ADA4891-1/ADA4891-2/ADA4891-3/ADA4891-4

Rev. B | Page 19 of

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VIDEO RECONSTRUCTION FILTER

MULTIPLEXER

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.

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.

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.

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

SELECT

HCO4

453Ω

453Ω

10µF

0.1µF

10µF

0.1µF

49.9Ω

453Ω

10µF

0.1µF

10µF

0.1µF

ADA4891-3

ADA4891-3

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

R3

125Ω

R6

6.8Ω

+5V

R7

68.1Ω

R1

C1

51pF

C3

15pF

C4

1nF

R4

1kΩ

R5

1kΩ

R8

75Ω

C2

51pF

Figure 64. Two-to-One Multiplexer Using Two ADA4861-3 Devices

Figure 62. 15 MHz Video Reconstruction Filter Schematic

The select signal and the output waveforms for this circuit are

shown in Figure 65.

–39

–36

–33

–30

–27

–24

–21

–18

–15

–12

–9

–6

–3

0

0.03

0.1

1

10

100

FREQUENCY (MHz)

1µs/DIV

1V/DIV

1µs/DIV

5V/DIV

SELECT

OUTPUT

Figure 65. ADA4861-3 Mux Output

Figure 63. Video Reconstruction Filter Frequency Performance