AD9621AN ,Wideband Voltage Feedback AmplifierSpecifications subject to change without notice.–2– REV. 0AD96211ABSOLUTE MAXIMUM RATINGS THEORY OF ..
AD9627BCPZ-125 , 12-Bit, 80/105/125/150 MSPS, 1.8 V Dual Analog-to-Digital Converter
AD9630AN ,Low Distortion 750 MHz Closed-Loop Buffer AmpSPECIFICATIONSOutput Offset Voltage +25
AD9621AN
Wideband Voltage Feedback Amplifier
REV.0
CONNECTION DIAGRAM
FEATURES
350 MHz Small Signal Bandwidth
130 MHz Large Signal BW (4 V p-p)
High Slew Rate: 1200 V/ms
Fast Settling: 11 ns to 0.01%/7 ns to 0.1%63 V Supply Operation
APPLICATIONS
ADC Input Driver
Differential Amplifiers
IF/RF Amplifiers
Pulse Amplifiers
Professional Video
DAC Current-to-Voltage
Baseband and Video Communications
Pin Diode Receivers
Active Filters/lntegrators/Log Amps
GENERAL DESCRIPTIONThe AD9621 is one of a family of very high speed and wide
bandwidth amplifiers utilizing a voltage feedback architecture.
These amplifiers define a new level of performance for voltage
feedback amplifiers, especially in the categories of large signal
bandwidth, slew rate, settling, and low noise.
Proprietary design architectures have resulted in an amplifier
family that combines the most attractive attributes of both cur-
rent feedback and voltage feedback amplifiers. The AD9621 ex-
hibits extraordinarily accurate and fast pulse response
characteristics (7 ns settling to 0.1%) as well as extremely wide
small and large signal bandwidth previously found only in cur-
rent feedback amplifiers. When combined with balanced high
impedance inputs and low input noise current more common to
voltage feedback architectures, the AD9621 offers performance
not previously available in a monolithic operational amplifier.
*. Patent 5,150,074 and others pending.Other members of the AD962X amplifier family are the
AD9622 (G = +2), AD9623 (G = +4), and the AD9624
(G = +6). A separate data sheet is available from Analog De-
vices for each model. Each generic device has been designed for
a different minimum stable gain setting, allowing users flexibility
in optimizing system performance. Dynamic performance speci-
fications such as slew rate, settling time, and distortion vary
from model to model. The table below summarizes key perfor-
mance attributes for the AD962X family and can be used as a
selection guide.
The AD9621 is offered in industrial and military temperature
ranges. Industrial versions are available in plastic DIP, SOIC,
and cerdip; MIL versions are packaged in cerdips.
PRODUCT HIGHLIGHTS1. Wide Large Signal Bandwidth
2. High Slew Rate
3. Fast Settling
4. Output Short-Circuit Protected
Wideband Voltage
Feedback Amplifier
AD9621–SPECIFICATIONS
DC ELECTRICAL CHARACTERISTICSNOTES
(6VS = 65 V, RLOAD = 100 Ω; AV = 1, unless otherwise noted)
ABSOLUTE MAXIMUM RATINGS1Supply Voltages (±VS) . . . . . . . . . . . . . . . . . . . . . . . . . . .±6 V
Common-Mode Input Voltage . . . . . . . . . . . . . . . . . . . . . .±VS
Differential Input Voltage . . . . . . . . . . . . . . . . . . . . . . . . . .6 V
Continuous Output Current2 . . . . . . . . . . . . . . . . . . . . .90 mA
Operating Temperature Ranges
AN, AQ, AR . . . . . . . . . . . . . . . . . . . . . . . .–40°C to +85°C . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .–55°C to +125°C
Storage Temperature
Ceramic . . . . . . . . . . . . . . . . . . . . . . . . . . .–65°C to +150°C
Plastic . . . . . . . . . . . . . . . . . . . . . . . . . . . .–65°C to +125°C
Junction Temperature
Ceramic3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .+175°C
Plastic3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .+150°C
Lead Soldering Temperature (1 minute)4 . . . . . . . . . .+220°C
NOTES
1Absolute maximum ratings are limiting values to be applied individually, and
beyond which the serviceability of the circuit may be impaired. Functional
operability is not necessarily implied. Exposure to absolute maximum rating
conditions for an extended period of time may affect device reliability.
2Output is short-circuit protected; for maximum reliability, 90 mA continuous
current should not be exceeded.
3Typical thermal impedances (part soldered onto board; no air flow):
Ceramic DIP:θJA = 100°C/W; θJC = 30°C/W
Plastic SOIC:θJA = 125°C/W; θJC = 45°C/W
Plastic DIP:θJA = 90°C/W; θJC = 45°C/W
4Temperature shown is for surface mount devices, mounted by vapor phase
soldering. Throughhole devices (ceramic and plastic DIPs) can be soldered at
+300°C for 10 seconds.
ORDERING GUIDEAD9621AQ
AD9621AR
EXPLANATION OF TEST LEVELS
Test Level–100% production tested.–100% production tested at +25°C, and sample tested at
specified temperatures. AC testing of “A” grade devices
done on sample basis.
III–Sample tested only.–Parameter is guaranteed by design and characterization
testing.–Parameter is a typical value only.–All devices are 100% production tested at +25°C. 100%
production tested at temperature extremes for extended
temperature devices; sample tested at temperature ex-
tremes for commercial/industrial devices.
THEORY OF OPERATIONThe AD9621 is a wide bandwidth, unity gain stable voltage
feedback amplifier. Since its open-loop frequency response fol-
lows the conventional 6 dB/octave roll-off, its gain bandwidth
product is basically constant. Increasing its closed-loop gain re-
sults in a corresponding decrease in small signal bandwidth. The
AD9621 typically maintains a 55 degree unity loop gain phase
margin. This high margin minimizes the effects of signal and
noise peaking.
Feedback Resistor ChoiceAt minimum stable gain (+1), the AD9621 provides optimum
dynamic performance with RF ≅ 51 Ω. This resistor acts only as
a parasitic suppressor against damped RF oscillations that can
occur due to lead (input, feedback) inductance and parasitic ca-
pacitance. For settling accuracy to 0.1% or less, this resistor
should not be required if layout guidelines are closely followed.
This value for RF provides the best combination of wide band-
width, low parasitic peaking, and fast settling time.
When the AD9621 is used in the transimpedance (I-to-V)
mode, such as for photo-diode detection, the value for RF and
diode capacitance (CI) are usually known. See Figure 1. Gener-
ally, the value of RF selected will be in the kΩ range, and a shunt
capacitor (CF) across RF will be required to maintain good am-
plifier stability. The value of CF required to maintain < 1 dB of
peaking can be estimated as: ≅[(2ωοCIRF−1)ωοRF]12|RF≥1kΩ
where ωo is equal to the unity gain bandwidth product of the
amplifier in RAD/sec, and CI is the equivalent total input ca-
pacitance at the inverting input. Typically ωo is 700 × 106
RAD/sec (See Open Loop Frequency Response curve).
As an example, choosing RF of 10 kΩ and CI of 5 pF, requires
CF to be 1.1 pF (Note: CI includes both the source and parasitic
circuit capacitance). The bandwidth of the amplifier can be esti-
mated using the CF calculated as: dB≅1.6πRFCF
For general voltage gain applications, the amplifier bandwidth
can be estimated as: dB≅ωοRF
This estimation loses accuracy for gains approaching +2/–1 or
lower due to the amplifier’s damping factor. For these “low
gain” cases, the bandwidth will actually extend beyond the cal-
culated value. See Closed Loop BW plots.
As a rule of thumb, capacitor CF will not be required if: RG()CI≤NG
4ωο
AD9621phase margin (55°), low noise current (3.6 pA/√Hz), and slew
rate (1200 V/μs) give higher performance capabilities to these
applications over previous voltage feedback designs.
With a settling time of 11 ns to 0.01% and 7 ns to 0.1%, the de-
vice is an excellent choice for DAC I/V conversion. The same
characteristics, along with low harmonic distortion, make it a
good choice for ADC buffering/amplification. With its superb
linearity at relatively high signal frequencies, it is an ideal driver
for ADCs up to 14 bits.
Layout ConsiderationsAs with all wide bandwidth components, printed circuit layout
is critical to obtain best dynamic performance with the AD9621.
The ground plane in the area of the amplifier and its associated
components should cover as much of the component side of the
board as possible (or first interior layer of a multi layer surface
mount board).
The ground plane should be removed in the area of the inputs
and RF and RG to minimize stray capacitance at the input. The
same precaution should be used for CB, if used. Each power
supply trace should be decoupled close to the package with a
0.1 μF ceramic capacitor, plus a 6.8 μF tantalum nearby.
All lead lengths for input, output, and feedback resistor should
be kept as short as possible. All gain setting resistors should be
chosen for low values of parasitic capacitance and inductance,
i.e., microwave resistors and/or carbon resistors.
Microstrip techniques should be used for lead lengths in excess
of one inch. Sockets should be avoided if at all possible because
of their high series inductance. If sockets are necessary, indi-
vidual pin sockets such as AMP p/n 6-330808-3 should be used.
These contribute far less stray reactance than molded socket
assemblies.
An evaluation board is available from Analog Devices for a
nominal charge.
Pulse ResponseUnlike a traditional voltage feedback amplifier in which slew
speed is dictated by its front end dc quiescent current and gain
bandwidth product, the AD9621 provides “on demand” trans-
conductance current that increases proportionally to the input
“step” signal amplitude. This results in slew speeds (1200 V/μs)
comparable to wideband current feedback designs. This, com-
bined with relatively low input noise current (3.6 pA/√Hz), gives
the AD9621 the best attributes of both voltage and current feed-
back amplifiers.
Bootstrap Capacitor (CB)In most applications, the CB capacitor will not be required.
Under certain conditions, it can be used to further enhance set-
tling time performance.
The CB capacitor (0.001 μF) connects to the internal high im-
pedance nodes of the amplifier. Using this capacitor will reduce
the large signal (4 V) step output settling time by 3 to 5 ns for
0.05% or greater accuracy. For settling accuracy less than
0.05% or for smaller step sizes, its effect will be less apparent.
Under heavy slew conditions, this capacitor forces the internal
signal (initial step) amplitude to be controlled by the “on”
(slewed) transistor, preventing its complement from completely
turning off. This allows for faster settling time of these internal
nodes and also the output.
In the frequency domain, total (high frequency) distortion will
be approximately the same with or without CB. Typically, the
3rd harmonic will be greater than the 2nd without CB. This will
be reversed with CB in place.
APPLICATIONSThe AD9621 is a voltage feedback amplifier and is well suited
for such applications as photo-detector preamp, active filters,
and log amplifiers. The device’s wide bandwidth (350 MHz),
Figure 1.Transimpedance
Configuration
Figure 3.
Diagram
Figure 2.Inverting Gain Connection
Diagram
Figure 4.Open-Loop Gain and
Phase
Figure 7.Harmonic Distortion
vs. Frequency
Figure 10.Frequency Response
vs. RLOAD
Figure 5.Inverting Frequency
Response
Figure 8.Third Order Intercept
Figure 11.Short-Term Settling
Time
AD9621Figure 16.Large Signal Pulse
Response
Figure 17.Small Signal Pulse
Response
MECHANICAL INFORMATIONDimensions shown in inches and (mm).
Cerdip (Suffix Q)
Plastic DIP (Suffix N)
Plastic SOIC (Suffix R)
