LMH6559_06 Datasheet, PDF (13/22 Pages) National Semiconductor (TI) – High-Speed, Closed-Loop Buffer

Application Notes
USING BUFFERS
A buffer is an electronic device delivering current gain but no
voltage gain. It is used in cases where low impedances need
to be driven and more drive current is required. Buffers need
a flat frequency response and small propagation delay. Fur-
thermore, the buffer needs to be stable under resistive,
capacitive and inductive loads. High frequency buffer appli-
cations require that the buffer be able to drive transmission
lines and cables directly.
IN WHAT SITUATION WILL WE USE A BUFFER?
In case of a signal source not having a low output impedance
one can increase the output drive capability by using a
buffer. For example, an oscillator might stop working or have
frequency shift which is unacceptably high when loaded
heavily. A buffer should be used in that situation. Also in the
case of feeding a signal to an A/D converter it is recom-
mended that the signal source be isolated from the A/D
converter. Using a buffer assures a low output impedance,
the delivery of a stable signal to the converter, and accom-
modation of the complex and varying capacitive loads that
the A/D converter presents to the OpAmp. Optimum value is
often found by experimentation for the particular application.
The use of buffers is strongly recommended for the handling
of high frequency signals, for the distribution of signals
through transmission lines or on pcb’s, or for the driving of
external equipment. There are several driving options:
• Use one buffer to drive one transmission line (see Figure
1)
• Use one buffer to drive to multiple points on one trans-
mission line (see Figure 2)
• Use one buffer to drive several transmission lines each
driving a different receiver. (see Figure 3)
FIGURE 1.
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FIGURE 3.
In these three options it is seen that there is more than one
preferred method to reach an (end) point on a transmission
line. Until a certain point the designer can make his own
choice but the designer should keep in mind never to break
the rules about high frequency transport of signals. An ex-
planation follows in the text below.
TRANSMISSION LINES
Introduction to transmission lines. The following is an over-
view of transmission line theory. Transmission lines can be
used to send signals from DC to very high frequencies. At all
points across the transmission line, Ohm’s law must apply.
For very high frequencies, parasitic behavior of the PCB or
cables comes into play. The type of cable used must match
the application. For example an audio cable looks like a coax
cable but is unusable for radar frequencies at 10GHz. In this
case one have to use special coax cables with lower attenu-
ation and radiation characteristics.
Normally a pcb trace is used to connect components on a
pcb board together. An important considerations is the
amount of current carried by these pcb traces. Wider pcb
traces are required for higher current densities and for ap-
plications where very low series resistance is needed. When
routed over a ground plane, pcb traces have a defined
Characteristic Impedance. In many design situations char-
acteristic impedance is not utilized. In the case of high
frequency transmission, however it is necessary to match
the load impedance to the line characteristic impedance
(more on this later). Each trace is associated with a certain
amount of series resistance and series inductance plus each
trace exhibits parallel capacitance to the ground plane. The
combination of these parameters defines the line’s charac-
teristic impedance. The formula with which we calculate this
impedance is as follows:
Z₀ = √(L/C)
In this formula L and C are the value/unit length, and R is
assumed to be zero. C and L are unknown in many cases so
we have to follow other steps to calculate the Z₀. The char-
acteristic impedance is a function of the geometry of the
cross section of the line. In (Figure 4) we see three cross
sections of commonly used transmission lines.
FIGURE 2.
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