MAX9512ATE 데이터 시트보기 (PDF) - Maxim Integrated

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MAX9512ATE

Video Filter Amplifier with SmartSleep and Y/C Mixer Circuit

Maxim Integrated 

Video Filter Amplifier with SmartSleep

and Y/C Mixer Circuit

Inputs

The MAX9512 video inputs YIN and CIN should be

directly connected to the output of the video current

DAC. DC-coupling ensures that the input signals are

ground referenced such that the sync tip of the luma

signal is within 50mV of ground and the blank level of

the chroma signal is between 0.5V and 0.65V.

Video Reconstruction Filter

The MAX9512 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 4.2MHz bandwidth, and PAL signals

have 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 ease the filtering requirements, most

modern video systems oversample by two times, clock-

ing the video current DAC at 27MHz.

Y/C Mixer

The Y/C mixer adds the luma and chroma signals

together to create a composite video signal. Since

chroma is a phase modulated carrier at 3.58MHz for

NTSC and 4.43MHz for PAL, the chroma signal is AC-

coupled into the Y/C mixer so that the variation in blank

level from one video source to another video source

does not affect the DC bias of the composite video sig-

nal. The highpass corner frequency of the chroma AC-

coupling circuit is 300kHz.

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 about

300mV of headroom from either supply rail. The

MAX9512 has an internal level shift circuit that positions

the sync tip at approximately 300mV at the output. The

blank level of the chroma output is positioned at

approximately 1.3V if the blank level of the chroma

input signal is 0.5V. The blank level of the chroma out-

put is positioned at approximately 1.5V if the blank level

of the chroma input signal is 0.6V.

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 MAX9512 draws less than 1µA

supply current. All the amplifier outputs become high

impedance. The effective output resistance at the video

outputs is 28kΩ, due to the internal feedback resistors

to ground.

Applications Information

Reducing Power Consumption in

the Video DACs

YIN and CIN have high-impedance input buffers and

can work with source resistances as high as 300Ω. To

reduce power dissipation in the video DACs, the DAC

output resistor can be scaled up in value. The refer-

ence resistor that sets the reference current inside the

video DACs must also be similarly scaled up. For

instance, if the output resistor is 37.5Ω, the DAC must

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

resistor is increased to 300Ω, 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,

300Ω 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 MAX9512 as close to the video

DAC outputs as possible.

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