Rainbow Electronics MAX4029 - Manual (Page 8)

MAX4028/MAX4029
Therefore, a 6k Ω resistor will produce a 1.13V key-
clamp voltage as shown in Figure 2. The clamp voltage
(V CLAMP ) is measured at the output; the voltage at the
input is V CLAMP (sync tip or key clamp) divided by the
gain (+2V/V) + V
BE
.
In order for these clamps (sync tip or key) to work proper-
ly, the input must be coupled with a 0.1µF capacitor (typ)
with low leakage (<1µA to 2µA, max). Without proper cou-
pling, the clamp voltage will change during the horizontal
line time causing the “black level” to vary, changing the
image brightness from left to right on the display. In addi-
tion to the capacitor, a low resistance ( ≤75Ω) is required
on the source side to return the capacitor to ground. The
clamps used here are active devices with the coupling
capacitor serving two functions; first, as a charge reser-
voir 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 imped-
ance at 2k
Ω typically. Multiple devices can be paral-
leled to construct larger switch matrices by connecting
the outputs of several devices together and disabling
all but one of the paralleled amplifiers’ outputs.
Triple/Quad, 2:1 Video
Multiplexer-Amplifiers with Input Clamps
8 _______________________________________________________________________________________
KEY-CLAMP REFERENCE VOLTAGE
vs. R KEYREF
MAX4028 fig02
R KEYREF (k Ω)
KEY-CLAMP REFERENCE VOLTAGE (V)
11 9 7 5 3
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
0
1 13
Figure 2. Key-Clamp Reference Voltage vs. R KEYREF
Table 1. MAX4028 Clamp Settings for Video Formats
INPUT FORMAT CLAMP/KEY
1 Cvbs1 Clamp
2 Cvbs2 Clamp
3 Cvbs3 Clamp
INPUT FORMAT CLAMP/KEY
1 G’ Clamp
2 B’ Clamp
3 R’ Clamp
R, G, B have sync on all.
INPUT FORMAT CLAMP/KEY
1 Gs Clamp
2 B Key
3 R Key
Gs, B, R have sync only on Green.
INPUT FORMAT CLAMP/KEY
1 Y Clamp
2 C Key
3 Cvbs Clamp
INPUT FORMAT CLAMP/KEY
1 Y Clamp
2 Pb Key
3 Pr Key