CA3256
Any combination of switch input circuits can be configured
A
B
C
A
B
C
with multiple, parallel, line-drive outputs. The video switch
amplifier circuit of Figure 8 illustrates how the CA3256 may
be configured in pairs to provide an 8-to-1 video switch
amplifier using a 3-bit address to select the input. It is also
possible to use the fifth channel input to tie signals to a
common bus line for distribution from the selected amplifier;
however, distributed capacitance loading will result in
reduced bandwidth. The 4 plus 1 combination of input-signal
switching provides for a wide assortment of video switch
circuit configurations.
EN
1
+
V
IN1
1
2
3
4
-
V
V
V
IN2
IN3
IN4
V
CH
1-4
OUT
+12V
-5V
V+
GND
V-
CA3256
CH 5
5
While the BiMOS process does provide some compromises
for both the switch and the amplifier, the combined system is
capable of the performance needed in most high-quality,
switching applications. As an integrated system, many of the
problems in PC-board layout are simplified, and there is a
reduction in component count. In its simplest form, with
+12V and -5V supplies, the CA3256 may be DC connected
at the input and output; the LED indicators need not be
connected. Under these conditions, the circuit may be as
simple as the one in Figure 8.
A
B
C
EN
2
+
V
V
V
V
-
IN5
IN6
IN7
IN8
1
2
3
4
CH
5-8
+12V
-5V
V+
GND
V-
5
NC
CA3256
TRUTH TABLE
Summary
CH
C
0
0
0
0
1
1
1
1
A
0
0
1
1
0
0
1
1
B
0
1
0
1
0
1
0
1
While each video-switch amplifier is designed for a specific
application and, to that end, is tailored as far as performance
to a given set of specifications, the circuit-designer’s goal is
generally the same in every case: to make the best possible
switch for the lowest cost. In this respect, the CA3256 IC
switch and amplifier discussed provide an excellent choice
for a cost-effective high-performance video-switch amplifier,
by taking advantage of the complementary features of both
high-speed CMOS and bipolar integrated circuits.
1
2
3
4
5
6
7
8
FIGURE 8. AN 8-TO-1 VIDEO SWITCH AMPLIFIER USING TWO
CA3256 DEVICES
8-11
HARRIS [ HARRIS CORPORATION ]
