MAX4028EWE 查看數據表（PDF） - Maxim Integrated
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MAX4028EWE Datasheet PDF : 12 Pages
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MAX4028/MAX4029
Triple/Quad, 2:1 Video
Multiplexer-Amplifiers with Input Clamps
Therefore, a 6kΩ resistor will produce a 1.13V
key-clamp voltage as shown in Figure 2. The clamp
voltage (VCLAMP) is measured at the output; the voltage
at the input is VCLAMP (sync tip or key clamp) divided by
the gain (+2V/V) + VBE.
In order for these clamps (sync tip or key) to work
properly, the input must be coupled with a 0.1µF capac-
itor (typ) with low leakage (<1µA to 2µA, max). Without
proper coupling, the clamp voltage will change during the
horizontal line time causing the “black level” to vary, chang-
ing the image brightness from left to right on the display. In
addition to the capacitor, a low resistance (≤ 75Ω) is
required on the source side to return the capacitor to
KEY-CLAMP REFERENCE VOLTAGE
vs. RKEYREF
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0
1
3
5
7
9 11 13
RKEYREF (kΩ)
Figure 2. Key-Clamp Reference Voltage vs. RKEYREF
ground. The clamps used here are active devices with the
coupling capacitor serving two functions; first, as a charge
reservoir to maintain the clamp voltage, and second, as
the compensation capacitor for the clamp itself. If an input
is not used, it must be terminated to avoid causing oscilla-
tions that could couple with another input.
In general, a sync-tip clamp is used for composite video
(Cvbs), gamma-corrected primaries (R’G’B’), and the
luma signal (Y) in S-video. A key clamp is preferred for
component color difference signals (Pb and Pr), linear
primaries (RGB in PCs), and chroma (C) in S-video. The
rule is to sync tip clamp a signal if sync is present and
key clamp all others. Several examples are given in the
Clamp/Key-Clamp Settings for Video Formats section.
Clamp/Key-Clamp Settings for Video Formats
Tables 1 and 2 provide the clamp settings on the MAX4028/
MAX4029 to interface with various video formats.
Low-Power, High-Impedance Disable Mode
All parts feature a low-power, high-impedance disable
mode that is activated by driving the DISABLE input
low. Placing the amplifier in disable mode reduces
the quiescent supply current and places the output
impedance at 2kΩ typically. Multiple devices can be
paralleled to construct larger switch matrices by
connecting the outputs of several devices together
and disabling all but one of the paralleled amplifiers’
outputs.
Table 1. MAX4028 Clamp Settings for Video Formats
INPUT
FORMAT
CLAMP/KEY
1
Cvbs1
Clamp
2
Cvbs2
Clamp
3
Cvbs3
Clamp
INPUT
1
2
3
FORMAT
Y
C
Cvbs
CLAMP/KEY
Clamp
Key
Clamp
INPUT
FORMAT
1
G’
2
B’
3
R’
R, G, B have sync on all.
INPUT
FORMAT
1
Gs
2
B
3
R
Gs, B, R have sync only on Green.
CLAMP/KEY
Clamp
Clamp
Clamp
CLAMP/KEY
Clamp
Key
Key
INPUT
1
2
3
FORMAT
Y
Pb
Pr
CLAMP/KEY
Clamp
Key
Key
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