AD9221/AD9223/AD9220
degrade slightly as the input common-mode voltage deviates
from its optimum level of 2.5 V.
AD817:
AD826:
AD818:
AD828:
AD812:
50 MHz Unity GBW, 70 ns Settling to 0.01%, +5 V
to ±15 V Supplies
Best Applications: Sample Rates < 7 MSPS, Low-
Noise, 5 V p-p Input Range
Alternative AC Interface
Figure 42 shows a flexible ac coupled circuit which can be con-
figured for different input spans. Since the common-mode
voltage of VINA and VINB are biased to midsupply indepen-
dent of VREF, VREF can be pin-strapped or reconfigured to
achieve input spans between 2 V and 5 V p-p. The AD9221/
AD9223/AD9220’s CMRR along with the symmetrical coupling
R-C networks will reject both power supply variations and noise.
The resistors, R, establish the common-mode voltage. They may
have a high value (e.g., 5 kΩ) to minimize power consumption
and establish a low cutoff frequency. The capacitors, C1and
C2, are typically a 0.1 µF ceramic and 10 µF tantalum capacitor
in parallel to achieve a low cutoff frequency while maintaining a
low impedance over a wide frequency range. RS isolates the
buffer amplifier from the A/D input. The optimum performance
is achieved when VINA and VINB are driven via «Immetrical
networks. The f–3 dB point can be approximated by the equation,
Limits: THD above 100 kHz
Dual Version of AD817
Best Applications: Differential and/or Low Imped-
ance Input
Drivers, Low Noise
Limits: THD above 100 kHz
130 MHz @ G = +2 BW, 80 ns Settling to 0.01%,
+5 V to ±15 V Supplies
Best Applications: Sample Rates < 7 MSPS, Low
Noise, 5 V p-p Input Range, Gains ≥ +2
Limits: THD above 100 kHz
Dual Version of AD818
Best Applications: Differential and/or Low Impedance
Input
Drivers, Low Noise, Gains ≥ +2
Limits: THD above 100 kHz
f
–3 dB = 1/(2 × π × R/2 × (C1 + C2))
+5V
Dual, 145 MHz Unity GBW, Single-Supply Cur-
rent Feedback, +5 V to ±15 V Supplies
Best Applications: Differential and/or Low Imped-
ance Input Drivers, Sample Rates < 7 MSPS
Limits: THD above 1 MHz
+5V
R
C1
V
R
S
IN
VINA
C2
R
AD9221/
AD9223/
AD9220
–5V
AD8011: f–3 dB = 300 MHz, +5 V or ±5 V Supplies, Current
Feedback
R
R
S
VINB
+5V
C1
C2
R
Best Applications: Single-Supply, AC/DC-Coupled,
Good AC Specs, Low Noise, Low Power (5 mW)
Limits: THD above 5 MHz, Usable Input/Output
Range
Figure 42. AC-Coupled Input-Flexible Input Span, VCM = 2 V
AD8013: Triple, f–3 dB = 230 MHz, +5 V or ±5 V Supplies,
Current Feedback, Disable Function
OP AMP SELECTION GUIDE
Op amp selection for the AD9221/AD9223/AD9220 is highly
dependent on a particular application. In general, the perfor-
mance requirements of any given application can be character-
ized by either time domain or frequency domain parameters. In
either case, one should carefully select an op amp which pre-
serves the performance of the A/D. This task becomes challeng-
ing when one considers the AD9221/AD9223/AD9220’s high
performance capabilities coupled with other extraneous system
level requirements such as power consumption and cost.
Best Applications: 3:1 Multiplexer, Good AC Specs
Limits: THD above 5 MHz, Input Range
AD9631: 220 MHz Unity GBW, 16 ns Settling to 0.01%, ±5 V
Supplies
Best Applications: Best AC Specs, Low Noise, AC-
Coupled
Limits: Usable Input/Output Range, Power
Consumption
The ability to select the optimal op amp may be further compli-
cated by either limited power supply availability and/or limited
acceptable supplies for a desired op amp. Newer, high performance
op amps typically have input and output range limitations in
accordance with their lower supply voltages. As a result, some
op amps will be more appropriate in systems where ac-coupling
is allowable. When dc-coupling is required, op amps without
headroom constraints such as rail-to-rail op amps or ones where
larger supplies can be used should be considered. The following
section describes some op amps currently available from Analog
Devices. The system designer is always encouraged to contact
the factory or local sales office to be updated on Analog Devices
latest amplifier product offerings. Highlights of the areas where
the op amps excel and where they may limit the performance of
the AD9221/AD9223/AD9220 is also included.
AD8047: 130 MHz Unity GBW, 30 ns Settling to 0.01%,
±5 V Supplies
Best Applications: Good AC Specs, Low Noise,
AC-Coupled
Limits: THD > 5 MHz, Usable Input Range
AD8041: Rail-to-Rail, 160 MHz Unity GBW, 55 ns Settling
to 0.01%, +5 V Supply, 26 mW
Best Applications: Low Power, Single-Supply Sys-
tems, DC-Coupled, Large Input Range
Limits: Noise with 2 V Input Range
AD8042: Dual AD8041
Best Applications: Differential and/or Low Imped-
ance Input Drivers
Limits: Noise with 2 V Input Range
–16–
REV. D
ADI [ ADI ]
