ADA4851-1_06 AD [Analog Devices], ADA4851-1_06 Datasheet - Page 15

• Manufacturer Part Number: ADA4851-1_06
• Description: Low Cost, High Speed, Rail-to-Rail, Output Op Amps
• Manufacturer: AD [Analog Devices]

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 illustrates the effects of supply
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 applications, the ADA4851-x 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
Rev. D | Page 15 of 20
OUT
An example of an 8 MHz, 3-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 3 poles; two
are active with a third passive pole (R6 and C4) placed at the
output. C3 improves the filter rolloff. 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
Figure 47. Video Reconstruction Filter Frequency Performance
0.03
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
8.239 626MHz
C4
1nF
68.1Ω
R7
100
R8
75Ω
V
OUT