MAX9505 Maxim Integrated Products, MAX9505 Datasheet - Page 10

• Manufacturer Part Number: MAX9505
• Description: DirectDrive Video Amplifier
• Manufacturer: Maxim Integrated Products

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DirectDrive Video Amplifier with
Reconstruction Filter and Analog Switch
Figure 3. AC-Coupled Output
Typically, the black level of the video signal created by
the video DAC is around 300mV. The MAX9505 shifts
the black level to ground at the output. Therefore, the
active video is above ground, and sync is below
ground. The amplifier needs a negative supply for its
output stage to remain in its linear region when driving
sync below ground.
The MAX9505 has an integrated charge pump and linear
regulator to create a low-noise negative supply from the
positive supply voltage. The charge pump inverts the
positive supply to create a raw negative voltage that is
then fed into the linear regulator, which outputs -2V. The
linear regulator filters out the charge-pump noise.
The actual level of the video signal varies less with a
DirectDrive output than an AC-coupled output. The
video signal average can change greatly depending
upon the picture content. With an AC-coupled output,
the average will change according to the time constant
formed by the series capacitor and series resistance
(usually 150Ω). For example, Figure 3 shows an AC-
coupled video signal alternating between a completely
black screen and a completely white screen. Notice the
excursion of the video signal as the screen changes.
With the DirectDrive amplifier, the black level is held at
ground. The video signal is constrained between
-0.3V to +0.7V. Figure 4 shows the video signal from a
DirectDrive amplifier with the same input signal as the
AC-coupled system.
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0V
0V
Comparison Between DirectDrive Output
2ms/div
and AC-Coupled Output
Video Amplifier
INPUT
500mV/div
OUTPUT
500mV/div
The MAX9505 features an internal 4.4Ω (max) analog
normally open switch that interfaces with a 4-pole multi-
media jack in which one of the poles can be set to a
video input, a video output, or a microphone input. The
switch has three modes of operation: open, normal
switch, and extended range switch. In normal switch
mode, the analog range is from 0V to V
range switch mode, the analog range is from -2V to
V
log switch operating modes (Table 1).
Before the video signal from the DAC can be amplified,
it must be lowpass filtered to smooth the steps and to
reduce the spikes created whenever the DAC output
changes value. In the frequency domain, the steps and
spikes cause images of the video signal to appear at
multiples of the sampling clock. The MAX9505 contains
a six-pole Butterworth lowpass filter. The passband
extends to 5.5MHz, and the minimum attenuation is
35dB at 27MHz.
The MAX9505 uses two logic inputs to set the device
into one of the following modes: 1) shutdown mode, 2)
extended-range switch mode, 3) video output mode,
and 4) normal switch mode.
In video output mode, the video circuitry is on, the
charge pump is on, and the analog switch is open. In
normal switch mode, the analog switch is closed, and
the video circuitry and charge pump are off. In the
extended-range switch mode, the charge pump is on
and the analog switch is closed. Shutdown mode
reduces the supply current to 10nA typically (see Table
1 and Typical Application Circuits).
Figure 4. DirectDrive Output
DD.
Logic inputs MODE1 and MODE0 control the ana-
0V
0V
Video Reconstruction Filter
2ms/div
Operating Modes
Analog Switch
DD
. In extended-
INPUT
500mV/div
OUTPUT
500mV/div