MAX11508/MAX11509
Low-Cost, 3-Channel, HD/PS/SD/BP
Video Filters with Buffer
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Detailed Description
The MAX11508/MAX11509 integrated filters offer 3
channels of SD, PS, or HD video, and include a bypass
mode useful for high-bandwidth 1080p and RGB video
signals. The MAX11508 6th-order lowpass filters provide
-3dB bandwidths of 9MHz (SD), 16MHz (PS), 33MHz
(HD), and 60MHz (BP). The MAX11509 6th-order low-
pass filters provide -3dB bandwidths of 10MHz (SD),
17MHz (PS), 34MHz (HD), and 60MHz (BP).
Two control inputs, FSEL0 and FSEL1, select the filter
mode for all filters (Table 1). Input bias circuitry on 2 fil-
ter channels can be enabled through the BIAS input to
offer simple connection to bipolar video signals such as
C, Pb, and Pr. Shutdown mode reduces device current
to 1µA (typ) and is enabled by forcing SHDN low.
Figure 1 shows a simplified block diagram of the
MAX11508/MAX11509.
The MAX11508 provides a flat passband response and
the MAX11509 provides a +0.8dB high-frequency
boost at 5MHz (SD), 8.5MHz (PS), and 20MHz (HD) to
help with system rolloff. No frequency boost is included
in bypass mode. Typical voltage waveforms are shown
in Figures 2 and 3.
Inputs
Transparent Clamps
All inputs feature transparent clamps to allow either AC-
or DC-coupling of the inputs. The clamp remains inac-
tive while the input signal is above ground, offering true
DC input coupling. When the signal goes below
ground, as is the case when it is AC-coupled, the
clamp sets the sync tip close to the ground level.
Input Coupling
The choice of AC- or DC-coupling the input depends
on the video source. Many DACs have a current output
and are terminated to ground with a resistor; such sig-
nals are conveniently DC-coupled. Use AC-coupling
when the DC level of the video signal is unknown or
outside the specified input range of the MAX11508/
MAX11509, such as SCART or V
CC
-terminated DAC
outputs. A bias network within 2 channels of the
MAX11508/MAX11509, for use with bipolar signals,
connects to the input node when BIAS is forced low.
Figures 4 and 5 show how the bias network operates.
DC-Coupled Inputs
When the input is DC-coupled, the voltage must remain
above zero, but not to exceed 1.4V (typ).
AC-Coupling and BIAS
When the input is AC-coupled, the transparent clamps
are active and set the lowest point of the signal at
ground. This is appropriate for unipolar signals such as
CVBS, Y, R, G, or B, with sync pulses (Figure 4). Force
BIAS high when coupling unipolar signals.
For bipolar signals, such as Pb and Pr, a bias network is
provided within the MAX11508/MAX11509 for inputs
Pb/B_IN and Pr/R_IN. Force BIAS low to connect the bias
network to these inputs (Figure 5). The internal network
biases AC-coupled inputs to a fixed DC voltage, typically
0.59V, to ensure that the transparent clamp remains off.
Table 1. Frequency-Selection Truth Table
