Single, Dual, Triple, and Quad Standard-Definition

Video Filter Amplifiers with AC-Coupled Input Buffers

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Detailed Description

The MAX9586–MAX9589 filter and amplify the video

DAC output in applications such as set-top boxes and

televisions. These devices consist of input clamps,

input bias circuits, lowpass filters, and gain of 2V/V out-

put amplifiers capable of driving a standard 150Ω

video load to ground.

Inputs

The input stages of the MAX9586–MAX9589 are either

sync-tip clamps or bias circuits. Sync-tip clamps accept

AC-coupled CVBS or luma video signals with sync puls-

es. The sync-tip voltage is internally set to 300mV. Bias

circuit inputs accept AC-coupled chroma, a subcarrier

modulated with the color information. The bias voltage

of the bias circuits is approximately 500mV.

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.

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 (MAX9586)

The MAX9586 draws less than 1µA supply current

when SHDN is low. In shutdown mode, the amplifier

output becomes high impedance.

Applications Information

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 MAX9586–MAX9589 operate from a single-supply

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

external components as close as possible to the device.