非常详细的阻抗匹配设计

Trace Termination
PWB traces [or cables] should be terminated (using one of the schemes listed below) when the trace length exceeds the following: Length > tr / [ 2 x t pr ]. Where tr = Signal rise time, t pr = Signal propagation rate .For a general approximation this page uses: 150ps/inch for FR4 [Board Material], and 130pS/inch for Polimide [Board Material]. For example, using FR4 [150ps/inch] a trace with a 1.1nS rise time would need to be terminated if it exceeded 3.3 inches. The four main ways to terminate a signal trace are shown below. Calculations for Signal propagation rate [by board type], and reflection amplitude and frequency are shown after the termination examples. As a side note: A Printed Wiring Board [PWB] trace really has no resistance [because it's very low], this page deals with Printed Wiring Board trace impedance. The resistance of a Printed Wiring Board trace has more to do with voltage drop over the signal line [trace] and nothing to do with signal reflections ~ which this page deals with. This page uses the terms Printed Wiring Board, or PWB, and Printed Circuit Card, or PCC, and Circuit Card Assembly, CCA Interchangeably. Unused IC input pins which require a Resistor pull-up are not discussed on this page. Also the information provided works for terminating a cable in addition to a board trace.
Series Trace Termination
Series [Source] Termination should only be used with one load on the line, but only requires one resistor [R], placed near the source. Source Termination also works well when the driver impedance is less than the characteristic impedance of the line. It also causes no DC path. The Series Termination resistor [R] should be selected so that the combination of the resistor [R] and the output resistance [Zs] of the driver matches the trace impedance [Zo]. Source Termination may effect the rise time of the signal because of the RC time constant [R of resistor, C of the cable]. The driver is specified with a rise time based on some unit load [say 10pF], the rise time will be reduce to the R * C time constant with the series resistor. As the rise time is decreased the over all current demand from the driving pin is also reduced. Reducing the rise time has one benefit, reducing the Instantaneous current demand on the driver [reducing ground bounce and EMI] A series terminated line does not stop reflections [the load is still not terminated, un-matched], but it does reduce [damp] the amplitude of the ringing. If a source resistor is used with multiple loads, only the last device will see a clean edge, all the other loads will see a stair step edge [until the first reflection comes back]. The step voltage seen is equal to VO * [Zo / [R + Zs + Zo]]. The voltage drop
-(t/RC)
rising
over
the
capacitor
[by
time;
t]
is
given
by:
VC = 1 - e
Parallel Trace Termination


Parallel Termination dissipates the most power (at low clock rates), but only requires one resistor. Parallel Termination will work with any number of loads. The termination resistor [R] is still selected to match the trace impedance [Zo] and may be taken to GND or Vcc [the Power Supply]. The large power dissipation occurs at low switching rates, while at faster clock rates the driver is switching all the time any how. VOH = the Voltage Output when High [watch the amount of current you can source], and VOL = Voltage Output when Low [watch the amount of current you can sink]. A reflection will occur when the termination resistor [R] does not match the trace impedance [Zo], some people set the termination resistor a bit higher then Zo to reduce the amplitude of the reflection [because the trace impedance is to low to match].
Thevenin Trace Termination
Thevenin Termination [or Split Termination] allows the selection of the correct voltage and impedance of the line, but don't use with floating outputs. This Termination scheme also provides a constant DC path, but the resistor values are normally twice as large as with Parallel Termination. The two resistors [R1 and R2] (in parallel) should be chosen to equal the line impedance [Zo], and the Thevenin voltage should be chosen to provide VT for the logic family being used. So the constant current demand calculation is Vcc / [R1 + R2], and the demand from the driving device is Vo / R1. ECL devices will pull the lower resistor to Vee, and not ground. Use the equations below to solve for the ECL values:
R1 = Zo * [Vcc - Vee / VT - Vee] R2 = Zo * [Vcc - Vee / Vcc - VT] Zo = [R1 * R2] / [R1 + R2] = Trace Impedance VT = [R1 * Vee] + [R2 * Vcc] / [R1 + R2] A capacitor may also be used to eliminate steady state DC current flow. See AC Trace Termination below.


The SCSI Bus uses R1 = 330 ohms, and R2 = 220 Ohms, with no blocking capacitor. The VME Bus uses R1 = 470 ohms, and R2 = 330 Ohms, with no blocking capacitor. The GPIB Bus uses R1 = 6.2k ohms, and R2 = 3k Ohms, with no blocking capacitor. In some cases resistor packages are used to supply a number of terminations; MIL-PRF-83401/9 defines 8 Thevenin Terminations in a SIP package. Refer to this Resistor dictionary page for a definition of Thevenin. Also refer to Resistor Network Manufacturers, or Resistor Array Schematics and common values.
AC Trace Termination
AC Termination of a line results in the lowest power drain, but also requires two parts. Current only flows while the capacitor is charging. The termination resistor [R] is still selected to match the trace impedance [Zo], while the capacitor is selected by: Xc = [3 * Tr] / Zo. The capacitor value may be traded off to select a lower value [below 200pF] for low power consumption, or higher values for a cleaner waveform but a higher power consumption at higher frequencies. Xc = 1 / [ 2 * 3.1415 * F * C] = Capacitive Reactance F = Frequency of the signal, and C = the value of the Capacitor Tr = Rise Time of the signal [in nS], and Zo = Trace Impedance
Differential Trace Termination


Differential lines also require a termination resistor if the line length exceeds the data rate, from the equation at the top of the page. The termination is placed at the destination. To reduce the current consumption AC termination may be used [but not very common]. AC Termination of a line results in the lowest power drain, but also requires two parts. Current only flows while the capacitor is charging. The termination resistor [R] is still selected to match the trace impedance [Zo], while the capacitor is selected by: Xc = [3 * Tr] / Zo.
Half-Duplex Circuits, which transmit in both directions need to be terminated at both ends of the trace. So that the destination at each end has a termination resistor. Only two termination resistors are to be used. If there are other loads [transceiver] on the bus they should be left un-terminated.













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