AD8134ACPZ-R2 datasheet(15/20 Pages) AD

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Data Sheet

AD8134

Rev. B | Page 15 of 20

SYNC-ON-COMMON-MODE

The AD8134 drives RGB video signals over UTP cable. The

balance of the differential outputs is trimmed to ensure low

radiated energy from each of the twisted pairs. The common-

mode outputs of each of the R, G, and B differential outputs

are set using the circuit in Figure 32. This circuit embeds the

horizontal and vertical sync pulses on the three common-mode

outputs in a way that also results in low radiated energy. For a

more detailed description of the sync scheme, see the

Applications section.

The sync-on-common-mode circuit generates a current based

on the SYNC LEVEL input pin (Pin 18). With SYNC LEVEL

input tied to VS−, the common-mode output of all drivers is set

at (VS+ + VS−)/2. Using a resistor divider, a voltage can be

applied between VS− and SYNC LEVEL that determines the

maximum deviation of the common-mode outputs from their

midsupply level. If, for instance, SYNC LEVEL − VS− = 0.5 V

and the supply voltage is 5 V, then the common-mode outputs

fall within an envelope of 2.5 V ± 0.5 V. The state of each VOUT,cm

output based on the HSYNC and VSYNC inputs is determined by

the equations defined in the Applications section.

On a single 5 V supply, the sync-on-common-mode circuit can

be used by directly applying the HSYNC and VSYNC signals to the

respective AD8134 inputs. The logic thresholds of the HSYNC and

VSYNC inputs are nominally set at (VS+ − VS−)/4, using a resistor

divider with an impedance of approximately 200 kΩ. This

allows the inputs to be driven beyond the rails without logic

inversion and maintains fast switching speeds. The robustness

of the HSYNC and VSYNC inputs therefore allows them to be driven

directly off the output of a computer video card without concern of

overdriving the inputs. The input path from HSYNC and VSYNC

inputs to the switches in the current mode level-shifting circuit

are well matched to eliminate false switching transients. This

maximizes common-mode balance and minimizes radiated

energy.

The sync-on-common-mode circuit can be used with ±5 V

supplies, but in this case, the HSYNC and VSYNC logic signals

require level-shifting. Level-shifting details are provided in the

Applications section.

VS–

BLUE VOCM

SYNC LEVEL

HSYNC

VSYNC

VS+

MIRROR

RED VOCM

GREEN VOCM

Figure 32. Sync-On-Common-Mode Simplified Circuit

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