Datasheet ADA4891-1, ADA4891-2, ADA4891-3, ADA4891-4 (Analog Devices) - 20

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ADA4891-1/ADA4891-2/ADA4891-3/ADA4891-4 Data Sheet VIDEO RECONSTRUCTION FILTER MULTIPLEXER
A common application for active filters is at the output of video The ADA4891-3 has a disable pin used to power down the digital-to-analog converters (DACs)/encoders. The filter, or more amplifier to save power or to create a mux circuit. If two or appropriately, the video reconstruction filter, is used at the output more ADA4891-3 outputs are connected together and only one of a video DAC/encoder to eliminate the multiple images that output is enabled, then only the signal of the enabled amplifier are created during the sampling process within the DAC. For appears at the output. This configuration is used to select from portable video applications, the ADA4891-1/ADA4891-2/ various input signal sources. Additionally, the same input signal ADA4891-3/ADA4891-4 is an ideal choice due to its lower is applied to different gain stages, or differently tuned filters, to power requirements and high performance. make a gain-step amplifier or a selectable frequency amplifier. For active filters, a good rule of thumb is that the −3 dB band- Figure 64 shows a schematic of two ADA4891-3 devices used width of the amplifiers be at least 10 times higher than the corner to create a mux that selects between two inputs. One input is a frequency of the filter. This ensures that no initial roll-off is 1 V p-p, 3 MHz sine wave; the other input is a 2 V p-p, 1 MHz introduced by the amplifier and that the pass band is flat until sine wave. the cutoff frequency.
+2.5V
An example of a 15 MHz, 3-pole, Sallen-Key, low-pass video
0.1µF 10µF
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
49.9Ω ADA4891-3
attenuation at 29.7 MHz (see Figure 63). The filter has three
1V p-p
poles: two poles are active, with a third passive pole (R6 and C4)
3MHz 0.1µF 10µF
placed at the output. C3 improves the filter rol -off. R6, R7, and
49.9Ω –2.5V
R8 make up the video load of 150 Ω. Components R6, C4, R7,
453Ω
R8, and the input termination of the network analyzer form a
VOUT 453Ω
6 dB attenuator; therefore, the reference level is roughly 0 dB,
+2.5V 49.9Ω
as shown in Figure 63.
49.9Ω C2 0.1µF 10µF 51pF 2V p-p 49.9Ω ADA4891-3 R2 R3 1MHz 47Ω 125Ω +5V R6 R7 6.8Ω 68.1Ω 0.1µF 10µF V V IN C1 OUT R1 51pF C4 R8 1nF 75Ω –2.5V R4 453Ω 1kΩ 453Ω HCO4
087
C3 R5 15pF SELECT 1kΩ
062 08054- Figure 64. Two-to-One Multiplexer Using Two ADA4891-3 Devices 08054- Figure 62. 15 MHz Video Reconstruction Filter Schematic The select signal and the output waveforms for this circuit are shown in Figure 65.
0 –3 1V/DIV 1µs/DIV –6 –9 OUTPUT –12 B) d –15 ( –18 UDE T –21 NI –24 AG M SELECT –27 –30 –33 –36
088
5V/DIV 1µs/DIV –39
08054-
0.03 0.1 1 10 100
059 Figure 65. ADA4891-3 Mux Output
FREQUENCY (MHz)
08054- Figure 63. Video Reconstruction Filter Frequency Performance Rev. F | Page 20 of 24 Document Outline FEATURES APPLICATIONS GENERAL DESCRIPTION CONNECTION DIAGRAMS REVISION HISTORY SPECIFICATIONS 5 V OPERATION 3 V OPERATION ABSOLUTE MAXIMUM RATINGS MAXIMUM POWER DISSIPATION ESD CAUTION TYPICAL PERFORMANCE CHARACTERISTICS APPLICATIONS INFORMATION USING THE ADA4891-1/ADA4891-2/ADA4891-3/ ADA4891-4 WIDEBAND, NONINVERTING GAIN OPERATION WIDEBAND, INVERTING GAIN OPERATION RECOMMENDED VALUES EFFECT OF RF ON 0.1 dB GAIN FLATNESS DRIVING CAPACITIVE LOADS TERMINATING UNUSED AMPLIFIERS DISABLE FEATURE (ADA4891-3 ONLY) SINGLE-SUPPLY OPERATION VIDEO RECONSTRUCTION FILTER MULTIPLEXER LAYOUT, GROUNDING, AND BYPASSING POWER SUPPLY BYPASSING GROUNDING INPUT AND OUTPUT CAPACITANCE INPUT-TO-OUTPUT COUPLING LEAKAGE CURRENTS OUTLINE DIMENSIONS ORDERING GUIDE AUTOMOTIVE PRODUCTS