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⍀
transmissionlineconnections atboththeinput andthe
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 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 (someoftheseholesarenotvisibleinthefigures
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
surfacemount capacitor,closestto thedevicepin, with
anadjacent0.1 FX7Rcapacitor. Otherconfigurations
arepossibleanditmaybefoundthatasingle0.01For
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
regulatorsare locatedmore than afew inchesfrom 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-sideprintedcircuitboard.Thecircuit is suitable
for both the ZXFV301 and ZXFV302 and either device
maybefitted. Figures5and6showthecircuitdiagram,
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
instrumentsvia 50⍀cables.The outputcircuit 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:
ZXFV301
ISSUE 2 - 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
210F 16V Tant Elec size
C
Integrated Circuits
1U1 ᎏZetex ZXF 301N16
or ZXFV302N16
Miscellaneous
5ᎏBNC Socket, PCB
straight flange,
e.g. Tyco
B35N14H999X99
1ᎏ3-way PCB screw
terminal block IMO
20.501/3SB
1SW1 ᎏDIL switch, 8 way
10 TP1 to TP10 ᎏPCB test terminal,
red, W.Hughes
100-107