Datasheet
Data Sheet ADA4858-3
Rev. B | Page 15 of 20
R
220µA
G
220µA
220µA
V1
74AC86
74AC86
74AC86
U1
ADA4858-3
75Ω
R
301Ω
301Ω
75Ω
4.7nF
V2
U2
75Ω
G
301Ω
301Ω
75Ω
4.7nF
V3
U3
75Ω
B
301Ω
301Ω
75Ω
4.7nF
B
H
ADCMP371AKSZ
200kΩ
0.1µF
+5V
2.8kΩ7.15kΩ
NTA4153
NTA4153
NTA4153
07714-100
Figure 43. AC-Coupled Video Input with DC Restored Output
DC RESTORE FUNCTION
Having a charge pump gives the ability to take an ac-coupled
input signal and restore its dc 0 V reference. The simplest way
of accomplishing this is to use the blanking interval and the H-
sync signal to set the 0 V reference. Use the H-sync to sample the
dc level during the blanking interval to charge a capacitor and
hold the charge during the video signal. Figure 43 shows the
schematic of the dc restored circuit.
The H-sync coming out of the video source can be either positive
or negative. This is why a polarity correction circuit is used to
produce only a positive going H-sync. The H-sync is fed to a
comparator that produces a high voltage if H-sync is negative and
a low voltage if the H-sync is positive. The H-sync is then fed to
an XOR with the output of the comparator. If the original H-sync
was negative, the output of the XOR is positive because of the
logic high coming from the comparator, causing the XOR to act
as an inverter. However, if the original H-sync is positive, it stays
the same because the output of the comparator is low and the
XOR acts as a buffer.
The result is a positive going H-sync triggering the MOSFET
during the blanking interval. This shorts the 4.7 nF capacitor to
ground, which causes it to charge up by the dc level of the current
signal. When the H-sync goes low, the MOSFET opens and the
capacitor holds the charge during the video signal, making the
output signal referenced to ground or 0 V level.
CLAMP AMPLIFIER
In some applications, a current output DAC driving a resistor
may not have a negative supply available. In such case, the YPbPr
video signal may be shifted up by 300 mV to avoid clamping the
sync tip. These applications require a signal dc clamp on the output
of the video driver to restore the dc level to 0 V reference. The
ADA4858-3 has a charge pump that allows the output to swing
negative; twice the sync tip (−600 mV) in G = 2 configuration.
Figure 44 shows the ADA4858-3 in a difference amplifier
configuration. The video signal is connected to the noninverting
side, and a dc bias of 600 mV is injected on the inverting side.
07714-101
U1
ADA4858-3
R12
75Ω
Y
R1
301Ω
R2
301Ω
R7
75Ω
V
CC
= 5V
DAC1
Y
U2
R13
75Ω
Pb
R3
301Ω
R4
301Ω
R8
75Ω
V
CC
= 5V
DAC2
Pb
U3
R14
75Ω
Pr
R5
301Ω
R6
301Ω
R9
75Ω
V
CC
= 5V
DAC3
Pr
V1
R10
44.2kΩ
R11
6.02kΩ
ADA4860-1
V
CC
= 5V
V
CC
= 5V
C1
0.1µF
C2
10µF
Figure 44. Clamp Amp