Datasheet

+
-
V
+
V
-
R
IN
BUFFER INTERNAL CONNECTIONS
C
L
-1
C
L
-2
R
SERIES
= 1mtr
(2 x 10
8
) x (10 x 10
-9
)
2
S =
R = Z0
Z0
B
A =1X
INPUT
E
A
V
TRANSMISSION LINE LENGTH
BEGIN END
TIME (ns)
0
1
2
3
4
5
0
V/2
LMH6559
SNOSA57C –APRIL 2003–REVISED MARCH 2013 www.ti.com
Figure 42.
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 5ns 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 traveling 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
(6)
As calculated before in the Driving More Than One Input section 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 connecting 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 capacitance such as C
L
-1 which is connected directly to
the output stage is active in the loop gain as seen in Figure 43. 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.
Figure 43.
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 series resonant. At the resonant frequency the reactive component vanishes leaving only the
ohmic resistance (R-1 or R-2) of the series L/C combination. (see Figure 44).
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