LMH6559
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SNOSA57C –APRIL 2003–REVISED MARCH 2013
Driving More Than One Input
Another transmission line possibility is to route the trace via several points along a transmission line (Figure 37)
This is only possible if care is taken to observe certain restrictions. Failure to do so will result in impedance
discontinuities that will cause distortion of the signal. In the configuration of Figure 37 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 Connecting A
Load Using A Transmission Line section 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
signal 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 (5)
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 38). 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 beginning 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 41 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
endpoint 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 41 trace D). Once the wavefront
reaches the series termination resistor, provided the termination value is Z0, the wavefront undergoes total
absorption by the termination. This is only true if the output impedance of the buffer/driver is low in comparison to
the characteristic impedance Z0. At this moment the voltage in the whole transmission line has the nominal value
of 2V (see Figure 41 trace E). If the three transmission lines each have a different length the particular point in
time at which the voltage at the series termination resistor jumps to 2V is different for each case. However, this
transient is not transferred to the other lines because the output of the buffer is low and this transient is highly
attenuated by the combination of the termination resistor and the output impedance of the buffer. A simple
calculation illustrates the point. Assume that the output impedance is 5Ω. For the frequency of interest the
attenuation is VB/VA= 55/5 = 11, where A and B are the points in Figure 38. In this case the voltage caused by
the reflection is 2/11 = 0.18V. This voltage is transferred to the remaining transmission lines in sequence and
following the same rules as before this voltage is seen at the end points of those lines. The lower the output
resistance the higher the decoupling between the different lines. Furthermore one can see that at the endpoint of
these transmission lines there is a normal transient equal to the original transient at the beginning point. However
at all other points of the transmission line there is a step voltage at different distances from the startpoint
depending at what point this is measured (see trace D).
Measuring The Length Of A Transmission Line
An open transmission line can be used to measure the length of a particular transmission line. As can be seen in
Figure 42 the line of interest has a certain length. A transient is applied at T = 0 and at that point in time the
wavefront starts traveling with an amplitude of V/2 towards the end of the line where it is reflected back to the
startpoint.
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