AD9225-EB Analog Devices Inc, AD9225-EB Datasheet - Page 14

• Manufacturer Part Number: AD9225-EB
• Description: BOARD EVAL FOR AD9225
• Manufacturer: Analog Devices Inc

Specifications of AD9225-EB

• Rohs Status: RoHS non-compliant
• Number Of Adc's: 1
• Number Of Bits: 12
• Sampling Rate (per Second): 25M
• Data Interface: Parallel
• Inputs Per Adc: 1 Differential
• Input Range: 4 Vpp
• Power (typ) @ Conditions: 335mW @ 25MSPS
• Voltage Supply Source: Single Supply
• Operating Temperature: -40°C ~ 85°C
• Utilized Ic / Part: AD9225

AD9225
OP AMP SELECTION GUIDE
Op amp selection for the AD9225 is highly dependent on the
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 care-
fully select an op amp that preserves the performance of the ADC.
This task becomes challenging when the AD9225’s high perfor-
mance capabilities are coupled with other extraneous system level
requirements such as power consumption and cost.
The ability to select the optimal op amp may be further compli-
cated by either limited power supply availability and/or limited
acceptable supplies for a desired op amp. Newer, high performance
op amps typically have input and output range limitations in
accordance 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 head-
room constraints such as rail-to-rail op amps or the ones where
larger supplies can be used should be considered. The following
section describes some op amps currently available from Analog
Devices, Inc. The system designer is always encouraged to contact
the factory or local sales office to be updated on Analog Devices’
latest amplifier product offerings. Highlights of the areas where the
op amps excel and where they may limit the performance of the
AD9225 is also included.
AD8055:
AD8056:
AD9631:
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. In systems that do not need to be
dc-coupled, an RF transformer with a center tap is the best method
to generate differential inputs for the AD9225. It provides all the
benefits of operating the ADC in the differential mode without
contributing additional noise or distortion. An RF transformer also
has the added benefit of providing electrical isolation between the
signal source and the ADC.
An improvement in THD and SFDR performance can be realized
by operating the AD9225 in the differential mode. The perfor-
mance enhancement between the differential and single-ended
mode is most noteworthy as the input frequency approaches and
goes beyond the Nyquist frequency (i.e., f
f
Low cost. Best used for driving single-ended ac-
coupled configuration.
Limit: THD is compromised when output is not
swinging about 0 V.
Dual Version of above amp.
Perfect for single-ended to differential configuration
(see Figure 12). Harmonics cancel each other in
differential drive, making this amplifier highly rec-
ommended for a single-ended input signal source.
Handles input signals past the 20 MHz Nyquist
frequency.
f
Moderate cost.
Good for single-ended drive applications when
signal is anywhere between 0 V and 3 V.
Limits: THD is compromised above 8 MHz.
–3 dB
–3 dB
= 300 MHz.
= 250 MHz.
IN
> f
S
/2).
–14–
The circuit shown in Figure 12 is an ideal method of applying a
differential dc drive to the AD9225. We have used this configura-
tion to drive the AD9225 from 2 V to 4 V spans at frequencies
approaching Nyquist with performance numbers matching those
listed in the Specifications tables (gathered through a transformer).
The dc input is shifted to a dc point swinging symmetrically about
the reference voltage. The optional resistor will provide additional
current if more reference drive is required.
The driver circuit shown in Figure 12 is optimized for dc coupling
applications requiring optimum distortion performance. This
differential op amp driver circuit is configured to convert and level
shift a 2 V p-p single-ended, ground referenced signal to a 4 V p-p
differential signal centered at the VREF level of the ADC. The
circuit is based on two op amps that are configured as matched
unity gain difference amplifiers. The single-ended input signal is
applied to opposing inputs of the difference amplifiers, thus provid-
ing differential drive. The common-mode offset voltage is applied
to the noninverting resistor leg of each difference amplifier provid-
ing the required offset voltage. The common-mode offset can be
varied over a wide span without any serious degradation in distor-
tion performance as shown in Figures 14 and 15, thus providing
some flexibility in improving output compression distortion from
some ± 5 V op amps with limited positive voltage swing.
To protect the AD9225 from an undervoltage fault condition from
op amps specified for ±5 V operation, two diodes to AGND can be
inserted between each op amp output and the AD9225 inputs.
The AD9225 will inherently be protected against any overvoltage
condition if the op amps share the same positive power supply (i.e.,
AVDD) as the AD9225. Note that the gain accuracy and com-
mon-mode rejection of each difference amplifier in this driver
circuit can be enhanced by using a matched thin-film resistor
network (i.e., Ohmtek ORNA5000F) for the op amps. Recall that
the AD9225’s small signal bandwidth is 105 MHz and therefore,
any noise falling within the baseband bandwidth of the AD9225
will degrade its overall noise performance.
The noise performance of each unity gain differential driver circuit
is limited by its inherent noise gain of 2. For unity gain op amps
ONLY, the noise gain can be reduced from 2 to 1 beyond the
input signals passband by adding a shunt capacitor, C
each op amp’s feedback resistor. This will essentially establish a
low-pass filter, which reduces the noise gain to 1 beyond the filter’s
f
Note that the pole established by this filter can also be used as the
real pole of an antialiasing filter.
0V
–3 dB
while simultaneously bandlimiting the input signal to f
Figure 12. Direct Coupled Drive Circuit with
AD8056 Dual Op Amp
VREF
*OPTIONAL
500
500
500
500
500
500
500
500
50
50
10 F
VINA
VINB
AD9225
F
, across
CML
Rev. C
0.1 F
–3 dB
+V
.
R*