AD8128ACPZ-R2 View Datasheet(PDF) - Analog Devices
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Description
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AD8128ACPZ-R2 Datasheet PDF : 12 Pages
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AD8128
APPLICATIONS INFORMATION
KVM APPLICATIONS
In KVM applications, cable equalization typically occurs at the
root of the KVM network. In a star configuration, a driver is
located at each of the end nodes and a receiver/equalizer is
located at the single root node. In a daisy-chain configuration,
each of the end nodes are connected to one another, and one of
them is connected to the root. Similarly, the drivers are placed
on the nodes, and the receivers/equalizers are placed at the root.
In both of these aforementioned configurations, three AD8128
receiver/equalizers can be used at the root node to equalize the
transmitted red (R), green (G), and blue (B) channels for up to
100 meters of cable. Since the skew between two pairs of cables
in CAT-5 is less than 1%, the control pins can be tied together
and used as a single set of controls.
If the common-mode levels of the inputs permit using the
AD8128 as a receiver (see the Input Common-Mode Voltage
Range Considerations section), the input signal must be
terminated by a 100 Ω shunt resistor between the pairs, or by
two 50 Ω shunt resistors with a common-mode tap in the middle.
This CM tap can extract the sync information from the signal if
sync-on-common-mode is used.
VDIFF
VCM
50Ω
CAT-5
50Ω
VCM
VDIFF
VCM
VIN+
VIN–
HPF
HPF
LPF
AD8128
VOUT
VPEAK VGAIN
VOFFSET
Figure 18. Single Receiver Configuration for CAT-5 Equalizer
DC CONTROL PINS
The AD8128 uses two control pins (VGAIN and VPEAK) to adjust
the equalization based on the length of the cable and one pin
(VOFFSET) to adjust the dc output offset. VGAIN is a user-adjustable
0 V to 1 V broadband gain control pin, and VPEAK is a 0 V to 1 V
adjustable high frequency gain pin to equalize for the skin effect
in CAT-5 cable. The values of both VPEAK and VGAIN are linearly
correlated to the length of the cable to be equalized. A simple
formula can approximate the desired values for both of these pins.
VGAIN
length(m)
425m/V
(3)
VPEAK
length(m)
110m/V
(4)
Data Sheet
While these equations give a close approximation of the desired
value for each pin, to achieve optimal performance, it can be
necessary to adjust these values slightly.
Figure 19 and Figure 20 illustrate circuits that adjusts the
control pins on the AD8128. In Figure 19, a 1 kΩ potentiometer
adjusts the control pin voltage between the specified range of 0 V to
1 V. In Figure 20, a 2 kΩ potentiometer controls the offset pin from
−2.5 V to +2.5 V. For both of these configurations, a ±5V supply
is assumed.
+5V
4kΩ
1kΩ
CONTROL PIN
VGAIN OR VPEAK
0.01µF
Figure 19. Circuit to Control VGAIN and VPEAK (0 V to 1 V)
+5V
1kΩ
2kΩ
OFFSET
0.01µF
1kΩ
–5V
Figure 20. Circuit to Control VOFFSET (±2.5 V)
Rev. A | Page 10 of 12
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