APPLICATIONS INFORMATION

Introduction

A typical circuit application is outlined in Figure 1,

above, where two devices are combined to provide

8-to-1 multiplexing. A more detailed basic application

circuit for 4 dc-coupled channels is given in Figure 3,

and an AC coupled circuit for 8 channels is shown in

Figure 4. These circuits are suitable for 75

transmission line connections at both the input and the

output and are useful for distribution of wide-band

signals such as video via cables. The 75

reverse

terminating resistor at the output gives the correct

matching condition to a terminated video cable. The

amplifier load is then 150 .

The wide bandwidth of this device necessitates some

care in the layout of the printed circuit. Partly for this

reason, an Evaluation Circuit board is available and is

described in a later paragraph. 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 o r 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 (some of these holes are not visible in the figures

for the Evaluation Circuit Board – artworks and NC drill

output can be provided if required).

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 1000 pF NPO size 0805 ceramic

surface mount capacitor, closest to the device pin, with

an adjacent 0.1 F X7R capacitor. Other configurations

are possible and it may be found that a single 0.01 For

0.1

F X7R capacitor, size 0805 or smaller, 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

F 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.

Evaluation Circuit

An evaluation circuit is available, constructed on a

double-side printed circuit board. The circuit is suitable

for both the ZXFV301 and ZXFV302 and either device

may be fitted. Figures 5 and 6 show the circuit diagram,

and the layout of components and copper. A parts list

is provided below. This layout serves as a useful

example for many applications, showing the practical

implementation of the advice given above in the

Introduction.

BNC connector sockets allow connection to test

instruments via 50

cables. The output circuit includes

a resistor matching circuit to present a load of 150

to

the device and simultaneously provide 50

output

impedance. The attenuation of this matching circuit is

15.45 dB. As the device has unity voltage gain, the

overall loss when loaded by 75

is also 15.45 dB.

EVALUATION CIRCUIT PARTS LIST:

ZXFV302

ISSUE 4 - NOVEMBER 2002

5

QTY CCT-REF

VALUE

DESCRIPTION

Resistors, surface mount

4

R1,R2,R3,R4

51R

0805

4

R5,R6,R7,R8

22k

0805

1

R9

120R

0805

1

R10

62R

0805

1

R11

10R

0805

5

R12 to R16

47k

0805

Capacitors, surface mount

6

100nF

25V ceramic 0805

X7R

2

1nF

50V ceramic 0805

NPO

210 F

16V Tant Elec size

C

Integrated Circuits

1U1

Zetex ZXF 301N16

or ZXFV302N16

Miscellaneous

5

BNC Socket, PCB

straight flange,

e.g. Tyco

B35N14H999X99

1

3-way PCB screw

terminal block IMO

20.501/3SB

1SW1

DIL switch, 8 way

10

TP1 to TP10

PCB test terminal,

red, W.Hughes

100-107