HA5024 Intersil Corporation, HA5024 Datasheet - Page 8

• Manufacturer Part Number: HA5024
• Description: Quad 125MHz Video Current Feedback Amplifier with Disable
• Manufacturer: Intersil Corporation

The circuit shown in Figure 8 is a simplified schematic of the
enable/disable function. The large value resistors in series with
the DISABLE pin makes it appear as a current source to the
driver. When the driver pulls this pin low current flows out of the
pin and into the driver. This current, which may be as large as
350 A when external circuit and process variables are at their
extremes, is required to insure that point “A” achieves the
proper potential to disable the output.The driver must have the
compliance and capability of sinking all of this current.
When V
dedicated TTL gate. The maximum low level output voltage
of the TTL gate, 0.4V, has enough compliance to insure that
the amplifier will always be disabled even though D
turn on, and the TTL gate will sink enough current to keep
point “A” at its proper voltage. When V
the DISABLE pin should be driven with an open collector
device that has a breakdown rating greater than V
Referring to Figure 8, it can be seen that R
resistor to +V
where the enable/disable function is not required on all circuits
some circuits can be permanently enabled by letting the
DISABLE pin float. If a driver is used to set the enable/disable
level, be sure that the driver does not sink more than 20 A
when the DISABLE pin is at a high level. TTL gates, especially
CMOS versions, do not violate this criteria so it is permissible to
control the enable/disable function with TTL.
Typical Applications
Four Channel Video Multiplexer
Referring to the amplifier U
cable in its characteristic impedance of 75 , and R
terminates the cable in its characteristic impedance. The
amplifier is set up in a gain configuration of +2 to yield an
overall network gain of +1 when driving a double terminated
cable. The value of R
gain is desired. R
inhibiting the amplifier until the switch, S
position 1. At position 1 the switch pulls the disable pin up to
the plus supply rail thereby enabling the amplifier. Since all
of the actual signal switching takes place within the amplifier,
its differential gain and phase parameters, which are 0.03%
and 0.03 degrees respectively, determine the circuit’s
performance. The other three circuits, U
operate in a similar manner.
ENABLE/DISABLE INPUT
FIGURE 8. SIMPLIFIED SCHEMATIC OF ENABLE/DISABLE
CC
+V
is +5V the DISABLE pin may be driven with a
CC
CC
FUNCTION
R
15K
7
if the DISABLE pin is left open. In those cases
5
holds the disable pin at ground thus
3
R
15K
6
can be changed if a different network
D
1
1A
8
in Figure 9, R
R
Q
R
CC
8
P3
10
1B
1
6
is greater than +5V
, is thrown to
will act as a pull-up
through U
1
terminates the
R
Q
CC
A
33
P18
1
4
back
will not
.
1D
,
HA5024
When the plus supply rail is 5V the disable pin can be driven by
a dedicated TTL gate as discussed earlier. If a multiplexer IC or
its equivalent is used to select channels its logic must be break
before make. When these conditions are satisfied the
HA5024IP is often used as a remote video multiplexer, and the
multiplexer may be extended by adding more amplifier ICs.
Low Impedance Multiplexer
Two common problems surface when you try to multiplex
multiple high speed signals into a low impedance source
such as an A/D converter. The first problem is the low source
impedance which tends to make amplifiers oscillate and
causes gain errors. The second problem is the multiplexer
which supplies no gain, introduces all kinds of distortion and
limits the frequency response. Using op amps which have an
enable/disable function, such as the HA5024, eliminates the
multiplexer problems because the external mux chip is not
needed, and the HA5024 can drive low impedance (large
capacitance) loads if a series isolation resistor is used.
VIDEO
INPUT
VIDEO
INPUT
VIDEO
INPUT
NOTES:
18. U
19. All resistors in
20. S
21. Use ground plane.
(NOTE 17)
#1
#3
#4
R
R
+5V IN
FIGURE 9. FOUR CHANNEL VIDEO MULTIPLEXER
R
75
R
75
75
75
11
16
1
1
1
6
is HA5024IP.
is break before make.
(NOTE 17)
(NOTE 17)
0.1 F
(NOTE 17)
100
100
100
681
100
R
R
681
681
681
R
R
13
18
3
8
U
U
U
U
1A
1B
13
12
1C
18
19
1D
3
2
8
9
15
R
R
+
681
+
681
+
681
+
681
-
-
-
-
R
R
6
-5V
14
+5V
17
12
17
4
2
7
7
10
1
20
11
10 F
2000
2000
2000
R
R
2000
R
15
20
+5V
R
R
R
R
75
5
R
75
75
75
14
19
10
4
9
-5V IN
0.1 F
VIDEO OUTPUT
TO
75 LOAD
3
2
4
ALL
OFF
1 R
S
1
100
10 F
21
-5V
+5V