AD9243
REV. A
–15–
of the A/D is 0 V to 5 V. Other input ranges could be selected
by changing VREF but the A/D’s distortion performance will
degrade slightly as the input common-mode voltage deviates
from its optimum level of 2.5 V.
Alternative AC Interface
Figure 35 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 AD9243’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, C1
and 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. R
S
isolates the buffer amplifier from the
A/D input. The optimum performance is achieved when VINA
and VINB are driven via symmetrical networks. The high pass
f
–3 dB
point can be approximated by the equation,
f
–3 dB
= 1/(2 × π × R/2 × (C1 + C2))
C2
VINA
VINB
AD9243
C1
R
+5V
–5V
R
S
V
IN
C1
C2
R
R
S
+5V
R
R
+5V
Figure 35. AC-Coupled Input-Flexible Input Span,
V
CM
= 2.5 V
OP AMP SELECTION GUIDE
Op amp selection for the AD9243 is highly dependent on a
particular application. In general, the performance requirements
of any given application can be characterized by either time
domain or frequency domain parameters. In either case, one
should carefully select an op amp which preserves the perfor-
mance of the A/D. This task becomes challenging when one
considers the AD9243’s high performance capabilities coupled
with other external system level requirements such as power
consumption and cost.
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 AD9243 are also included.
AD812: 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
Limits: THD above 1 MHz
AD8011: f
–3 dB
= 300 MHz, +5 V or ±5 V Supplies, Current
Feedback
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
AD8013: Triple, f
–3 dB
= 230 MHz, +5 V or ±5 V supplies,
Current Feedback, Disable Function
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
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
REFERENCE CONFIGURATIONS
The figures associated with this section on internal and external
reference operation do not show recommended matching series resistors
for VINA and VINB for the purpose of simplicity. Please refer to
section “Driving the Analog Inputs, Introduction” for a discussion of
this topic. Also, the figures do not show the decoupling network asso-
ciated with the CAPT and CAPB pins. Please refer to the section “Ref-
erence Operation” for a discussion of the internal reference circuitry
and the recommended decoupling network shown in Figure 27.
USING THE INTERNAL REFERENCE
Single-Ended Input with 0 to 2 3 VREF Range
Figure 36 shows how to connect the AD9243 for a 0 V to 2 V or
0 V to 5 V input range via pin strapping the SENSE pin. An
intermediate input range of 0 to 2 × VREF can be established
using the resistor programmable configuration in Figure 38 and
connecting VREF to VINB.
In either case, both the common-mode voltage and input span
are directly dependent on the value of VREF. More specifically,
the common-mode voltage is equal to VREF while the input
span is equal to 2 × VREF. Thus, the valid input range extends
from 0 to 2 × VREF. When VINA is ≤ 0 V, the digital output
will be 0000 Hex; when VINA is ≥ 2 × VREF, the digital output
will be 3FFF Hex.
