AD9223 Analog Devices, AD9223 Datasheet - Page 16

• Manufacturer Part Number: AD9223
• Description: 12-Bit, 3.0 MSPS A/D Converter
• Manufacturer: Analog Devices

Specifications of AD9223

• Resolution (bits): 12bit
• # Chan: 1
• Sample Rate: 3MSPS
• Interface: Par
• Analog Input Type: Diff-Uni,SE-Uni
• Ain Range: 2 V p-p,5V p-p
• Adc Architecture: Pipelined
• Pkg Type: SOIC,SOP

AD9221/AD9223/AD9220
AC COUPLING AND INTERFACE ISSUES
For applications where ac coupling is appropriate, the op amp’s
output can be easily level shifted to the common-mode voltage,
VCM, of the AD9221/AD9223/AD9220 via a coupling capacitor.
This has the advantage of allowing the op amp’s common-mode
level to be symmetrically biased to its midsupply level (i.e.,
(V
to their power supplies typically provide the best ac performance
as well as the greatest input/output span. Thus, various high
speed/performance amplifiers that are restricted to +5 V/–5 V
operation and/or specified for 5 V single-supply operation can be
easily configured for the 5 V or 2 V input span of the AD9221/
AD9223/AD9220. The best ac distortion performance is achieved
when the A/D is configured for a 2 V input span and common-
mode voltage of 2.5 V. Note that differential transformer coupling,
which is another form of ac coupling, should be considered for
optimum ac performance.
Simple AC Interface
Figure 15 shows a typical example of an ac-coupled, single-ended
configuration. The bias voltage shifts the bipolar, ground-refer-
enced input signal to approximately VREF. The value for C1
and C2 will depend on the size of the resistor, R. The capacitors,
C1 and C2, are typically a 0.1 µF ceramic and 10 µF tanta-
lum capacitor in parallel to achieve a low cutoff frequency
while maintaining a low impedance over a wide frequency
range. The combination of the capacitor and the resistor form a
high-pass filter with a high-pass –3 dB frequency determined
by the equation,
The low impedance VREF voltage source both biases the VINB
input and provides the bias voltage for the VINA input. Figure 15
shows the VREF configured for 2.5 V; thus the input range
of the A/D is 0 V to 5 V. Other input ranges could be selected
by changing VREF, but the A/D’s distortion performance will
degrade slightly as the input common-mode voltage deviates
from its optimum level of 2.5 V.
Alternative AC Interface
Figure 16 shows a flexible ac-coupled circuit that can be config-
ured for different input spans. Since the common-mode voltage
of VINA and VINB are biased to midsupply independent of
VREF, VREF can be pin-strapped or reconfigured to achieve
input spans between 2 V and 5 V p-p. The AD9221/AD9223/
AD9220’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 kΩ) to minimize power consumption
and establish a low cutoff frequency. The capacitors, C1 and
C2, are typically 0.1 µF ceramic and 10 µF tantalum capacitors
+VREF
–VREF
CC
0V
+ V
EE
)/2). Op amps that operate symmetrically with respect
V
IN
f
Figure 15. AC-Coupled Input
3
dB
=
–5V
+5V
1 2
/
(
× ×
π
C2
R
C1
C2
×
(
C
C1
R
1
+
C
R
R
2
S
S
)
)
VINA
VINB
VREF
SENSE
AD9221/
AD9223/
AD9220
–16–
in parallel to achieve a low cutoff frequency while maintaining a
low impedance over a wide frequency range. R
buffer amplifier from the A/D input. The optimum performance
is achieved when VINA and VINB are driven via «Immetrical
networks. The f
Op Amp Selection Guide
Op amp selection for the AD9221/AD9223/AD9220 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
performance of the A/D. This task becomes challenging when
one considers the AD9221/AD9223/AD9220’s high perfor-
mance capabilities 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 perfor-
mance 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
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
Devices’ latest amplifier product offerings. Highlights of the
areas where the op amps excel and where they may limit the
performance of the AD9221/AD9223/AD9220 is also included.
AD817:
AD826:
AD818:
Figure 16. AC-Coupled Input-Flexible Input Span,
V
V
IN
CM
= 2 V
50 MHz Unity GBW, 70 ns Settling to 0.01%, +5 V
to ± 15 V Supplies
Best Applications: Sample Rates < 7 MSPS, Low
Noise, 5 V p-p Input Range
Limits: THD above 100 kHz
Dual Version of AD817
Best Applications: Differential and/or Low Imped-
ance Input
Drivers, Low Noise
Limits: THD above 100 kHz
130 MHz @ G = +2 BW, 80 ns Settling to 0.01%,
+5 V to ± 15 V Supplies
Best Applications: Sample Rates < 7 MSPS, Low
Noise, 5 V p-p Input Range, Gains ≥ +2
Limits: THD above 100 kHz
f
–
–5V
+5V
–3 dB
3
+5V
dB
=
point can be approximated by the equation,
1 2
R
/
(
× ×
R
C2
π
C1
R
C2
/
+5V
2
R
R
×
C1
(
R
R
C
S
S
1
+
C
VINA
VINB
2
AD9220
AD9221/
AD9223/
S
)
)
isolates the
REV. E