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AD9631AR-REEL7

P1-P3P4-P6P7-P9P10-P12P13-P15P16-P18P19-P20
AD9631/AD9632 Data Sheet
Rev. D | Page 16 of 20
PULSE RESPONSE
Unlike a traditional voltage feedback amplifier, where the slew
speed is dictated by its front end dc quiescent current and gain
bandwidth product, the AD9631/AD9632 provide on-demand
current that increases proportionally to the input step signal
amplitude. This results in slew rates (1300 V/µs) comparable
to wideband current feedback designs. This, combined with
relatively low input noise current (2.0 pA/√Hz), gives the
AD9631/AD9632 the best attributes of both voltage and
current feedback amplifiers.
LARGE SIGNAL PERFORMANCE
The outstanding large signal operation of the AD9631 and
AD9632 is due to a unique, proprietary design architecture. To
maintain this level of performance, the maximum 550 V × MHz
product must be observed (for example, @ 100 MHz, V
OUT
≤
5.5 V p-p).
POWER SUPPLY BYPASSING
Adequate power supply bypassing can be critical when optimiz-
ing the performance of a high frequency circuit. Inductance in
the power supply leads can form resonant circuits that produce
peaking in the amplifier’s response. In addition, if large current
transients must be delivered to the load, then bypass capacitors
(typically greater than 1 µF) will be required to provide the best
settling time and lowest distortion. A parallel combination of at
least 4.7 µF, and between 0.1 µF and 0.01 µF, is recommended.
Some brands of electrolytic capacitors will require a small series
damping resistor ≈4.7 Ω for optimum results.
DRIVING CAPACITIVE LOADS
The AD9631/AD9632 were designed primarily to drive non-
reactive loads. If driving loads with a capacitive component is
desired, the best frequency response is obtained by the addition
of a small series resistance as shown in Figure 62. Figure 63
shows the optimum value for R
SERIES
vs. capacitive load. It is
worth noting that the frequency response of the circuit when
driving large capacitive loads will be dominated by the passive
roll-off of R
SERIES
and C
L
.
Figure 62. Driving Capacitive Loads
Figure 63. Recommended R
SERIES
vs. Capacitive Load
R
F
R
SERIES
R
IN
R
IN
C
L
R
L
1kΩ
AD9631/
AD9632
00601-062
40
30
20
10
0 10 205 15 25
R
SERIES
(Ω)
C
L
(pF)
00601-063
Data Sheet AD9631/AD9632
Rev. D | Page 17 of 20
APPLICATIONS INFORMATION
The AD9631/AD9632 are voltage feedback amplifiers well
suited for applications such as photodetectors, active filters,
and log amplifiers. The wide bandwidth (320 MHz), phase
margin (65°), low current noise (2.0 pA/√Hz), and slew rate
(1300 V/µs) of the devices give higher performance capabilities
to these applications over previous voltage feedback designs.
With a settling time of 16 ns to 0.01% and 11 ns to 0.1%, the
devices are an excellent choice for DAC I/V conversion. The
same characteristics along with low harmonic distortion make
them a good choice for ADC buffering/amplification. With
superb linearity at relatively high signal frequencies, the
AD9631/AD9632 are ideal drivers for ADCs up to 12 bits.
OPERATION AS A VIDEO LINE DRIVER
The AD9631/AD9632 have been designed to offer outstanding
performance as video line drivers. The important specifications
of differential gain (0.02%) and differential phase (0.02°) meet
the most exacting HDTV demands for driving video loads.
Figure 64. Video Line Driver
ACTIVE FILTERS
The wide bandwidth and low distortion of the AD9631/
AD9632 are ideal for the realization of higher bandwidth active
filters. These characteristics, while being more common in
many current feedback op amps, are offered in the AD9631/
AD9632 in a voltage feedback configuration. Many active
filter configurations are not realizable with current feedback
amplifiers.
A multiple feedback active filter requires a voltage feedback
amplifier and is more demanding of op amp performance than
other active filter configurations, such as the Sallen-Key. In
general, the amplifier should have a bandwidth that is at least
10 times the bandwidth of the filter if problems due to phase
shift of the amplifier are to be avoided.
Figure 65 is an example of a 20 MHz low-pass multiple feedback
active filter using an AD9632.
Figure 65. Active Filter Circuit
Choose
F
O
= cutoff frequency = 20 MHz
α = damping ratio = 1/Q = 2
H = absolute value of circuit gain =
1
1
4
=
−
R
R
Then
12 CFk
O
π
=
2
)1(14
α
+
=
HC
C2
HK
R1
2
α
=
)1(2 +
=
HK
R3
α
)1(RHR4 =
+V
S
0.1µF
0.1µF
10µF
10µF
–V
S
V
OUT
V
IN
AD9631/
AD9632
75Ω
75Ω
75Ω
274Ω 274Ω
75Ω
CABLE
75Ω
CABLE
00601-064
0.1µF
10µF
0.1µF
10µF
V
OUT
V
IN
+5V
–5V
AD9632
100Ω
C1
50pF
R3
78.7Ω
R4
154Ω
R1
154Ω
C2
100pF
00601-065
AD9631/AD9632 Data Sheet
Rev. D | Page 18 of 20
ANALOG-TO-DIGITAL CONVERTER (ADC) DRIVER
As ADCs move toward higher speeds with higher resolutions,
there becomes a need for high performance drivers that will not
degrade the analog signal to the converter. It is desirable from a
system’s standpoint that the ADC be the element in the signal
chain that ultimately limits overall distortion. Figure 66 is such
an example.
Figure 66. AD9631 Used as Driver for an ADC Signal Chain
LAYOUT CONSIDERATIONS
The specified high speed performance of the AD9631/AD9632
requires careful attention to board layout and component
selection. Proper RF design techniques and low-pass parasitic
component selection are mandatory.
The PCB should have a ground plane covering all unused
portions of the component side of the board to provide a low
impedance path. Remove the ground plane from the area near
the input pins to reduce stray capacitance.
Use chip capacitors for supply bypassing (see Figure 59 and
Figure 60). Connect one end to the ground plane, and the other
within 1/8 inch of each power pin. Connect an additional large
(0.47 μF to 10 μF) tantalum electrolytic capacitor in parallel,
though not necessarily so close, to supply current for fast, large
signal changes at the output.
The feedback resistor should be located close to the inverting
input pin to keep the stray capacitance at this node to a mini-
mum. Capacitance variations of less than 1 pF at the inverting
input will significantly affect high speed performance.
Use stripline design techniques for long signal traces (greater
than about 1 inch). These should be designed with a
characteristic impedance of 50 Ω or 75 Ω and be properly
terminated at each end.
00601-066
+5V
–5V
130Ω
140Ω
ANALOG IN
AD9631
ADC
P1-P3P4-P6P7-P9P10-P12P13-P15P16-P18P19-P20

AD9631AR-REEL7

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Buy AD9631AR-REEL7
Manufacturer:
Analog Devices Inc.
Description:
High Speed Operational Amplifiers Ultra Low Distortion Stable
Lifecycle:
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