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

2.8kΩ

7.15kΩ

NTA4153

NTA4153

NTA4153

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.

U1

ADA4858-3

R12

75Ω

Y

R1

301Ω

R2

301Ω

R7

75Ω

DAC1

Y

U2

R13

75Ω

Pb

R3

301Ω

R4

301Ω

R8

75Ω

DAC2

Pb

U3

R14

75Ω

Pr

R5

301Ω

R6

301Ω

R9

75Ω

DAC3

Pr

V1

R10

44.2kΩ

R11

6.02kΩ

ADA4860-1

C1

0.1µF

C2

10µF

Figure 44. Clamp Amp