AD9240 Analog Devices, AD9240 Datasheet - Page 14

• 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
AC Coupling with Op Amps
As previously stated, a dual op amp differential driver may be
more suitable in applications in which the spectral content of the
input signal falls below the cutoff frequency of a suitable RF
transformer and/or the cost of an RF transformer and a low
distortion driver for the transformer is prohibitive.
The ac-coupled differential driver shown in Figure 33 is best
suited for 5 V systems in which the input signal is ground
referenced. In this case, V
achieve performance similar to an RF transformer over the
AD9240’s full Nyquist bandwidth of 5 MHz. However, unlike
the RF transformer, the lower cutoff frequency can be arbitrarily
set low by adjusting the RC time constant formed by C
R
tribution of op amp-generated noise at higher frequencies. Low
cost, high performance dual op amps operating from 5 V such
as the AD8056 and AD8058, are excellent choices for this appli-
cation and are capable of maintaining 78 dB SNR and 83 dB
THD at 1 MHz (5 V span). An optional resistor R
added to U1B to achieve a similar group delay as U1A, potentially
improving overall distortion performance. A resistor divider net-
work formed by R
AVDD/2 to achieve its optimum distortion performance.
DC Coupling with Op Amps
The dc-coupled differential driver in Figure 34 is best suited for
optimum distortion performance is desired. This driver circuit
provides the ability to level-shift the input signal to within the
common-mode range of the AD9240. The two op amps are
configured as matched differential amps with the input signal
applied to opposing inputs to provide the differential output.
The common-mode offset voltage is applied to the noninverting
resistor network, which provides the proper level shifting. The
AD9631 is given as the amplifier of choice in this application
due to its superior distortion performance for relatively large
output swings and wide bandwidth. If cost or space are factors,
the AD8056 dual op amp will save on both, but at the cost of
slightly increased distortion with large signal levels. Figure 34
also illustrates the use of protection diodes, which are used to
protect the AD9240 from any fault condition in which the op
amps outputs inadvertently go above V
B
5 V systems in which the input signal is ground referenced and
/2. C
SOURCE
N
, in combination with R
V
CM
Figure 33. AC Coupling of Op Amps
R
O
B
centers the inputs of the AD9240 around
U1A
U1B
CM
will be 0 V. This driver circuit can
S
, can be used to limit the con-
R
R
S
S
C
N
DD
or below GND.
0.1 F
0.1 F
C
C
C
C
O
R
5k
R
5k
B
B
can be
V
V
CC
CC
C
R
5k
R
5k
and
B
B
–14–
Single Supply DC-Coupled Driver
The circuit of Figure 33 can be easily modified for a single
supply, dc-coupled application. This is done by biasing V
AVDD/2, the normal common-mode level in a single supply
system. Since the outputs of the op amps are centered at
AVDD/2, the ac coupling network of C
With this done, the differential driving pair can now be run from
a single supply.
SINGLE-ENDED MODE OF OPERATION
The AD9240 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 de-
grade 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 requirements. Both dc
and ac coupling provide this necessary function, but each
method results in different interface issues which may influence
the system design and performance.
DC COUPLING AND INTERFACE ISSUES
Many applications require the analog input signal to be dc
coupled to the AD9240. An operational amplifier can be con-
figured to rescale and level-shift the input signal so it is compat-
ible with the selected input range of the A/D. The input range
to the A/D should be selected on the basis of system perfor-
mance objectives as well as the analog power supply availability
since this will place certain constraints on the op amp selection.
Many of the new high performance op amps are specified for
only 5 V operation and have limited input/output swing capa-
bilities. Hence, the selected input range of the AD9240 should
be sensitive to the headroom requirements of the particular op
amp to prevent clipping of the signal. Also, since the output of a
dual supply amplifier can swing below –0.3 V, clamping its
output should be considered in some applications.
In some applications, it may be advantageous to use an op amp
specified for single supply +5 V operation since it will inherently
limit its output swing to within the power supply rails. Rail-to-
rail output amplifiers such as the AD8041 allow the AD9240 to
be configured with larger input spans which improves the noise
performance.
V
IN
Figure 34. Differential Driver with Level-Shifting
390
390
390
390
AD9631
AD9631
390
390
390
390
0.1 F
0.1 F
AVDD
220
0.1 F
AVDD
220
V
V
CML
CML
2.5k
100
1 F
C
–VIN
+VIN
and R
OP113
0.1 F
33
33
B
can be removed.
VINA
VINB
CML
AD9240
REV.
CM
to
B