AD8134 Data Sheet
Rev. B | Page 18 of 20
–OUT R
+OUT R
–OUT G
+OUT G
–OUT B
+OUT B
V
OCM
WEIGHTING EQUATIONS:
RED V
OCM
=
K
(V
SYNC
– H
SYNC
) + V
MIDSUPPLY
GREEN V
OCM
=
K
(–2V
SYNC
) + V
MIDSUPPLY
BLUE V
OCM
=
K
(V
SYNC
+ H
SYNC
) + V
MIDSUPPLY
+IN R
–IN R
V
SYNC
H
SYNC
SYNC LEVEL
+IN G
–IN G
+IN B
–IN B
R
1.5kΩ
1.5kΩ
AD8134
750Ω
750Ω
G
1.5kΩ
1.5kΩ
750Ω
750Ω
V
OCM
V
OCM
V
OCM
B
1.5kΩ
1.5kΩ
750Ω
750Ω
OPD
×2
04770-003
2
2
2
Figure 34. AD8134 Sync-On-Common-Mode Encoding Scheme
0
0.5
1.0
1.5
2.0
4.0
3.5
4.5
2.5
3.0
5.0
0.98 0.99 1.00 1.01 1.03
1.04 1.051.02 1.06 1.07
TIME (µs)
H
SYNC
V
SYNC
2.0
2.1
2.2
2.3
2.4
2.9
2.8
3.0
2.5
2.6
2.7
3.1
R
G
B
04770-004
Figure 35. AD8134 Sync-On-Common-Mode Signals in Single 5 V Application
The transmitted common-mode sync signal magnitudes are
scaled by applying a dc voltage to the SYNC LEVEL input,
referenced to the negative supply. The difference between the
voltage applied to the SYNC LEVEL input and the negative
supply sets the peak deviation of the encoded sync signals about
the midsupply common-mode voltage. For example, with the
SYNC LEVEL input set at V
S−
+ 500 mV, the deviation of the
encoded sync pulses about the nominal midsupply common-
mode voltage is typically ±500 mV. The equations in Figure 34
describe how the V
SYNC
and H
SYNC
signals are encoded on each
color’s midsupply common-mode signal. In these equations, the
weights of the V
SYNC
and H
SYNC
signals are ±1 (+1 for high, −1
for low), and the constant K is equal to the peak deviation of the
encoded sync signals.
Figure 35 shows how the sync signals appear on each common-
mode voltage in a single 5 V supply application when the
voltage applied to the SYNC LEVEL input is 500 mV. A typical
setting for the SYNC LEVEL voltage is 500 mV above the
negative supply.
LEVEL-SHIFTING SYNC PULSES ON ±5 V SUPPLIES
The vertical and horizontal sync pulses received from a
computer video port are generally referenced to ground. When
using ±5 V supplies, these pulses must be level-shifted before
being applied to the negative-supply referenced V
SYNC
and H
SYNC
inputs because these inputs are referenced to the negative
supply. The circuit shown in Figure 36 provides the proper sync
pulse level-shifting for a negative supply voltage of −5 V. The
vertical and horizontal sync pulses each require a level-shift
circuit.
6.04kΩ 2.21kΩ
LEVEL-SHIFTED
SYNC PULSE
TO AD8134
GROUND-REFERENCED
SYNC PULSE
1kΩ
V
S
–
2N3906
04770-035
Figure 36. Level-Shifting Sync Pulses on ±5 V Supplies
