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AD8178
Rev. 0 | Page 27 of 40
APPLICATIONS INFORMATION
OPERATING MODES
Depending on the state of the CMENC logic input, the AD8178
can be set in either of two differential-in, differential-out operat-
ing modes. In addition, monitors can be driven directly by tapping
the outputs single-ended and making use of the decoded H and
V sync outputs.
Middle-of-Cat-5-Run Application, CM Encoding Turned Off
In this application, the AD8178 is placed somewhere in the middle
of a Cat-5 run. By tying CMENC low, the CM of each RGB differ-
ential pair is removed through the device (or turned off), and
the overall CM at the output is defined by the reference value
VOCM_CMENCOFF. In this mode of operation, CM noise is
removed, while the intended differential RGB signals are buffered
and passed to the outputs. The AD8178 is placed in this operation
mode when used in a sync-on color scheme.
Figure 46 shows
the voltage levels and CM handling for a single input channel
connected to a single output channel in a middle-of-Cat-5-run
application with CM encoding turned off.
CMR
CMB
DIFF. R
DIFF. B
INPUT
OVERALL
CM
CMG
DIFF. G
CMENC
AD8178
CMR
CMB
DIFF. R
DIFF. B
OUTPUT
OVERALL
CM
CMG
DIFF. G
VOCM_CMENCOFF
0
660
8-
0
21
Figure 46. AD8178 in a Middle-of-Cat-5-Run Application, CM Encoding Off
(Note that in this application, the H and V outputs, though asserted, are not used.)
Input VBLK and Input VOCM_CMENCOFF allow the user
complete flexibility in defining the output CM level and the
amount of overlap between the positive and negative phases,
thus maximizing output headroom usage. Whenever VBLK
differs from VOCM_CMENCOFF by more than ±100 mV,
a differential voltage, Δdiff, is added at the outputs according
to the expression Δdiff = 2 × (VBLK VOCM_CMENCOFF.)
Conversely, whenever the difference between VBLK and
VOCM_CMENCOFF is less than ±100 mV, no differential
voltage is added at the outputs.
Middle-of-Cat-5-Run Application, CM Encoding Turned On
In this application, the AD8178 is also placed somewhere in the
middle of a Cat-5 run, although the common-mode handling is
different. By tying CMENC high, the CM of each RGB input is
passed through the part with a gain of +1, while at the same time,
the overall output CM is stripped and set equal to the voltage
applied at the VOCM_CMENCON pin. The AD8178 is placed in
this operation mode when used with a sync-on CM scheme.
Although asserted, the H and V outputs are not used in this
application.
Figure 47 shows the voltage levels and CM handling for a single
input channel connected to a single output channel in a middle-
of-Cat-5-run application with CM encoding turned on.
CMR
CMB
DIFF. R
DIFF. B
INPUT
OVERALL
CM
CMG
DIFF. G
CMENC
AD8178
CMR
CMB
DIFF. R
DIFF. B
OUTPUT
OVERALL
CM
CMG
DIFF. G
VOCM_CMENCON
0
6608
-022
Figure 47. AD8178 in Middle-of-Cat-5-Run Application, CM Encoding On
(Note that in this application, the H and V outputs, though asserted, are not used.)
In this operation mode, the difference Δdiff = 2 × (VBLK
VOCM_CMENCOFF) still adds an output differential voltage,
End-of-Cat-5-Run Application, CM Encoding Turned Off—
Driving a Monitor Directly
In this application, the AD8178 is placed at the end of a Cat-5
run to drive a monitor directly: the differential outputs are
tapped single-ended to drive the monitor inputs, CMENC is
tied to logic low to remove sync-on-CM information at the
output of the part, and the decoded H and V sync outputs are
tied to the sync inputs of the monitor.
The differential-in, differential-out gain of +4 provides a
differential-in, single-ended out gain of +2 at the output pins of
the AD8178. This yields the correct differential-in, single-ended
out gain of +1 at the input of the monitor.
The relationship between the incoming sync-on CM signaling
and the H and V syncs is defined according to
Table 16.
Table 16. H and V Sync Truth Table (VPOS/VNEG = ±2.5 V)
CMR
CMG
CMB
H
V
0.5
0
Low
High
0
0.5
Low
0.5
0
High
Low
0
0.5
High
The following two statements are equivalent to the truth table
(Table 16) in producing H and V for all allowable CM inputs:
H sync is high when the CM of Blue is larger than the CM
of Red.
V sync is high when the combined CM of Red and Blue is
larger than the CM of Green.