AD9221/AD9223/AD9220
DRIVING THE ANALOG INPUTS
INTRODUCTION
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. Fig-
ure 37 depicts the common-mode rejection of the three devices.
The AD9221/AD9223/AD9220 has a highly flexible input struc-
ture allowing it to interface with single-ended or differential
input interface circuitry. The applications shown in sections
Driving the Analog Inputs and Reference Configurations, along
with the information presented in Input and Reference Over-
view 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.
As is typical of most CMOS devices, exceeding the supply
limits will turn on internal parasitic diodes resulting in transient
currents within the device. Figure 38 shows a simple means of
clamping an ac or dc coupled single-ended input with the addi-
tion of two series resistors and two diodes. An optional capaci-
tor 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 degra-
dation in overall performance. A similar clamping circuit could
also be used for each input if a differential input signal is being
applied.
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 applica-
tions. Also, many communication applications which require a
dc coupled input for proper demodulation can take advantage of
the excellent single-ended distortion performance of the AD9221/
AD9223/AD9220. The input span should be configured such
that the system’s performance objectives and the headroom
requirements of the driving op amp are simultaneously met.
OPTIONAL
AC COUPLING
CAPACITOR
AVDD
D2
V
CC
R
30⍀
R
S1
S2
1N4148
AD9221/
AD9223/
AD9220
20⍀
D1
1N4148
Alternatively, the differential mode of operation with a trans-
former coupled input provides the best THD and SFDR perfor-
mance over a wide frequency range. This mode of operation
should be considered for the most demanding spectral-based
applications which allow ac coupling (e.g., Direct IF to Digital
Conversion).
V
EE
Figure 38. Simple Clamping Circuit
SINGLE-ENDED MODE OF OPERATION
The AD9221/AD9223/AD9220 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 degrade 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 require-
ments. Both dc and ac coupling provide this necessary func-
tion, but each method results in different interface issues which
may influence the system design and performance.
Single-ended operation requires that VINA be ac or dc coupled
to the input signal source while VINB of the AD9221/AD9223/
AD9220 be biased to the appropriate voltage corresponding to a
midscale code transition. Note that signal inversion may be
easily accomplished by transposing VINA and VINB. The rated
specifications for the AD9221/AD9223/AD9220 are characterized
using single-ended circuitry with input spans of 5 V and 2 V as well
as VINB = 2.5 V.
DC COUPLING AND INTERFACE ISSUES
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
AD9221/AD9223/AD9220 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
Many applications require the analog input signal to be dc
coupled to the AD9221/AD9223/AD9220. 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.
20
Many of the new high performance op amps are specified for
only ± 5 V operation and have limited input/output swing
capabilities. Hence, the selected input range of the AD9221/
AD9223/AD9220 should be sensitive to the headroom require-
ments of the particular op amp to prevent clipping of the sig-
nal. Also, since the output of a dual supply amplifier can swing
below –0.3 V, clamping its output should be considered in
some applications.
30
40
AD9223
AD9221
50
60
AD9220
70
In some applications, it may be advantageous to use an op amp
specified for single supply +5 V operation since it will inher-
ently limit its output swing to within the power supply rails.
An amplifier like the AD8041, AD8011, and AD817 are useful
for this purpose. Rail-to-rail output amplifiers such as the
AD8041 allow the AD9221/AD9223/AD9220 to be configured
for larger input spans which improves the noise performance.
80
90
0.1
1
10
FREQUENCY– MHz
100
Figure 37. AD9221/AD9223/AD9220 Input CMR vs. Input
Frequency
–14–
REV. D
ADI [ ADI ]
