circuit.
• Whatever form of delay you choose, incorporate its uncertainty in delay
into your timing margin calculations.

43. Canceling Parasitic Capacitance of a Clock Repeater