APPLICATIONS INFORMATION

Introduction

A typical circuit application is shown in Figure 1, above.

This is suitable for 75 ohm transmission line

connections at both the input and the output and is

useful for distribution of wide-band signals such as

video and xDSL via cables.

The 75ohms reverse

terminating resistor R4 gives the correct matching

condition to a terminated video cable. The amplifier

load is then 150 ohms in parallel with the local feedback

network.

The wide bandwidth of this device necessitates some

care in the layout of the printed circuit. A continuous

ground plane is required under the device and its

signal connection paths, to provide the shortest

possible ground return paths for signals and power

supply filtering.

A double-sided or multi-layer PCB

construction is required, with plated-through via holes

providing closely spaced low-inductance connections

from some components to the continuous ground

plane.

For the power supply filtering, low inductance surface

mount capacitors are normally required. It has been

found that very good RF decoupling is provided on

each supply using a 1000pF NPO size 0805 or smaller

ceramic surface mount capacitor, closest to the device

pin, with an adjacent 0.1uF X7R capacitor.

Other

configurations are possible and it may be found that a

single 0.01uF X7R capacitor on each supply gives good

results. However this should be supported by larger

decoupling capacitors elsewhere on the printed circuit

board.

Values of 1 to 10 µF are recommended,

particularly where the voltage regulators are located

more than a few inches from the device. These larger

capacitors are recommended to be solid tantalum

electrolytic or ceramic types.

Note particularly that the inverting input of this current

feedback type of amplifier is sensitive to small

amounts of capacitance to ground which occur as part

of the practical circuit board layout. This capacitance

affects bandwidth, frequency response peaking and

pulse overshoot.

Therefore to minimise this

capacitance, the feedback components R2 and R3 of

Figure1 should be positioned as close as possible to

the inverting input connection.

The frequency response and pulse response will vary

according to particular values of resistors and layout

capacitance.

The response can be tailored for the

application to some extent by choice of the value of

feedback resistor.

Figure 2 shows an oscilloscope

display of the pulse response for a practical double

sided printed circuit board where RF=510ohms.

ZXFV201

ISSUE 1 - FEBRUARY 2002

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