CVBS Video Filter Amplifier with SmartSleep

and Bidirectional Video Support

18

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Inputs

The MAX9513 CVBSIN input should be directly con-

nected to the output of the video current DAC. DC-cou-

pling ensures that the input signals are ground

referenced such that the sync tip of CVBSIN is within

50mV of ground.

The EXTCVBSIN input can only handle signals with a

sync pulse, such as composite video and luma.

Because the DC level of an external video input signal

is unknown, the external video signal is AC-coupled

through a 0.1µF capacitor, and a sync tip clamp sets

the internal DC level of the MAX9513. A 2:1 multiplexer

under the control of the INT/EXT input selects either the

signal at CVBSIN or EXTCVBSIN.

Video Reconstruction Filter

The MAX9513 filter passband is 6.75MHz, which makes

the device suitable for the higher bandwidth video sig-

nals from a DVD chip. Broadcast video signals actually

require less bandwidth because of channel limitations:

NTSC signals have a 4.2MHz bandwidth, and PAL sig-

nals have a 5MHz bandwidth. Video signals from a DVD

player are not channel limited; therefore, the bandwidth

of DVD video signals can push right against the Nyquist

limit of 6.75MHz. (Recommendation ITU-R BT.601-5

specifies 13.5MHz as the sampling rate for standard-

definition video). Therefore, the maximum bandwidth of

the signal is 6.75MHz. To reduce the filtering require-

ments, most modern video systems oversample the

input by two times, thus clocking the video current DAC

at 27MHz.

Outputs

The video output amplifiers can both source and sink

load current, allowing output loads to be DC- or AC-

coupled. The amplifier output stage needs approxi-

mately 300mV of headroom from either supply rail. The

MAX9513 has an internal level-shift circuit that positions

the sync tip at approximately 300mV at the output.

If the supply voltage is greater than 3.135V (5% below

a 3.3V supply), each amplifier can drive two DC-cou-

pled video loads to ground. If the supply is less than

3.135V, each amplifier can drive only one DC-coupled

or AC-coupled video load.

Shutdown

When SHDN is low, the MAX9513 typically draws less

than 10nA supply current. All the amplifier outputs

become high impedance. The effective output resis-

tance at the video outputs is 28k

Ω, due to the internal

feedback resistors to ground.

Applications Information

Reducing Power Consumption

in the Video DACs

CVBSIN has a high-impedance input buffer and can

work with source resistances as high as 300

Ω. To

reduce power dissipation in the video DACs, the DAC

output resistor can be scaled up in value. The reference

resistor that sets the reference current inside the video

DACs must also be similarly scaled up. For instance, if

the output resistor is 37.5

Ω, the DAC must source

26.7mA when the output is 1V. If the output resistor is

increased to 300

Ω, the DAC only needs to source

3.33mA when the output is 1V.

There is parasitic capacitance from the DAC output to

ground. That capacitance in parallel with the DAC out-

put resistor forms a pole that can potentially roll off the

frequency response of the video signal. For example,

300

Ω in parallel with 50pF creates a pole at 10.6MHz.

To minimize this capacitance, reduce the area of the

signal trace attached to the DAC output as much as

possible, and place the MAX9513 as close to the video

DAC outputs as possible.

AC-Coupling the Outputs

The outputs can be AC-coupled since the output stage

can source and sink current as shown in Figure 9.

Coupling capacitors should be 220µF or greater to keep

the highpass filter formed by the 150

Ω equivalent resis-

tance of the video transmission line to a corner frequen-

cy of 4.8Hz or below. The frame rate of PAL systems is

25Hz, and the frame rate of NTSC systems is 30Hz. The

corner frequency should be well below the frame rate.

SmartSleep only works with DC-coupled loads.

Power-Supply Bypassing and Ground

The MAX9513 operates from a single-supply voltage

down to 2.7V, allowing for low-power operation. Bypass

components as close as possible to the device.