Datasheet

Data Sheet ADA4891-1/ADA4891-2/ADA4891-3/ADA4891-4
Rev. E | Page 19 of 24
VIDEO RECONSTRUCTION FILTER
A common application for active filters is at the output of video
digital-to-analog converters (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 ADA4891 is an ideal choice due
to its lower power requirements and high performance.
For active filters, a good rule of thumb is that the −3 dB band-
width of the amplifiers be at least 10 times higher than the corner
frequency of the filter. This ensures that no initial roll-off is
introduced by the amplifier and that the pass band is flat until
the cutoff frequency.
An example of a 15 MHz, 3-pole, Sallen-Key, low-pass video
reconstruction filter is shown in Figure 62. This circuit features
a gain of +2, a 0.1 dB bandwidth of 7.3 MHz, and over 17 dB
attenuation at 29.7 MHz (see Figure 63). The filter has three
poles: two poles are active, with a third passive pole (R6 and C4)
placed at the output. C3 improves the filter roll-off. R6, R7, and
R8 make up the video load of 150 Ω. Components R6, C4, R7,
R8, and the input termination of the network analyzer form a
6 dB attenuator; therefore, the reference level is roughly 0 dB,
as shown in Figure 63.
R2
47Ω
V
IN
R3
125Ω
R6
6.8Ω
+5V
R7
68.1Ω
R1
C1
51pF
C3
15pF
C4
1nF
R4
1k
Ω
R5
1kΩ
R8
75Ω
V
OUT
C2
51pF
08054-062
Figure 62. 15 MHz Video Reconstruction Filter Schematic
–39
–36
–33
–30
–27
–24
–21
–18
–15
–12
–9
–6
–3
0
0.03
0.1 1 10 100
MAGNITUDE (dB)
FREQUENCY (MHz)
08054-059
Figure 63. Video Reconstruction Filter Frequency Performance
MULTIPLEXER
The ADA4891-3 has a disable pin used to power down the
amplifier to save power or to create a mux circuit. If two or
more ADA4891-3 outputs are connected together and only one
output is enabled, then only the signal of the enabled amplifier
appears at the output. This configuration is used to select from
various input signal sources. Additionally, the same input signal
is applied to different gain stages, or differently tuned filters, to
make a gain-step amplifier or a selectable frequency amplifier.
Figure 64 shows a schematic of two ADA4891-3 devices used
to create a mux that selects between two inputs. One input is a
1 V p-p, 3 MHz sine wave; the other input is a 2 V p-p, 1 MHz
sine wave.
49.9Ω
453Ω
+2.5V
–2.5V
+2.5V
–2.5V
49.9Ω
49.9Ω
49.9Ω
1V p-p
3MHz
2V p-p
1MHz
V
OUT
SELECT
HCO4
453Ω
453Ω
10µF
0.1µF
10µF
0.1µF
49.9Ω
453Ω
10µF
0.1µF
10µF
0.1µF
08054-087
ADA4891-3
ADA4891-3
Figure 64. Two-to-One Multiplexer Using Two ADA4861-3 Devices
The select signal and the output waveforms for this circuit are
shown in Figure 65.
1µs/DIV1V/DIV
1µs/DIV5V/DIV
SELECT
OUTPUT
08054-088
Figure 65. ADA4861-3 Mux Output