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參數資料
| 型號： | AD9224-EB |
| 廠商： | Analog Devices, Inc. |
| 英文描述： | Complete 12-Bit 40 MSPS Monolithic A/D Converter |
| 中文描述： | 完整的12位40 MSPS的單片A / D轉換 |
| 文件頁數： | 14/24頁 |
| 文件大小： | 309K |
| 代理商： | AD9224-EB |
AD9224
–14–
REV. A
Alternative AC Interface
Figure 22 shows a flexible ac-coupled circuit that can be con-
figured for different input spans. Since the common-mode
voltage of VINA and VINB are biased to midsupply (V
CM
)
independent of VREF, VREF can be pin strapped or reconfig-
ured to achieve input spans between 2 V and 4 V p-p. The
AD9224’s CMRR, along with the symmetrical coupling R-C
networks, will reject both power supply variations and noise.
V
CM
establishes the common-mode voltage. V
CM
’s source im-
pedance is 5 k
. The capacitors, C1 and C2, are typically a
0.1
μ
F ceramic and 10
μ
F tantalum capacitor in parallel to
achieve a low cutoff frequency while maintaining a low imped-
ance over a wide frequency range. R
S
isolates the buffer ampli-
fier from the A/D input. The optimum performance is preserved
because VINA and VINB are driven via symmetrical R-C net-
works. The f
–3 dB
point can be approximated by the equation,
f
–3
dB
=
1
2
π×
6
K
+
(
C
1
+
C
2)
VINA
VINB
AD9224
1k
V
R
S
V
IN
C2
0.1
m
F
R
S
VCM
1k
V
C2
0.1
m
F
C1
10
m
F
C1
10
m
F
C3
0.1
m
F
Figure 22. AC-Coupled Input-Flexible Input Span,
V
CM
= 2.5 V
OP AMP SELECTION GUIDE
Op amp selection for the AD9224 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 that preserves the perfor-
mance of the A/D. This task becomes challenging when one
considers the AD9224’s high performance capabilities coupled
with other extraneous 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 perfor-
mance op amps typically have input and output range limita-
tions 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 avail-
able from Analog Devices. The system designer is always en-
couraged 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 AD9224 is also included.
When single-ended, dc coupling is needed. The use of the
AD8056 in a differential configuration (Figure 23) is highly
recommended.
AD8055:
f
–3 dB
= 300 MHz.
Low cost. Best used for driving single-ended ac
coupled configuration.
Limit: THD is compromised when output is not
swinging about 0 V.
AD8056:
Dual Version of above amp.
Perfect for single-ended to differential configuration
(see Figure 23). Harmonics cancel each other in
differential drive, making this amplifier highly recom-
mended for a single-ended input signal source. Handles
input signals past the 20 MHz Nyquist frequency.
AD9631:
f
–3 dB
= 250 MHz.
Moderate cost.
Good for single-ended drive applications when signal
is anywhere between 0 V and 3 V.
Limits: THD is compromised above 8 MHz.
DIFFERENTIAL MODE OF OPERATION
Since not all applications have a signal preconditioned for differ-
ential operation, there is often a need to perform a single-ended-
to-differential conversion. In systems that do not need to be dc
coupled, an RF transformer with a center tap is the best method
to generate differential inputs for the AD9224. It provides all
the benefits of operating the A/D in the differential mode with-
out contributing additional noise or distortion. An RF transformer
also has the added benefit of providing electrical isolation be-
tween the signal source and the A/D.
An improvement in THD and SFDR performance can be real-
ized by operating the AD9224 in the differential mode. The
performance enhancement between the differential and single-
ended mode is most noteworthy as the input frequency approaches
and goes beyond the Nyquist frequency (i.e., f
IN
> F
S
/2).
The circuit shown in Figure 23 is an ideal method of applying a
differential dc drive to the AD9224. We have used this configu-
ration to drive the AD9224 from 2 V to 4 V spans at frequencies
approaching Nyquist, with performance numbers matching
those shown on the Specification pages of this data sheet (gath-
ered through a transformer). The dc input is shifted to a dc
point swinging symmetrically about the reference voltage. The
optional resistor will provide additional current if more refer-
ence drive is required.
VINA
VINB
CML
+V
R*
0.1
m
F
500
V
VREF
0V
10
m
F
500
V
500
V
500
V
500
V
500
V
500
V
500
V
*OPTIONAL
50
V
50
V
AD9224
Figure 23. Direct Coupled Drive Circuit with AD8056 Dual
Op Amps
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