LM1205 National Semiconductor, LM1205 Datasheet - Page 9

• Manufacturer Part Number: LM1205
• Description: 130 MHz/85 MHz RGB Video Amplifier System with Blanking
• Manufacturer: National Semiconductor

Figure 5 shows the block diagram of a typical analog RGB
Figure 5 block labeled ‘‘VIDEO AMPLIFICATION WITH
Figure 6 is a detailed block diagram of the green channel of
Applications Information
color monitor The RGB monitor is used with CAD CAM
work stations PC’s arcade games and in a wide range of
other applications that benefit from the use of color display
terminals The RGB color monitor characteristics may differ
in such ways as sweep rates screen size CRT color trio
spacing (dot pitch) or in video amplifier bandwidths but will
still be generally configured as shown in Figure 5 Separate
horizontal and vertical sync signals may be required or they
may be contained in the green video input signal The video
input signals are usually supplied by coax cable which is
terminated into 75
coupled to the video amplifiers These input signals are ap-
proximately 1V peak to peak in amplitude and at the input of
the high voltage video section approximately 5V peak to
peak At the cathode of the CRT the video signals can be as
high as 60V peak to peak One important requirement of the
three video amplifiers is that they match and track each
other over the contrast and brightness control range The
GAIN AND DC CONTROL’’ describes the function of the
LM1205 LM1207 which contains the three matched video
amplifiers contrast control and brightness control The
LM1205 LM1207 also provides the capability to blank at the
cathode of the CRT
Functional Description
the LM1205 LM1207 along with the recommended external
components The IC pin numbers are circled and all external
components are shown outside the dashed line The other
two video channels are identical to the green channel only
the numbers to the pins unique to each channel are differ-
ent The input video is normally terminated into 75
at the monitor input and internally AC
FIGURE 5 Typical RGB Color Monitor Block Diagram
(Continued)
The
9
termination resistor depends on the impedance of the coax
cable being used 75
used in video applications The video signal is AC coupled
through a 10
standard for the DC level of a video signal therefore the
signal must be AC coupled to the LM1205 LM1207 Internal
to the LM1205 LM1207 is a 2 8V reference giving the input
video an offset voltage of 2 8V This voltage was selected to
give the input video enough DC offset to guarantee that the
lowest voltage of the video signal at pin 6 is far enough
above ground to keep the LM1205 LM1207 in the active
region The 200
and for current limiting during any voltage surge that may
occur at the input driving pin 6 above V
signal is buffered by
verting amplifier is shown with a ‘‘
the amplifier designation The output of
contrast and drive attenuator sections
The contrast and drive control sections are virtually identi-
cal Both sections take a 0V to 4V input voltage 4V giving
the maximum gain for either the contrast or the drive This is
a high impedance input allowing for an easy interface to 5V
DACs One may also use 100k potentiometers with no deg-
radation in performance The contrast control section is
common to all three channels It converts the input voltage
at pin 12 to a couple of internal DC voltages that control the
gain of the contrast attenuator Referring to the Attenuation
vs Contrast Voltage under typical performance characteris-
tics note that a 4V control voltage results in no attenuation
of the video signal A 0 25V control voltage results in an
attenuation of 40 dB Again note that these internal control
voltages are common to all three channels To minimize
crosstalk these voltages go to pins 1 and 2 Minimizing
crosstalk is done by adding the RC network shown in the
block diagram (Figure 6)
F capacitor to the input pin 6 There is no
resistor at the input is for ESD protection
b
being the most common impedance
A1 In this circuit description an in-
b
’’ (minus sign) in front of
CC
TL H 11881 – 13
b
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The input video
A1 goes to the
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