LMH6560 datasheet(16/23 Pages) NSC

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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 50

Ω the backwards

traveling wavefront is subtracted from the incoming voltage

of 2V. Assume the load is 40

Ω. 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.

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 Z

0 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 Z

0. 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

about3*10

8m/sec, reduces through a transmission line or a

cable down to a value of about2*10

8m/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*10

8 * 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

20064246

FIGURE 6.

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