HA5024_06 Datasheet, PDF (8/16 Pages) Intersil Corporation – Quad 125MHz Video Current Feedback Amplifier with Disable

HA5024
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
350µA 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_CC is +5V the DISABLE pin may be driven with a
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 D₁ will not
turn on, and the TTL gate will sink enough current to keep
point “A” at its proper voltage. When V_CC is greater than +5V
the DISABLE pin should be driven with an open collector
device that has a breakdown rating greater than V_CC.
Referring to Figure 8, it can be seen that R₆ will act as a pull-up
resistor to +V_CC if the DISABLE pin is left open. In those cases
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 20µA
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.
+V_CC
R₆
15K
R₁₀
R₃₃
D₁
R₇
R₈
15K
Q_P18
A
Q_P3
ENABLE/DISABLE INPUT
FIGURE 8. SIMPLIFIED SCHEMATIC OF ENABLE/DISABLE
FUNCTION
Typical Applications
Four Channel Video Multiplexer
Referring to the amplifier U_1A in Figure 9, R₁ terminates the
cable in its characteristic impedance of 75Ω, and R₄ back
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 R₃ can be changed if a different network
gain is desired. R₅ holds the disable pin at ground thus
inhibiting the amplifier until the switch, S₁, is thrown to
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_1B through U_1D,
operate in a similar manner.
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
#1
R₁
75
(NOTE 17)
100Ω ₃
2 ⁺_-
1
U_1A 4
R₃
681
R₂
681
R₄
75
R₅
2000
(NOTE 17) 100Ω
8
9 ⁺_-
10
R₆
U_{1B 7}
75
R₇
R₈
681
681
R₉
75
R₁₀
2000
-5V
VIDEO
INPUT
#3
(NOTE 17)
100Ω ₁₃
15
12 ⁺_- ¹¹
R₁₄
75
R₁₁
U_1C 14
75
R₁₃
R₁₂
R₁₅
681 ²⁰⁰⁰
681
VIDEO OUTPUT
TO 75Ω LOAD
1 ^R21
100
+5V
2
3
S₁
4
ALL
OFF
+5V
VIDEO
INPUT
#4
(NOTE 17)
100Ω ₁₈ 6
19 ⁺_-
20
R₁₉
75
R₁₆
U_1D 17
75
R₁₈
R₁₇
681
R₂₀
2000
681
+5V IN
+5V -5V IN
-5V
0.1µF
10µF
0.1µF
10µF
NOTES:
18. U₁ is HA5024IP.
19. All resistors in Ω.
20. S₁ is break before make.
21. Use ground plane.
FIGURE 9. FOUR CHANNEL VIDEO MULTIPLEXER
8
3550.6
February 8, 2006