8

FN6104.1

August 8, 2005

clipped sync tip and degrading the video image. A larger

positive reference may offset the input above its positive

range.

The ISL59110 features an internal sync clamp and offset

function to level shift the entire video signal to the best level

before it reaches the input of the amplifier stage. These

features are also helpful to avoid saturation of the output

stage of the amplifier by setting the signal closer to the best

voltage range.

The simplified block diagram of the ISL59110 in Figure 24 is

divided into four sections. The first, Section A is the Sync

Clamp. The AC coupled video sync signal is pulled negative

by a current source at the input of the comparator amplifier.

When the sync tip goes below the comparator threshold the

output comparator is driven negative, The PMOS device

turns on clamping sync tip to near ground level. The network

triggers on the sync tip of video signal.

The Sallen Key Low Pass Filter

The Sallen Key in a classic low pass configuration illustrated

in Figure 24. This provides a very stable low pass function,

and in the case of the ISL59110, a three-pole roll-off at

around 8MHz. The three-pole function is accomplished with

an RC low pass network placed in series with and before the

Sallen Key. One pole provided by the RC network and poles

two and three provided by the Sallen Key for a nice three-

pole roll-off at around 8MHz. If more aggressive, multiple-

pole roll-offs are needed, multiple ISL59110 can be placed in

series. There will, of course, be a loss of bandwidth as

additional devices are added.

AC Output Coupling and the SAG Network

Composite video signals carry viable information at

frequencies as low as 30Hz up to 5MHz. When a video

system output is AC coupled it is critical that the filter

represented by the output coupling capacitor and the

surrounding resistance network provide a band pass

function with a low pass band low enough to exclude very

low frequencies down to DC, and with a high pass band pass

sufficiently high to include frequencies at the higher end of

the video spectrum.

FIGURE 25. SAG NETWORK AND AC COUPLING CAPACITORS

Typically this is accomplished with 220µF coupling capacitor,

a large and somewhat costly solution providing a low

frequency pole around 5Hz. If the size of this capacitor is

even slightly reduced we have found that the accompanying

phase shift in the 50Hz to 100Hz frequency range results in

field tilt resulting in a degraded video image.

The internal SAG network of the ISL59110 replaces the

220µF AC coupling capacitor with a network of two smaller

capacitors as shown in Figure 25. Additionally, the network is

designed to place a zero in the ~30Hz range, providing a

small amount of peaking to compensate the phase response

associated with field tilt.

DC Output Coupling

The ISL59110 internal sync clamp makes it possible to DC

couple the output to a video load, eliminating the need for

any AC coupling capacitors, saving board space and

additional expense for capacitors making the ISL59110 is

designed to be extremely attractive for portable video

applications Additionally, this solution completely eliminates

the issue of field tilt in the lower frequency. The trade off is

greater demand of supply current. Typical load current for

AC coupled is around 3mA compared to typical 6mA used

when DC coupling.

FIGURE 26. DC COUPLE

SAG

NETWORK

AC COUPLING

CAPACITOR

-

+

TELEVISION

OR VCR

ENABLE

ISL59110