AD9243
REV. A
–12–
DRIVING THE ANALOG INPUTS
INTRODUCTION
The AD9243 has a highly flexible input structure allowing it to
interface with single-ended or differential input interface cir-
cuitry. The applications shown in sections “Driving the Analog
Inputs” and “Reference Configurations” along with the infor-
mation 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 may be
appropriate for many data acquisition and imaging applications.
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 AD9243.
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.
Alternatively, the differential mode of operation provides the
best THD and SFDR performance over a wide frequency range.
A transformer coupled differential input should be considered
for the most demanding spectral-based applications which allow
ac coupling (e.g., Direct IF to Digital Conversion). The dc-
coupled differential mode of operation also provides an enhance-
ment in distortion and noise performance at higher input spans.
Furthermore, it allows the AD9243 to be configured for a 5 V
span using op amps specified for +5 V or ±5 V operation.
Single-ended operation requires that VINA be ac or dc coupled
to the input signal source while VINB of the AD9243 be biased
to the appropriate voltage corresponding to a midscale code
transition. Note that signal inversion may be easily accom-
plished by transposing VINA and VINB.
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
AD9243 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 shown in Figure 16.
As is typical of most CMOS devices, exceeding the supply limits
will turn on internal parasitic diodes resulting in transient cur-
rents within the device. Figure 28 shows a simple means of
clamping a dc coupled input with the addition of two series
resistors and two diodes. 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.
AVDD
R
S1
30V
V
CC
V
EE
D2
1N4148
D1
1N4148
R
S2
20V
AD9243
Figure 28. Simple Clamping Circuit
DIFFERENTIAL MODE OF OPERATION
Since not all applications have a signal preconditioned for
differential operation, there is often a need to perform a single-
ended-to-differential conversion. A single-ended-to-differential
conversion can be realized with an RF transformer or a dual op
amp differential driver. The optimum method depends on
whether the application requires the input signal to be ac or dc
coupled to AD9243.
AC Coupling via an RF Transformer
In applications that do not need to be dc coupled, an RF trans-
former with a center tap is the best method to generate differen-
tial inputs for the AD9243. It provides all the benefits of
operating the A/D in the differential mode without contributing
additional noise or distortion. An RF transformer has the added
benefit of providing electrical isolation between the signal source
and the A/D.
Figure 29 shows the schematic of the suggested transformer
circuit. The circuit uses a Mini-Circuits RF transformer, model
#T4-6T, which has an impedance ratio of four (turns ratio of
2). The schematic assumes that the signal source has a 50 Ω
source impedance. The 1:4 impedance ratio requires the 200 Ω
secondary termination for optimum power transfer and VSWR.
The centertap of the transformer provides a convenient means
of level shifting the input signal to a desired common-mode
voltage. Optimum performance can be realized when the centertap
is tied to CML of the AD9243 which is the common-mode bias
level of the internal SHA.
VINA
CML
VINB
AD9243
0.1mF
200V
MINI-CIRCUITS
T4-6T
50V
Figure 29. Transformer Coupled Input
Transformers with other turns ratios may also be selected to
optimize the performance of a given application. For example, a
given input signal source or amplifier may realize an improve-
ment in distortion performance at reduced output power levels
and signal swings. Hence, selecting a transformer with a higher
impedance ratio (i.e., Mini-Circuits T16-6T with a 1:16 imped-
ance ratio) effectively “steps up” the signal level, further reduc-
ing the driving requirements of the signal source.
