AD9240
REV.
–13–
DRIVING THE ANALOG INPUTS
INTRODUCTION
The AD9240 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 informa-
tion presented in the Input and Reference Overview section of
this data sheet, give examples of both single-ended and differen-
tial 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 AD9240.
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 AD9240 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 AD9240 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
AD9240 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 prod-
ucts 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 31 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
AD9240
Figure 31. 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 AD9240.
AC Coupling via an RF Transformer
An RF transformer with a center tap can be used to generate
differential inputs for the AD9240. It provides all of the benefits
of operating the ADC in the differential mode while contribut-
ing no additional noise and minimal distortion. As a result, an
RF transformer is recommended in high frequency applica-
tions, especially undersampling, in which the performance of
a dual op amp differential driver may not be adequate. An RF
transformer has the added benefit of providing electrical isola-
tion between the signal source and the ADC. However, since the
lower cutoff frequency of most RF transformers is nominally a
few 100 kHz, a dual op amp differential driver may be more suit-
able in ac-coupling applications, where the spectral content of the
input signal falls below the cutoff frequency of a suitable RF
transformer.
Figure 32 is a suggested transformer circuit using a Mini-
Circuits RF transformer, model #T4-6T, which has an imped-
ance ratio of four (turns ratio of 2). 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 de-
sired common-mode voltage. Optimum performance can be
realized when the centertap is tied to CML of the AD9240
which is the common-mode bias level of the internal SHA.
VINA
CML
VINB
AD9240
0.1mF
200V
MINI-CIRCUITS
T4-6T
50V
Figure 32. 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.
