ADA4851-2 Analog Devices, ADA4851-2 Datasheet - Page 19

• Manufacturer Part Number: ADA4851-2
• Description: Low Cost, High Speed, Rail-to-Rail, Output Op Amp (Dual)
• Manufacturer: Analog Devices

Specifications of ADA4851-2

• -3db Bandwidth: 130MHz
• Slew Rate: 375V/µs
• Vos: 600µV
• Ib: 2.2µA
• # Opamps Per Pkg: 2
• Input Noise (nv/rthz): 10nV/rtHz
• Vcc-vee: 2.7V to 12V
• Isy Per Amplifier: 3.2mA
• Packages: SOP

SINGLE-SUPPLY VIDEO AMPLIFIER
The ADA4851 family of amplifiers is well suited for portable
video applications. When operating in low voltage single-supply
applications, the input signal is limited by the input stage
headroom. For additional information, see the Headroom
Considerations section. Table 6 shows the recommended values
for voltage, input signal, various gains, and output signal swing
for the typical video amplifier shown in Figure 45.
Table 6. Recommended Values
Supply
Voltage
(V)
3
3
5
VIDEO RECONSTRUCTION FILTER
At higher frequencies, active filters require wider bandwidths to
work properly. Excessive phase shift introduced by lower frequency
op amps can significantly affect the filter performance.
A common application for active filters is at the output of video
DACs/encoders. The filter, or more appropriately, the video
reconstruction filter, is used at the output of a video DAC/
encoder to eliminate the multiple images that are created during
the sampling process within the DAC. For portable video appli-
cations, the ADA4851 family of amplifiers is an ideal choice due
to its lower power requirements and high performance.
R
G
V
IN
Input
Range
(V)
0 to 0.8
0 to 0.8
0 to 2.8
P
+V
D
S
U1
R
Figure 45. Video Amplifier
F
0.01μF
2.2μF
+
C1
C2
R
(kΩ)
1
0.499
1
G
V′
R
(kΩ)
1
1
1
F
75Ω
Gain
(V/V)
2
3
2
75Ω CABLE
V’
(V)
1.6
2.4
4.9
75Ω
V
V
(V)
0.8
1.2
2.45
OUT
OUT
Rev. J | Page 19 of 24
An example of an 8 MHz, three-pole, Sallen-Key, low-pass,
video reconstruction filter is shown in Figure 46. This circuit
features a gain of 3, has a 0.1 dB bandwidth of 8.2 MHz, and
over 17 dB attenuation at 27 MHz (see Figure 47). The filter has
three poles; two are active with a third passive pole (R6 and C4)
placed at the output. C3 improves the filter roll-off. R6, R7, and
R8 comprise the video load of 150 Ω. Components R6, C4, R7,
R8, and the input termination of the network analyzer form a
12.8 dB attenuator; therefore, the reference level is roughly
−3.3 dB, as shown in Figure 47.
VIDEO DAC
0.03
Figure 47. Video Reconstruction Filter Frequency Performance
ADA4851-1/ADA4851-2/ADA4851-4
I
OUT
Figure 46. 8 MHz Video Reconstruction Filter Schematic
37.4Ω
R1
0.1
47Ω
R2
5dB/REF –15dB
125Ω
51pF
R3
C1
FREQUENCY (MHz)
1
R5
1kΩ
51pF
C2
1: –3.3931dB
6.8pF
2kΩ
R4
C3
+3V
1
10
6.8Ω
R6
8.239626MHz
C4
1nF
68.1Ω
R7
100
R8
75Ω
V
OUT