ADA4851-1/ADA4851-2/ADA4851-4
Rev. J | Page 19 of 24
SINGLE-SUPPLY VIDEO AMPLIFIER
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.
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.
75Ω CABLE
V
OUT
75Ω
75Ω
V
IN
V′
R
G
R
F
+V
S
P
D
U1
C1
2.2μF
C2
0.01μF
05143-059
+
R2
47Ω
I
OUT
R3
125Ω
R6
6.8Ω
+3V
R7
68.1Ω
R1
37.4Ω
C1
51pF
C3
6.8pF
C4
1nF
R4
2kΩ
R5
1kΩ
R8
75Ω
V
OU
C2
51pF
VIDEO DAC
05143-061
Figure 45. Video Amplifier
Table 6. Recommended Values
Supply
Voltage
(V)
Input
Range
(V)
R
G
(kΩ)
R
F
(kΩ)
Gain
(V/V)
V’
(V)
Figure 46. 8 MHz Video Reconstruction Filter Schematic
V
OUT
(V)
05143-062
1: –3.3931dB 8.239626MHz5dB/REF –15dB
0.03 0.1 1 10 100
1
FREQUENCY (MHz)
3 0 to 0.8 1 1 2 1.6 0.8
3 0 to 0.8 0.499 1 3 2.4 1.2
5 0 to 2.8 1 1 2 4.9 2.45
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.
Figure 47. Video Reconstruction Filter Frequency Performance
