ADA4851-1/ADA4851-2/ADA4851-4

Rev. D | Page 15 of 20

SINGLE-SUPPLY VIDEO AMPLIFIER

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.

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.

75

Ω CABLE

75

Ω

75

Ω

V

′

U1

C1

2.2

μF

C2

0.01

μF

+

R2

47Ω

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

C2

51pF

VIDEO DAC

Figure 45. Video Amplifier

Table 6. Recommended Values

Supply

Voltage

(V)

Input

Range

(V)

(kΩ)

(kΩ)

Gain

(V/V)

V’

(V)

(V)

Figure 46. 8 MHz Video Reconstruction Filter Schematic

1: –3.3931dB

8.239 626MHz

5dB/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 applications, the ADA4851-x is an ideal choice due to its

lower power requirements and high performance.

Figure 47. Video Reconstruction Filter Frequency Performance