High Speed Schematic Design
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Transcript of High Speed Schematic Design
Presentation Contents
• Types of Terminators
• Terminator Resistor Selection and Cross-Talk
• Power System Distribution
• Selection Criteria of Bypass Capacitor
• Clock distribution fundamentals
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
Types of Terminators • End Terminations
• Series Terminations
• Middle Terminators
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
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.
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
End Terminations
Other Topologies Used with End Terminations
Series Terminators
Middle Terminators
AC Biasing for End Terminators
End Terminations for Differential Lines
Terminator Resistor Selection
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.
Terminator Resistor Selection and Cross-Talk
Cross-Talk
Cross-Talk
Cross-Talk
Power Regulators
LDO
Switching Regulators
• Buck
• Boost
• Buck-Boost
Power Modules
LDO
Pros:
Linear Operation
Easy to implement
No noise addition
Cost Effective
Less space on board
Cons:
Heats More
Less efficient
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
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.
Voltage reference used with single-ended logic
Common-path noise caused by a ground connection
Common-path inductance in power wiring
Single-plane power system
Differential signal transmission between gates
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.
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.
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.
Selection Criteria of Bypass Capacitor
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.
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.
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.
Clock Tree
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.
Single clock driver feeding two source-terminated lines
Fixed Delay Adjustments
Adjustable Delays
Automatically Programmable Delays
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.
Canceling Parasitic Capacitance of a Clock Repeater