ADA4851-2WYRMZ-R7 datasheet(19/24 Pages) AD

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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.

Ω CABLE

VOUT

VIN

′

+VS

2.2

μF

0.01

μF

47Ω

IOUT

125Ω

6.8Ω

+3V

68.1Ω

37.4Ω

51pF

6.8pF

1nF

2kΩ

1kΩ

75Ω

VOUT

51pF

VIDEO DAC

Figure 45. Video Amplifier

Table 6. Recommended Values

Supply

Voltage

(V)

Input

Range

(V)

(kΩ)

Gain

(V/V)

V’

(V)

Figure 46. 8 MHz Video Reconstruction Filter Schematic

VOUT

(V)

1: –3.3931dB

8.239626MHz

5dB/REF –15dB

0.03

0.1

100

FREQUENCY (MHz)

0 to 0.8

1.6

0.8

0 to 0.8

0.499

2.4

1.2

0 to 2.8

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

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