
Data Sheet
AD8141/AD8142
Rev. A | Page 17 of 24
The desired differential output in this example is 1.4 V p-p
because the terminated input signal is 0.7 V p-p and the closed-
loop gain = 2. The actual differential output voltage is equal to
(0.725 V p-p)(2 k/1038.3 ) = 1.4 V p-p. This illustrates how
the two aforementioned effects cancel for large RF and RG.
DRIVING A CAPACITIVE LOAD
A purely capacitive load can react with the output impedance of
the drivers to reduce phase margin, resulting in high frequency
ringing in the pulse response. The best way to minimize this effect
is to place the source termination resistors immediately at the
amplifier outputs to minimize parasitic capacitances formed by
unnecessarily long traces.
DISABLE
significantly reduce the power consumed while simultaneously
placing the outputs in high-Z states. The disable feature can be
Figure 22 for the disabled input-to-output isolation, output
impedance, and response performance. The threshold levels
for the disable pin are listed in
Table 1.An output glitch occurs whenever the disable feature is asserted or
AD8142 SYNC-ON-COMMON-MODE
T
he AD8142 includes on-chip, sync-on-common-mode circuitry
that encodes externally applied HSYNC and VSYNC signals onto the
common-mode output voltages of each of the R, G, and B drivers.
The circuit encodes the horizontal and vertical sync pulses in a
way that results in low radiated energy. A simplified circuit that
illustrates how the pulses are encoded is shown i
n Figure 41.For a more detailed description of the sync scheme, see the
The sync-on-common-mode circuit generates a current based
on the voltage applied to the SYNC LEVEL input pin (Pin 18)
with respect to the negative supply. With SYNC LEVEL input
tied to VS, the common-mode output of all drivers is set at 1.5 V
above the negative supply. 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 1.5 V ± 0.5 V. The state of each VOUT,cm
output based on the HSYNC and VSYNC inputs is determined by
For the positive supplies between 2.5 V and 5 V, the sync-on-
common-mode circuit can be used by directly applying standard
HSYNC and VSYNC signals to the respective AD8142 inputs. These inputs adhere to standard logic thresholds (s
ee Table 1 for the
exact levels). The HSYNC and VSYNC inputs, therefore, can be
driven directly off the output of a computer video card without
concern of being overdriven. The input path from the 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.
H
V
H
R
H
V
VS–
BLUE VOCM
SYNC LEVEL
HSYNC
VSYNC
VS+
H
V
H
R
V
MIRROR
V
RED VOCM
GREEN VOCM
09461-
039
Figure 41. Sync-On-Common-Mode Simplified Circuit