LMH6560 datasheet(17/23 Pages) NSC

17 / 23 page

Application Notes (Continued)

termination resistor, provided the termination value is Z

0, 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 Z

0.At

this moment the voltage in the whole transmission line has

the nominal value of 2V (see Figure 6 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 attenu-

ated by the combination of the termination resistor and the

output impedance of the buffer. A simple calculation illus-

trates the point. Assume that the output impedance is 5

Ω.

For the frequency of interest the attenuation is V

B/VA=55/5

=11, where A and B are the points in Figure 3. 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 volt-

age 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 7. 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.

To calculate the length of the line it is necessary to measure

immediately after the series termination resistor. The voltage

at that point remains at half nominal voltage, thus V/2, until

the reflection returns and the voltage jumps to V. During an

interval of 5 ns the signal travels to the end of the line where

the wave front is reflected and returns to the measurement

point. During the time interval when the wavefront is travel-

ing to the end of the transmission line and back the voltage

has a value of V/2. This interval is 10ns. The length can be

calculated with the following formula: S = (V*T)/2

(5)

As calculated before in the section ‘Driving more than one

input’ the signal travels 20cm/ns so in 5ns this distance

indicated distance is 1m. So this example is easily verified.

APPLYING A CAPACITIVE LOAD

The assumption of pure resistance for the purpose of con-

necting the output stage of a buffer or opamp to a load is

appropriate as a first approximation. Unfortunately that is

only a part of the truth. Associated with this resistor is a

capacitor in parallel and an inductor in series. Any capaci-

tance such as C1-1 which is connected directly to the output

stage is active in the loop gain as see in Figure 8. Output

capacitance, present also at the minus input in the case of a

buffer, causes an increasing phase shift leading to instability

or even oscillation in the circuit.

Unfortunately the leads of the output capacitor also contain

series inductors which become more and more important at

high frequencies. At a certain frequency this series capacitor

and inductor forms an LC combination which becomes se-

ries resonant. At the resonant frequency the reactive com-

ponent vanishes leaving only the ohmic resistance (R-1 or

R-2) of the series L/C combination. (see Figure 9).

Consider a frequency sweep over the entire spectrum for

which the LMH6559 high frequency buffer is active. In the

first instance peaking occurs due to the parasitic capaci-

tance connected at the load whereas at higher frequencies

the effects of the series combination of L and C become

20064247

FIGURE 7.

20064249

FIGURE 8.

20064250

FIGURE 9.

LMH6560 Datasheet(PDF) 17 Page - National Semiconductor (TI)

LMH6560 Datasheet(HTML) 17 Page - National Semiconductor (TI)

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23

Datasheet Download

Link URL