Datasheet

A
B
C
D
E
TRANSMISSION LINE LENGTH
TIME
0
2
4
6
8
10
BEGIN END
|
V/2
0
V/2
0
V/2
V
0
V
V/2
0
V
V/2
0
V
= 1.79V
(80 x 10
-3
x 40)
LMH6559
SNOSA57C –APRIL 2003–REVISED MARCH 2013 www.ti.com
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
interferences 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 41.
Using Serial And Parallel Termination
Many applications, such as video, use a series resistance between the driver and the transmission line (see
Figure 36). In this case the transmission line is terminated with the characteristic impedance at both ends of the
line. See Figure 41 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 Z
0
forms a two-to-one voltage divider. The
result is a loss of 6dB. For video applications, 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.
18 Submit Documentation Feedback Copyright © 2003–2013, Texas Instruments Incorporated
Product Folder Links: LMH6559