ADA4858-3 Analog Devices, ADA4858-3 Datasheet - Page 15

• Manufacturer Part Number: ADA4858-3
• Description: Single-Supply, High Speed, Triple Op Amp with Charge Pump
• Manufacturer: Analog Devices

Specifications of ADA4858-3

• -3db Bandwidth: 600MHz
• Slew Rate: 600V/µs
• Vos: 500µV
• Ib: 700nA
• # Opamps Per Pkg: 3
• Input Noise (nv/rthz): 4nV/rtHz
• Vcc-vee: 3V to 5.5V
• Isy Per Amplifier: 7mA
• Packages: CSP

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.
G
R
B
H
+5V
200kΩ
0.1µF
7.15kΩ
220µA
220µA
220µA
ADCMP371AKSZ
Figure 43. AC-Coupled Video Input with DC Restored Output
2.8kΩ
74AC86
74AC86
74AC86
Rev. A | Page 15 of 20
V1
V2
V3
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.
75Ω
75Ω
75Ω
DAC1
DAC2
DAC3
44.2kΩ
6.02kΩ
NTA4153
NTA4153
NTA4153
4.7nF
4.7nF
4.7nF
R10
R11
V
CC
= 5V
Y
Pb
Pr
ADA4860-1
V
CC
V1
ADA4858-3
= 5V
301Ω
301Ω
301Ω
U1
U2
U3
Figure 44. Clamp Amp
301Ω
301Ω
301Ω
75Ω
75Ω
75Ω
75Ω
75Ω
75Ω
R7
301Ω
R8
301Ω
R9
301Ω
R2
R4
R6
C1
0.1µF
R
G
B
V
V
V
CC
CC
CC
U1
U2
U3
= 5V
= 5V
= 5V
301Ω
301Ω
301Ω
C2
10µF
R1
R3
R5
ADA4858-3
ADA4858-3
R12
75Ω
R13
75Ω
R14
75Ω
Y
Pb
Pr