Data Sheet

ADA4856-3

Rev. B | Page 15 of 20

20 MHz ACTIVE LOW-PASS FILTER

The ADA4856-3 triple amplifier lends itself to higher order

active filters. Figure 42 shows a 20 MHz, 6-pole, Sallen-Key

low-pass filter.

U1

OP AMP

OUT1

+

–

R1

93.1Ω

R2

604Ω

C2

22pF

C1

33pF

U2

OP AMP

OUT2

+

–

R3

113Ω

R4

732Ω

C4

15pF

C3

33pF

U3

OP AMP

OUT3

+

–

R5

121Ω

R6

475Ω

C6

15pF

C5

47pF

Figure 42. 20 MHz, 6-Pole Low-Pass Filter

The filter has a gain of approximately 18 dB, which is set by

three fixed gain of 2 stages, and a flat frequency response out to

14 MHz. This type of filter is commonly used at the output of a

video DAC as a reconstruction filter. The frequency response of

the filter is shown in Figure 43.

20

10

0

–10

–20

–30

–40

–50

–60

1

10

100

200

FREQUENCY (MHz)

SIX POLES

TWO POLES

FOUR POLES

Figure 43. 20 MHz, Low-Pass Filter Frequency Response

VIDEO LINE DRIVER

The ADA4856-3 was designed to excel in video driver applications.

Figure 44 shows a typical schematic for a video driver operating

on bipolar supplies.

1

2

3

ADA4856-3

PD

0.1µF

0.1µF

0.1µF

0.1µF

10µF

0.1µF

+

0.1µF

10µF

7

5Ω

7

5Ω

7

5Ω

7

5Ω

7

5Ω

7

5Ω

16

15

14

13

5

6

7

12

11

10

9

4

8

Figure 44. Video Driver Schematic

In applications that require multiple video loads be driven

simultaneously, the ADA4856-3 can deliver. Figure 45 shows

the ADA4856-3 configured with triple video loads. Figure 46

shows the triple video load performance.

75Ω

CABLE

75Ω

CABLE

75Ω

75Ω

75Ω

0.1µF

0.1µF

10µF

10µF

75Ω

CABLE

75Ω

75Ω

75Ω

CABLE

75Ω

75Ω

+

–

ADA4856-3

0.1µF

Figure 45. Video Driver Schematic for Triple Video Loads

6.5

6.0

5.5

5.0

4.5

4.0

3.5

3.0

2.5

1

10

100

200

FREQUENCY (MHz)

Figure 46. Large Signal Frequency Response for Various Loads