AD9240 Analog Devices, AD9240 Datasheet - Page 16

• Manufacturer Part Number: AD9240
• Description: Complete 14-Bit, 10 MSPS Monolithic A/D Converter
• Manufacturer: Analog Devices

Specifications of AD9240

• Resolution (bits): 14bit
• # Chan: 1
• Sample Rate: 10MSPS
• 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

AD9240
degrade slightly as the input common-mode voltage deviates
from its optimum level of 2.5 V.
Alternative AC Interface
Figure 38 shows a flexible ac-coupled circuit which can be con-
figured for different input spans. Since the common-mode
voltage of VINA and VINB are biased to midsupply indepen-
dent of VREF, VREF can be pin-strapped or reconfigured to
achieve input spans between 2 V and 5 V p-p. The AD9240’s
CMRR along with the symmetrical coupling R-C networks will
reject both power supply variations and noise. The resistors, R,
establish the common-mode voltage. They may have a high value
(e.g., 5 k ) to minimize power consumption and establish a low
cutoff frequency. The capacitors, C1 and C2, are typically a
0.1 F ceramic and 10 F tantalum capacitor in parallel to
achieve a low cutoff frequency while maintaining a low imped-
ance over a wide frequency range. R
fier from the A/D input. The optimum performance is achieved
when VINA and VINB are driven via symmetrical networks.
The high pass f
OP AMP SELECTION GUIDE
Op amp selection for the AD9240 is highly dependent on a
particular application. In general, the performance requirements
of any given application can be characterized by either time
domain or frequency domain parameters. In either case, one
should carefully select an op amp that preserves the perfor-
mance of the A/D. This task becomes challenging when one
considers the AD9240’s high performance capabilities coupled
with other external system level requirements such as power
consumption and cost.
The ability to select the optimal op amp may be further compli-
cated by limited power supply availability and/or limited accept-
able supplies for a desired op amp. Newer, high performance op
amps typically have input and output range limitations in accor-
dance with their lower supply voltages. As a result, some op
amps will be more appropriate in systems where ac-coupling is
allowable. When dc-coupling is required, op amps without
headroom constraints such as rail-to-rail op amps or ones where
larger supplies can be used should be considered. The following
section describes some op amps currently available from Analog
Devices. The system designer is always encouraged to contact
the factory or local sales office to be updated on Analog De-
vices’ latest amplifier product offerings. Highlights of the areas
where the op amps excel and where they may limit the perfor-
mance of the AD9240 are also included.
V
V
Figure 38. AC-Coupled Input-Flexible Input Span,
IN
CM
= 2.5 V
+5V
–5V
f
+5V
–3 dB
–3 dB
= 1/(2
point can be approximated by the equation,
R
C2
C1
R
R/2
C2
S
+5V
isolates the buffer ampli-
(C1 + C2))
R
R
C1
R
R
S
S
VINA
VINB
AD9240
–16–
AD9631:
AD8047:
AD8042:
REFERENCE CONFIGURATIONS
For the purpose of simplicity, the figures associated with this
section on internal and external reference operation do not
show recommended matching series resistors for VINA and
VINB. Please refer to section Driving the Analog Inputs, Intro-
duction, for a discussion of this topic. The figures do not show
the decoupling network associated with the CAPT and CAPB
pins. Please refer to the Reference Operation section for a discus-
sion of the internal reference circuitry and the recommended
decoupling network shown in Figure 30.
USING THE INTERNAL REFERENCE
Single-Ended Input with 0 to 2
Figure 39 shows how to connect the AD9240 for a 0 V to 2 V or
0 V to 5 V input range via pin strapping the SENSE pin. An
intermediate input range of 0 to 2 VREF can be established
using the resistor programmable configuration in Figure 41 and
connecting VREF to VINB.
In either case, both the common-mode voltage and input span
are directly dependent on the value of VREF. More specifically,
the common-mode voltage is equal to VREF while the input
span is equal to 2 VREF. Thus, the valid input range extends
from 0 to 2
will be 0000 Hex; when VINA is 2
will be 3FFF Hex.
Shorting the VREF pin directly to the SENSE pin places the
internal reference amplifier in unity-gain mode and the result-
ant VREF output is 1 V. The valid input range is, therefore, 0 V
to 2 V. Shorting the SENSE pin directly to the REFCOM pin
configures the internal reference amplifier for a gain of 2.5 and
2xVREF
Figure 39. Internal Reference (2 V p-p Input Span,
V
CM
0V
= 1 V, or 5 V p-p Input Span, V
220 MHz Unity GBW, 16 ns Settling to 0.01%,
Best Applications: Best AC Specs, Low Noise,
AC-Coupled
Limits: Usable Input/Output Range, Power
Consumption
130 MHz Unity GBW, 30 ns Settling to 0.01%,
Best Applications: Good AC Specs, Low Noise,
AC-Coupled
Limits: THD > 5 MHz, Usable Input Range
Dual AD8041
Best Applications: Differential and/or Low Imped-
ance Input Drivers
Limits: Noise with 2 V Input Range
5 V Supplies
5 V Supplies
VREF. When VINA is
SHORT FOR 0 TO 2V
SHORT FOR 0 TO 5V
10 F
INPUT SPAN
INPUT SPAN
0.1 F
VREF Range
0 V, the digital output
VREF, the digital output
CM
VINA
VINB
VREF
SENSE
REFCOM
= 2.5 V)
AD9240
REV.
B