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参数资料
| 型号： | AD9224-EB |
| 厂商： | 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的单片A / D转换 |
| 文件页数： | 12/24页 |
| 文件大小： | 309K |
| 代理商： | AD9224-EB |
AD9224
–12–
REV. A
DRIVING THE ANALOG INPUTS
The AD9224 has a highly flexible input structure allowing it to
interface with single-ended or differential input interface cir-
cuitry. The applications shown in Driving the Analog Inputs and
Reference Configurations sections, along with the information
presented in Input and Reference Overview of this data sheet,
give examples of both single-ended and differential operation.
Refer to Tables I and II for a list of the different possible input
and reference configurations and their associated figures in the
data sheet.
The optimum mode of operation, analog input range, and asso-
ciated interface circuitry will be determined by the particular
applications performance requirements as well as power supply
options. For example, a dc-coupled single-ended input would be
appropriate for most data acquisition and imaging applications.
Also, many communication applications that require a dc coupled
input for proper demodulation can take advantage of the
single-ended distortion performance of the AD9224. The input
span should be configured so the system’s performance objec-
tives and the headroom requirements of the driving op amp are
simultaneously met.
Differential modes of operation (ac or dc coupled input) provide
the best THD and SFDR performance over a wide frequency
range.
Differential operation should be considered for the most de-
manding spectral based applications
(e.g., direct IF-to-digital con-
version). See Figures 23, 24 and section on Differential Mode of
Operation. Differential input characterization was performed for
this data sheet using the configuration shown in Figure 24.
Single-ended operation requires that VINA be ac or dc coupled
to the input signal source, while VINB of the AD9224 be biased
to the appropriate voltage corresponding to a midscale code transi-
tion. Note that signal inversion may be easily accomplished by
transposing VINA and VINB. Most of the single-ended specifi-
cations for the AD9224 were characterized using Figure 21
circuitry with input spans of 4 V and 2 V as well as V
CM
= 2.5 V.
Differential operation requires that VINA and VINB be simulta-
neously driven with two equal signals that are in and out of
phase versions of the input signal. Differential operation of the
AD9224 offers the following benefits: (1) Signal swings are
smaller and therefore linearity requirements placed on the input
signal source may be easier to achieve, (2) Signal swings are
smaller and therefore may allow the use of op amps which may
otherwise have been constrained by headroom limitations, (3)
Differential operation minimizes even-order harmonic products,
and (4) Differential operation offers noise immunity based on
the device’s common-mode rejection.
As is typical of most IC devices, exceeding the supply limits will
turn on internal parasitic diodes resulting in transient currents
within the device. Figure 18 shows a simple means of clamping
an ac or dc coupled single-ended input with the addition of two
series resistors and two diodes. An optional capacitor is shown
for ac coupled applications. Note that a larger series resistor
could be used to limit the fault current through D1 and D2 but
should be evaluated since it can cause a degradation in overall
performance. A similar clamping circuit could also be used for
each input if a differential input signal is being applied. The
diodes might cause nonlinearity in the signal. Careful evaluation
should be performed on the diodes used.
AVDD
AD9224
R
S1
30
V
V
CC
V
EE
OPTIONAL
AC COUPLING
CAPACITOR
D2
D1
R
S2
20
V
Figure 18. Simple Clamping Circuit
SINGLE-ENDED MODE OF OPERATION
The AD9224 can be configured for single-ended operation
using dc or ac coupling. In either case, the input of the A/D
must be driven from an operational amplifier that will not de-
grade the A/D’s performance. Because the A/D operates from a
single supply, it will be necessary to level shift ground-based
bipolar signals to comply with its input requirements. Both dc
and ac coupling provide this necessary function, but each method
results in different interface issues which may influence the
system design and performance.
Single-ended operation is often limited by the availability driv-
ing op amps. Very low distortion op amps that provide great
performance out to the Nyquist frequency of the converter are
hard to find. Compounding the problem, for dc coupled single-
ended applications, is the inability of the many high perfor-
mance amplifiers to maintain low distortions as their outputs
approach their positive output voltage limit (i.e., 1 dB compres-
sion point). For this reason, it is recommended that applications
requiring high performance dc coupling use the single-ended-to-
differential circuit shown in Figure 23.
DC COUPLING AND INTERFACE ISSUES
Many applications require the analog input signal to be dc coupled
to the AD9224. An operational amplifier can be configured to
rescale and level shift the input signal so that it is compatible
with the selected input range of the A/D. The input range to the
A/D should be selected on the basis of system performance
objectives as well as the analog power supply availability since
this will place certain constraints on the op amp selection.
Many of the new high performance op amps are specified for
only
±
5 V operation and have limited input/output swing capa-
bilities. The selected input range of the AD9224 should be consid-
ered with the headroom requirements of the particular op amp to
prevent clipping of the signal. Also, since the output of a dual
supply amplifier can swing below absolute minimum (–0.3 V),
clamping its output should be considered in some applications.
In some applications, it may be advantageous to use an op amp
specified for single supply +5 V operation since it will inherently
limit its output swing to within the power supply rails. Ampli-
fiers like the AD8041 and AD8011 are useful for this purpose
but their low bandwidths will limit the AD9224’s performance.
High performance amplifiers (
±
5 V) such as the AD9631,
AD9632, AD8056 or AD8055 allow the AD9224 to be config-
ured for larger input spans which will improve the ADC’s noise
performance.
Op amp circuits using a noninverting and inverting topologies
are discussed in the next section. Although not shown, the non-
inverting and inverting topologies can be easily configured as
part of an antialiasing filter by using a Sallen-Key or Multiple-
Feedback topology. An additional R-C network can be inserted
between the op amp’s output and the AD9224 input to provide
a filter pole.
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