LMH6560 datasheet(14/23 Pages) NSC

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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 Op Amp. 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

•

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)

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

cable 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 consideration 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 charac-

teristic impedance is not utilized. In the case of high fre-

quency 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 and also exhibits

parallel capacitance to the ground plane. The combination of

these parameters defines the line’s characteristic imped-

ance. The formula with which we calculate this impedance is

as follows:

0 = √(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 Z

0. 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.

20064237

FIGURE 1.

20064238

FIGURE 2.

20064239

FIGURE 3.

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