Interfacing to Video DACs that Produce

the output of the video DAC can work with the

MAX9532. Most video DACs source current into a

ground-referenced resistor, which converts the current

into a voltage. Figure 3 shows a video DAC that creates

a video signal from 0V to 1V across a 150

Ω resistor.

With a gain of 2V/V, the following video filter produces a

nominally. The video DAC in Figure 3 can be made to

work with the MAX9532 by scaling down the 150

Ω

resistor to a 75

Ω resistor, as shown in Figure 4. The

75

Ω resistor is one-half the size of the 150Ω resistor,

resulting in a video signal that is one-half the amplitude.

Video Source with a Positive DC Bias

In some applications, the video source generates a sig-

nal with a positive DC voltage bias, i.e., the sync tip of

the signal is well above ground. Figure 5 shows an

example in which the outputs of the luma (Y) DAC and

the chroma (C) DAC are connected together. Since the

DACs are current-mode, the output currents sum

together into the resistor, which converts the resulting

current into a voltage representing a composite video

signal.

When the chroma DAC is connected to an independent

output resistor to ground, the chroma signal, which is a

carrier at 3.58MHz for NTSC or at 4.43MHz for PAL,

generates a positive DC bias to keep the signal above

ground at all times. When the luma DAC is connected

to an independent output resistor to ground, the luma

signal usually does not have a positive DC bias, and

the sync tip is at approximately ground. When the chro-

ma and luma signals are added together, the resulting

composite video signal generates a positive DC bias.

Therefore, the signal must be AC-coupled into the

MAX9532 because the composite video signal is above

the nominal 0V to 0.7V DC-coupled input range.

Video Signal Routing

Minimize the length of the PCB trace between the out-

put of the video DAC and the input of the MAX9532 to

reduce coupling of external noise into the video signal.

If possible, shield the PCB trace.

DirectDrive Video Amplifier with

Short-to-Battery Protection

_______________________________________________________________________________________

9

LPF

DAC

IMAGE

PROCESSOR

ASIC

2V/V

150

Ω

0V TO 1V

GENERIC 2V/V CONFIGURATION

Across a 150

Ω Resistor Connected to Ground

LPF

DAC

IMAGE

PROCESSOR

ASIC

4V/V

75

Ω

0V TO 0.5V

MAX9532

Resistor Connected to Ground

VIDEO SIGNAL

MAX9532 POWER CONSUMPTION (mW)

WITH 150

Ω LOAD

MAX9532 POWER CONSUMPTION (mW)

WITH 100

Ω LOAD

All Black Screen

51.236

53.978

All White Screen

57.077

65.399

75% Color Bars

53.074

57.486

50% Flat Field

49.513

51.596

Table 1. Power Consumption of the MAX9532 with Different Video Signals

Note: The supply voltage is 3.3V.