ADA4891-2ARZ Analog Devices Inc, ADA4891-2ARZ Datasheet - Page 15

• Manufacturer Part Number: ADA4891-2ARZ
• Description: Lw Cst CMOS Hgh Spd RR Amp
• Manufacturer: Analog Devices Inc

Specifications of ADA4891-2ARZ

• Amplifier Type: General Purpose
• Number Of Circuits: 2
• Output Type: Rail-to-Rail
• Slew Rate: 210 V/µs
• -3db Bandwidth: 240MHz
• Current - Input Bias: 2pA
• Voltage - Input Offset: 2500µV
• Current - Supply: 4.4mA
• Current - Output / Channel: 150mA
• Voltage - Supply, Single/dual (±): 2.7 V ~ 5.5 V, ±1.35 V ~ 2.75 V
• Operating Temperature: -40°C ~ 125°C
• Mounting Type: Surface Mount
• Package / Case: 8-SOIC (3.9mm Width)
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Gain Bandwidth Product: -
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

TERMINATING UNUSED AMPLIFIERS
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.
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.
Figure 50. Terminating Unused Amplifier with Single Power Supply
Figure 49. Terminating Unused Amplifier with
Symmetrical Bipolar Power Supplies
2.5kΩ
2.5kΩ
ADA4891-2
ADA4891-2
+V
–V
S
S
+V
S
Rev. A | Page 15 of 20
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.
V
IN
–12
–15
–18
–21
–24
–27
–30
–33
–36
–39
Figure 52. Video Reconstruction Filter Frequency Performance
–3
–6
–9
0
0.03
Figure 51. 13 MHz Video Reconstruction Filter Schematic
R1
47Ω
R2
0.1
125Ω
51pF
R3
C1
FREQUENCY (MHz)
R5
1kΩ
ADA4891-1/ADA4891-2
51pF
C2
1
15pF
1kΩ
R4
C3
+5V
6.8Ω
R6
10
68.1Ω
C4
1nF
R7
R8
75Ω
100
V
OUT