AD9241 Analog Devices, AD9241 Datasheet - Page 12

• Manufacturer Part Number: AD9241
• Description: Complete 14-Bit, 1.25 MSPS Monolithic A/D Converter
• Manufacturer: Analog Devices

Specifications of AD9241

• Resolution (bits): 14bit
• # Chan: 1
• Sample Rate: 1.25MSPS
• Interface: Par
• Analog Input Type: Diff-Uni,SE-Uni
• Ain Range: (2Vref) p-p,2 V p-p,5V p-p,Uni (Vref) x 2,Uni 2.0V,Uni 5.0V
• Adc Architecture: Pipelined
• Pkg Type: QFP

AD9241
DRIVING THE ANALOG INPUTS
INTRODUCTION
The AD9241 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 Input and Reference Overview of this data
sheet, give examples of both single-ended and differential opera-
tion. 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 requiring a dc coupled
input for proper demodulation can take advantage of the excel-
lent single-ended distortion performance of the AD9241. The
input span should be configured so 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 that 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 AD9241 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 AD9241 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
AD9241 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 that 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 28 shows a simple means of clamp-
ing 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.
–12–
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. A single-ended-to-differential conversion
can be realized with an RF transformer or a dual op amp differ-
ential driver. The optimum method depends on whether the
application requires the input signal to be ac or dc coupled to
AD9241.
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 of generating differ-
ential inputs for the AD9241. 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
source impedance. The 1:4 impedance ratio requires the 200
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 AD9241 which is the common-mode bias
level of the internal SHA.
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.
Figure 29. Transformer Coupled Input
50
Figure 28. Simple Clamping Circuit
V
V
CC
EE
MINI-CIRCUITS
T4-6T
30
R
S1
200
AVDD
D2
1N4148
D1
1N4148
0.1µF
20
R
S2
VINA
CML
VINB
AD9243
AD9241
REV. 0