LMH6560 Datasheet, PDF (16/23 Pages) National Semiconductor (TI) – Quad, High-Speed, Closed-Loop Buffer

Application Notes (Continued)
(3)
The voltage wavefront of 2.45V will now set about traveling
back over the transmission line towards the source, thereby
resulting in a reflection caused by the mismatch. On the
other hand if the load is less then 50Ω the backwards
traveling wavefront is subtracted from the incoming voltage
of 2V. Assume the load is 40Ω. Then the voltage across the
load is:
(4)
This voltage is now traveling backwards through the line
toward the start point. In the case of a sinewave interfer-
ences develop between the incoming waveform and the
backwards-going reflections, thus distorting the signal. If
there is no load at all at the end point the complete transient
of 2V is reflected and travels backwards to the beginning of
the line. In this case the current at the endpoint is zero and
the maximum voltage is reflected. In the case of a short at
the end of the line the current is at maximum and the voltage
is zero.
FIGURE 6.
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Using Serial and Parallel Termination
Many applications, such as video, use a series resistance
between the driver and the transmission line (see Figure 1).
In this case the transmission line is terminated with the
characteristic impedance at both ends of the line. See Figure
6 trace B. The voltage traveling through the transmission line
is half the voltage seen at the output of the buffer, because
the series resistor in combination with Z₀ forms a two-to-on
voltage divider. The result is a loss of 6dB. For video appli-
cations, amplifier gain is set to 2 in order to realize an overall
gain of 1. Many operational amplifiers have a relatively flat
frequency response when set to a gain of two compared to
unity gain. In trace B it is seen that, if the voltage reaches the
end of the transmission line, the line is perfectly matched
and no reflections will occur. The end point voltage stays at
half the output voltage of the opamp or buffer.
Driving More Than One Input
Another transmission line possibility is to route the trace via
several points along a transmission line (see Figure 2). This
is only possible if care is taken to observe certain restric-
tions. Failure to do so will result in impedance discontinuities
that will cause distortion of the signal. In the configuration of
Figure 2 there is a transmission line connected to the buffer
output and the end of the line is terminated with Z₀. We have
seen in the section ’Connecting a load using a transmission
line’ that for the condition above, the signal throughout the
entire transmission line has the same value, that the value is
the nominal value initiated by the opamp output, and no
reflections occur at the end point. Because of the lack of
reflections no interferences will occur. Consequently the sig-
nal has every where on the line the same amplitude. This
allows the possibility of feeding this signal to the input port of
any device which has high ohmic impedance and low input
capacitance. In doing so keep in mind that the transient
arrives at different times at the connected points in the
transmission line. The speed of light in vacuum, which is
about 3 * 10⁸m/sec, reduces through a transmission line or a
cable down to a value of about 2 * 10⁸m/sec. The distance
the signal will travel in 1ns is calculated by solving the
following formula:
S = V*t
Where
S = distance
V = speed in the cable
T = time
This calculation gives the following result:
s = 2*10⁸ * 1*10^-9 = 0.2m
That is for each nanosecond the wave front shifts 20cm over
the length of the transmission line. Keep in mind that in a
distance of just 2cm the time displacement is already 100ps.
Using Serial Termination To More Than One
Transmission Line
Another way to reach several points via a transmission line is
to start several lines from one buffer output (see Figure 3).
This is possible only if the output can deliver the needed
current into the sum of all transmission lines. As can be seen
in this figure there is a series termination used at the begin-
ning of the transmission line and the end of the line has no
termination. This means that only the signal at the endpoint
is usable because at all other points the reflected signal will
cause distortion over the line. Only at the endpoint will the
measured signal be the same as at the startpoint. Referring
to Figure 6 trace C, the signal at the beginning of the line has
a value of V/2 and at T = 0 this voltage starts traveling
towards the end of the transmission line. Once at the end-
point the line has no termination and 100% reflection will
occur. At T = 10 the reflection causes the signal to jump to 2V
and to start traveling back along the line to the buffer (see
Figure 6 trace D). Once the wavefront reaches the series
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