Therefore, a 6k

Ω resistor will produce a 1.13V key-

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

Ω)

11

9

7

5

3

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

0

113

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

1G’

Clamp

2B’

Clamp

3R’

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