8
FN6104.5
February 12, 2014
presenting a 0V input to most single supply driven amplifiers
will saturate the output stage of the amplifier resulting in a
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 is 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, thereby saving board space and
additional expense for capacitors. This makes the ISL59110
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
R
6
R
5
R
4
R
7
C
5
C
4
R
OUT
R
L
SAG
NETWORK
AC COUPLING
CAPACITOR
-
+
TELEVISION
OR VCR
R
OUT
ENABLE
ISL59110
