MAX9507
Interfacing to Video DACs that Produce
Video Signals Larger than 0.25V
P-P
Devices designed to generate 1V
P-P
video signals at
the output of the video DAC can still work with the
MAX9507. Most video DACs source current into a
ground-referenced resistor, which converts the current
into a voltage. Figure 12 shows a video DAC that cre-
ates a video signal from 0 to 1V across a 150Ω resistor.
The following video filter amplifier has a 2V/V gain so
that the output is 2V
P-P
.
The MAX9507 expects input signals that are 0.25V
P-P
nominally. The same video DAC can be made to work
with the MAX9507 by scaling down the 150Ω resistor to
a 37.5Ω resistor, as shown in Figure 13. The 37.5Ω resis-
tor is one-quarter the size of the 150Ω resistor, resulting
in a video signal that is one-quarter the amplitude.
Changing Between Video Output and
Microphone Input on a Single Connector
Figure 14 shows how a single pole on a mobile phone
jack can be used for transmitting a video signal to a
television or receiving the signal from the microphone
of a headset. To transmit a video signal, open SW1 and
enable the video circuitry. To receive a signal from a
microphone, close SW1 and disable the video circuitry.
Switching Between Video
and Digital Signals
Figure 15 shows how the dual SPST analog switches
and the high-impedance output of the video amplifier
enable video transmission, digital transmission, and
digital reception all on a single pole of a connector. To
transmit a video signal, open SW1 and SW2 and enable
the video circuitry. To receive a digital signal, close
SW1, open SW2, and disable the video circuitry. To
transmit a digital signal, open SW1, close SW2, and
disable the video circuitry.
Selecting Between Two Video Sources
The analog switches can multiplex between two video
sources. For example, a mobile phone might have an
application processor with an integrated video encoder
and a mobile graphics processor with an integrated
video encoder, each creating a composite video signal
that is between 0 and 0.25V. Figure 16 shows this appli-
cation in which the MAX9507 chooses between two inter-
nal video sources. The two analog switches can be used
as a 2:1 multiplexer to select which video DAC output is
filtered, amplified, and driven out to the connector.
If the analog switches are in extended mode, then they
can also be used to select between two external video
signals, as shown in Figure 17. The external video sig-
nals are usually between -2V and +2V. The resistor net-
work divides the external signal by a factor of four,
thereby reducing the signal to between -0.5V and +0.5V
(see the
Anti-Alias Filter
section for an explanation on
why the resistor-divider network is necessary). In extend-
ed mode, the analog switch can easily handle this bipo-
lar input signal, even if the supply voltage is 1.8V.
1.8V DirectDrive Video Filter Amplifier with
Load Detection and Dual SPST Analog Switches
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