MAX9583AZT Ver la hoja de datos (PDF) - Unspecified

Número de pieza

componentes Descripción

Lista de partido

MAX9583AZT

Dual, Triple, and Quad Standard-Definition Video Filter Amplifiers with DC-Coupled Input Buffers

Unspecified 

Dual, Triple, and Quad Standard-Definition Video

Filter Amplifiers with DC-Coupled Input Buffers

Detailed Description

The MAX9583/MAX9584/MAX9585 filter and amplify the

video DAC output in applications such as set-top boxes

and televisions. These devices consist of input buffers,

lowpass filters, and gain of 2V/V output amplifiers capa-

ble of driving a standard 150Ω video load to ground.

Inputs

The video inputs should be directly connected to the

output of the video current DAC. DC coupling ensures

that the input signals are ground referenced so that the

sync tip of composite or luma signals is within 50mV of

ground and the blank level of the chroma signal is

between 0.5V and 0.65V. Since the input buffers are

identical, any standard-definition video signal can be

applied to those inputs provided the signal is between

ground and 1.05V when VDD = 2.7V. For example,

three composite video signals could be applied to INA,

INB, and INC of the MAX9584. The RGB set or the

YPbPr set can also be inputs to INA, INB, and INC of

the MAX9584.

Video Filter

The filter passband (±1dB) is typically 7MHz, which

makes the device suitable for standard-definition video

signals from all sources (e.g., broadcast and DVD).

Broadcast video signals are channel limited: NTSC sig-

nals have 4.2MHz bandwidth and PAL signals have

5MHz bandwidth. Video signals from a DVD player,

however, are not channel limited, so the bandwidth of

DVD video signals can approach 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.

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

devices have 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.

Applications Information

Reducing Power Consumption

in the Video DACs

The MAX9583/MAX9584/MAX9585 have high-imped-

ance input buffers that 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 reference resistor that sets the reference cur-

rent 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, then 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 create a pole at 10.6MHz. To

minimize this capacitance, reduce the area of the sig-

nal trace attached to the DAC output as much as possi-

ble, and place the MAX9583/MAX9584/MAX9585 as

close as possible to the video DAC outputs.

AC-Coupling the Outputs

The outputs can be AC-coupled since the output stage

can source and sink current as shown in Figure 1.

Coupling capacitors should be 220µF or greater to

keep the highpass filter, formed by the 150Ω equivalent

resistance of the video transmission line, to a corner

frequency 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.

Power-Supply Bypassing and Ground

The MAX9583/MAX9584/MAX9585 operate from a single-

supply voltage down to 2.7V, allowing for low-power opera-

tion. Bypass VDD to GND with a 0.1µF capacitor. Place all

external components as close as possible to the device.

8 _______________________________________________________________________________________

Share Link: