max9508atet Maxim Integrated Products, Inc., max9508atet Datasheet - Page 19

• Manufacturer Part Number: max9508atet
• Description: Max9508 Video Filter Amplifier With Smartsleep And Bidirectional Video Support
• Manufacturer: Maxim Integrated Products, Inc.

The MAX9508 filter passband is 6.75MHz, which makes
the device suitable for the higher bandwidth video sig-
nals from a DVD chip. Broadcast video signals actually
require less bandwidth because of channel limitations:
NTSC signals have a 4.2MHz bandwidth, and PAL sig-
nals have a 5MHz bandwidth. Video signals from a DVD
player are not channel limited; therefore, the bandwidth
of DVD video signals can push right against the Nyquist
limit of 6.75MHz. (Recommendation ITU-R BT.601-5
specifies 13.5MHz as the sampling rate for standard-
definition video). Therefore, the maximum bandwidth of
the signal is 6.75MHz. To ease the filtering require-
ments, most modern video systems oversample by two
times, clocking the video current DAC at 27MHz.
The video output amplifiers can both source and sink
load current, allowing output loads to be DC- or AC-
coupled. The amplifier output stage needs about
300mV of headroom from either supply rail. The
MAX9508 has an internal level-shift circuit that positions
the sync tip at approximately 300mV at the output. The
blank level of the chroma output is positioned at
approximately 1.3V if the blank level of the chroma
input signal is 0.5V. The blank level of the chroma out-
put is positioned at approximately 1.5V if the blank level
of the chroma input signal is 0.6V.
If the supply voltage is greater than 3.135V (5% below
a 3.3V supply), each amplifier can drive two DC-cou-
pled video loads to ground. If the supply is less than
3.135V, each amplifier can drive only one DC-coupled
or AC-coupled video load.
When SHDN is low, the MAX9508 draws typically less
than 10nA supply current. All the amplifier outputs
become high impedance. The effective output resis-
tance at the video outputs is 28kΩ, due to the internal
feedback resistors to ground.
______________________________________________________________________________________
SmartSleep and Bidirectional Video Support
Video Reconstruction Filter
CVBS/S-Video Filter Amplifier with
Shutdown
Outputs
YIN, CIN, and CVBSIN have high-impedance input
buffers and can work with source resistances as high as
300Ω. To reduce power dissipation in the video DACs,
the DAC output resistor can be scaled up in value. The
reference resistor that sets the reference current inside
the video DACs must also be similarly scaled up. For
instance, if the output resistor is 37.5Ω, the DAC must
source 26.7mA when the output is 1V. If the output resis-
tor is increased to 300Ω, the DAC only needs to source
3.33mA when the output is 1V.
There is parasitic capacitance from the DAC output to
ground. That capacitance in parallel with the DAC out-
put resistor forms a pole that can potentially roll off the
frequency response of the video signal. For example,
300Ω in parallel with 50pF creates a pole at 10.6MHz.
To minimize this capacitance, reduce the area of the
signal trace attached to the DAC output as much as
possible, and place the MAX9508 as close to the video
DAC outputs as possible.
The outputs can be AC-coupled since the output stage
can source and sink current as shown in
Coupling capacitors should be 220µF or greater to
keep the highpass filter formed by the 150Ω equivalent
resistance of the video transmission line to a corner fre-
quency of 4.8Hz or below. The frame rate of PAL sys-
tems is 25Hz, and the frame rate of NTSC systems is
30Hz. The corner frequency should be well below the
frame rate. SmartSleep only works with DC-coupled
loads.
Applications Information
Reducing Power Consumption
AC-Coupling the Outputs
in the Video DACs
Figure
10.
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