ISL59960_14 Datasheet, PDF (10/17 Pages) Intersil Corporation – An Automatic 960H and Composite Video Equalizer, Fully-Adaptive to 4000 Feet

ISL59960
Application Information
Unshielded Twisted Pair (UTP) Application
Circuit
Figure 9 shows the complete schematic for a 960MegaQ™
equalizer configured for unshielded twisted pair (UTP) cable. The
input signal is terminated into the network formed by R1, R2,
and R3. C1 and C2 AC-couple the signal into 960MegaQ™. To
protect the front-end circuitry, 5V transorbs (Z1 and Z2) should be
used instead of diodes because the signals on either differential
input may swing far enough below ground to turn on a diode and
distort the video.
On the output side, C5, R5, and C4 form a compensation
network, while R6 provides 75Ω source-termination for the video
output. 960MegaQ™ has an native gain of 6dB, so when VIDEO
OUT is terminated into 75Ω (the input to a DVR, TV, etc.), R6 and
the 75Ω terminator form a 2:1 divider, producing standard video
amplitude across the 75Ω terminator.
Coax Input Circuit
Figure 10 shows the input termination recommended for coaxial
cables. The differential termination resistance is now 75Ω to
match the characteristic impedance of the RG-59 coax cable. C3
bypasses high-frequency noise on the coax ground line to system
ground. This allows the coax ground to be independent of the
system at low frequencies (DC to 50/60Hz) to accommodate
differences in the ground potential of the remote video source(s).
The coax startup network (D1, R4, C4) prevents a rare start-up
condition that can occur when a high average-picture-level (e.g.,
white screen) video signal is present on the inputs before the
power has been applied.
VCC
C4
0.1µF
D1
10kΩ R4
1.0µF
Z1
5V
37.5Ω
R1 ^C1
1kΩ
TVS
37.5Ω
R3
R2 _C2
Z2
1.0µF
5V
C3
TVS
0.1µF
IN+
GND ^960MegaQTM
IN-
TVS = Transient Voltage Suppressor
a.k.a. Transorb
FIGURE 10. APPLICATION CIRCUIT FOR COAX CABLE
Dual UTP/Coax Input Circuit
If desired, it is also possible to support both UTP and coax cables
with the same PCB layout using two SPST switches that are
closed when in coax mode (Figure 11). Since UTP requires a
100Ω termination network while coax requires 75Ω, a switch to
introduce a shunt 300Ω resistor when in coax mode will change
the termination from 100Ω to 75Ω. A second switch is required
to engage C3. The addition of the coax startup circuit (D1, R4,
C4) can unbalance the capacitance of the differential pair and
degrade the CMRR in UTP applications. This in turn could cause
excess noise at long lengths of UTP. In UTP applications, if the
output signal is too noisy at long distances, an optional capacitor
Cx may be used to balance the capacitance of the differential
inputs. The value of Cx should be determined by calculating how
much trace capacitance is added by the coax startup circuit. A
typical value for a good layout is ~5pF. Note that only coax or UTP
should be connected at any one time - this circuit does not
multiplex between them.
VCC
C4
0.1µF
D1
UTP IN+ ^COAX
10kΩ
UTP IN-
300Ω
Z1
R5
5V
TVS SW1A
49.9Ω
49.9Ω
5.6pF
Cx* _Z2
5V
TVS
*optional
SW1B
C3
0.1µF
R4
1.0µF
C1
R1
R3
1kΩ
R2
C2
1.0µF
IN+
GND
IN-
Close all switches for
Coax
TVS = Transient Voltage Suppressor
a.k.a. Transorb
960MegaQ^TM
FIGURE 11. APPLICATION CIRCUIT FOR UTP/COAX CABLE
Using the ISL59960 with Steel Core Coax
Although the ISL59960 was designed for copper based cables, it
is capable of equalizing signals for coaxial applications that use
steel core cabling. Due to the higher DC and low-band resistance
of this cable type, the ISL59960 will not equalize to the same
distance as copper based cables. See Table 1 on page 9 for
maximum equalization distances.
Input Multiplexing
Placing a semiconductor multiplexer in front of this part may
increase high frequency attenuation and noise. However, a
low-capacitance mechanical relay may be acceptable. Note that
changing from one channel to another in Lock Until Reset mode
will require a reset (INVERT toggle) to trigger equalization of the
new channel (see “Lock Until RESET” on page 10).
For best performance, do not multiplex the inputs to the
equalizer - this can further degrade the signal. Instead, multiplex
at the output after equalization has been performed.
Stand-Alone Operation and Configuration
In its default stand-alone configuration, 960MegaQ™ features
two modes of automatic cable equalization: Lock Until Reset and
Continuous Update. Lock Until Reset is the recommended mode
for most applications.
LOCK UNTIL RESET
In the Lock Until Reset mode, once 960MegaQ™ finds the
optimum equalization and the LOCKED signal goes high, the
equalization is frozen and will not change until either the power
is cycled or the INVERT signal is toggled, which initiates a
re-equalization of the input signal. Re-equalization is usually only
necessary during device/system evaluation - in normal operation
960MegaQ™ powers-up, acquires and equalizes the signal, and
continues to equalize until/unless it is powered-down. If the
signal is lost in Lock Until Reset mode, the LOCKED pin will not
10
FN8358.0
December 18, 2012