AD9221/AD9223/AD9220
SFDR over a wide range of amplitudes required by the most
demanding communication applications. Similar performance is
achievable with the AD9221 and AD9223 at their correspond-
ing Nyquist frequency.
pin configures the internal reference amplifier for a gain of 2.5
and the resultant VREF output is 2.5 V. Thus, the valid input
range becomes 0 V to 5 V. The VREF pin should be bypassed
to the REFCOM pin with a 10 µF tantalum capacitor in parallel
with a low-inductance 0.1 µF ceramic capacitor.
90
AD9221/
VINA
SFDR – 5.0V p-p
80
AD9223/
AD9220
0V
SFDR – 2.0V p-p
70
VINB
10F
0.1F
VREF
60
50
SHORT FOR 0V TO 2V
INPUT SPAN
SENSE
SNR – 2.0V p-p
SHORT FOR 0V TO 5V
INPUT SPAN
40
SNR – 5.0V p-p
REFCOM
30
20
Figure 47. Internal Reference—2 V p-p Input Span, VCM
1 V, or 5 V p-p Input Span, VCM = 2.5 V
=
–50
–40
–30
–20
–10
0
INPUT AMPLITUDE – dBFS
Single-Ended or Differential Input, VCM = 2.5 V
Figure 46. AD9220 SFDR, SNR vs. Input Amplitude
(fIN = 5 MHz, fCLK = 10 MSPS, VCM = 2.5 V, Differential)
Figure 48 shows the single-ended configuration that gives the
best dynamic performance (SINAD, SFDR). To optimize
dynamic specifications, center the common-mode voltage of the
analog input at approximately by 2.5 V by connecting VINB to
a low-impedance 2.5 V source. As described above, shorting
the VREF pin directly to the SENSE pin results in a 1 V refer-
ence voltage and a 2 V p-p input span. The valid range for
input signals is 1.5 V to 3.5 V. The VREF pin should be by-
passed to the REFCOM pin with a 10 µF tantalum capacitor in
parallel with a low-inductance 0.1 µF ceramic capacitor.
Figure 46 also reveals a noteworthy difference in the SFDR and
SNR performance of the AD9220 between the 2 V p-p and 5 V
p-p input span options. First, the SNR performance improves
by 2 dB with a 5.0 V p-p input span due to the increase in dy-
namic range. Second, the SFDR performance of the AD9220
will improve for input signals below approximately –6.0 dBFS.
A 3 dB to 5 dB improvement was typically realized for input
signal levels between –6.0 dBFS and –36 dBFS. This improve-
ment in SNR and SFDR for a 5.0 V p-p span may be advanta-
geous for communication systems that have additional margin
or headroom to minimize clipping of the ADC.
This reference configuration could also be used for a differential
input in which VINA and VINB are driven via a transformer as
shown in Figure 45. In this case, the common-mode voltage,
VCM, is set at midsupply by connecting the transformers center
tap to CML of the AD9221/AD9223/AD9220. VREF can be
configured for 1 V or 2.5 V by connecting SENSE to either
VREF or REFCOM respectively. Note that the valid input
range for each of the differential input is one half of the single-
ended input and thus becomes VCM – VREF/2 to VCM + VREF/2.
REFERENCE CONFIGURATIONS
The figures associated with this section on internal and external
reference operation do not show recommended matching series resistors
for VINA and VINB for the purpose of simplicity. Please refer to
section Driving the Analog Inputs, Introduction for a discussion of
this topic. Also, the figures do not show the decoupling network asso-
ciated with the CAPT and CAPB pins. Please refer to the section “Ref-
erence Operation” for a discussion of the internal reference circuitry
and the recommended decoupling network shown in Figure 36.
3.5V
VINA
VINB
AD9221/
AD9223/
AD9220
1.5V
2.5V
USING THE INTERNAL REFERENCE
1V
VREF
Single-Ended Input with 0 to 2 ؋
VREF Range
SENSE
0.1F
10F
Figure 47 shows how to connect the AD9221/AD9223/AD9220
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 49 and connecting VREF to VINB.
REFCOM
Figure 48. Internal Reference—2 V p-p Input Span,
CM = 2.5 V
V
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 × VREF. When VINA is ≤ 0 V, the digital output
will be 000 Hex; when VINA is ≥ 2 × VREF, the digital output
will be FFF Hex.
Resistor Programmable Reference
Figure 49 shows an example of how to generate a reference
voltage other than 1 V or 2.5 V with the addition of two exter-
nal resistors and a bypass capacitor. Use the equation,
VREF = 1 V × (1 + R1/R2),
to determine appropriate values for R1 and R2. These resistors
should be in the 2 kΩ to 100 kΩ range. For the example shown,
R1 equals 2.5 kΩ and R2 equals 5 kΩ. From the equation
above, the resultant reference voltage on the VREF pin is
Shorting the VREF pin directly to the SENSE pin places the
internal reference amplifier in unity-gain mode and the resultant
VREF output is 1 V. Therefore, the valid input range is 0 V to
2 V. However, shorting the SENSE pin directly to the REFCOM
–18–
REV. D
ADI [ ADI ]
