ADA4891-1/ADA4891-2

Rev. A | Page 15 of 20

TERMINATING UNUSED AMPLIFIERS

An example of a 15 MHz, 3-pole Sallen-Key, low-pass, video

reconstruction filter is shown in Figure 51. This circuit features

a gain of 2, has a 0.1 dB bandwidth of 7.3 MHz, and over 17 dB

attenuation at 29.7 MHz (see Figure 52). 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 6 dB attenuator;

therefore, the reference level is roughly 0 dB, as shown in Figure 52.

Terminating unused amplifiers in a multiamplifier package is an

important step to ensuring proper operation of the functional

amplifier. Unterminated amplifiers can oscillate and draw excessive

power if left unattended. The recommended procedure for

terminating unused amplifiers is to connect any unused amplifiers

in a unity-gain configuration and connect the noninverting input

to midsupply voltage. With symmetrical bipolar power supplies,

this means connecting the noninverting input to ground, as shown

in Figure 49. In single power supply applications, a synthetic

midsupply source must be created. This can be accomplished

with a simple resistive voltage divider. Figure 50 shows the

proper connection for terminating an unused amplifier in a

single-supply configuration.

C2

51pF

ADA4891-2

Figure 49. Terminating Unused Amplifier with

Symmetrical Bipolar Power Supplies

2.5k

Ω

2.5k

Ω

ADA4891-2

Figure 50. Terminating Unused Amplifier with Single Power Supply

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 amplifiers

−3 dB bandwidth 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.

R2

47

Ω

R3

125

Ω

R6

6.8

Ω

+5V

R7

68.1

Ω

R1

C1

51pF

C3

15pF

C4

1nF

R4

1k

Ω

R5

1k

Ω

R8

75

Ω

Figure 51. 13 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

FREQUENCY (MHz)

Figure 52. Video Reconstruction Filter Frequency Performance