AD9221/AD9223/AD9220
–55
–65
–75
–85
–95
DIFFERENTIAL MODE OF OPERATION
Since not all applications have a signal preconditioned for differ-
ential operation, there is often a need to perform a single-ended-
to-differential conversion. In systems which do not need to be
dc coupled, an RF transformer with a center tap is the best
method to generate differential inputs for the AD9221/AD9223/
AD9220. It provides all the benefits of operating the A/D in the
differential mode without contributing additional noise or dis-
tortion. An RF transformer also has the added benefit of provid-
ing electrical isolation between the signal source and the A/D.
AD9221
AD9223
AD9220
Note that although a single-ended-to-differential op amp topol-
ogy would allow dc coupling of the input signal, no significant
improvement in THD performance was realized when compared
to the dc single-ended mode of operation up to the AD9221/
AD9223/AD9220’s Nyquist frequency (i.e., fIN < FS/2). Also,
the additional op amp required in the topology tends to in-
creases the total system noise, power consumption, and cost.
Hence, a single-ended mode of operation is recommended for
most applications requiring dc coupling.
1
10
100
FREQUENCY – MHz
Figure 44. AD9221/AD9223/AD9220 SFDR vs. Input Fre-
quency (VCM = 2.5 V, 2 V p-p Input Span, AIN = –0.5 dB)
Figure 45 shows the schematic of the suggested transformer
circuit. The circuit uses a minicircuits RF transformer, model
#T4-6T, which has an impedance ratio of four (turns ratio of
2). The schematic assumes that the signal source has a 50 Ω
source impedance. The 1:4 impedance ratio requires the 200 Ω
secondary termination for optimum power transfer and VSWR.
The center tap of the transformer provides a convenient means
of level shifting the input signal to a desired common-mode
voltage. Optimum performance can be realized when the center
tap is tied to CML of the AD9221/AD9223/AD9220 which is
the common-mode bias level of the internal SHA.
A dramatic improvement in THD and SFDR performance can
be realized by operating the AD9221/AD9223/AD9220 in the
differential mode using a transformer. Figure 43 shows a plot of
THD vs. Input Frequency for the differential transformer
coupled circuit for each A/D while Figure 44 shows a plot of
SFDR vs. Input Frequency. Both figures demonstrate the enhance-
ment in spectral performance for the differential-mode of opera-
tion. The performance enhancement between the differential
and single-ended mode is most noteworthy as the input frequency
approaches and goes beyond the Nyquist frequency (i.e., fIN
FS/2) corresponding to the particular A/D.
>
C
R
S
S
The figures are also helpful in determining the appropriate A/D
for Direct IF-Down Conversion or undersampling applications.
Refer to Analog Devices application notes AN-301 and AN-302
for an informative discussion on undersampling. One should
select the A/D that meets or exceeds the distortion performance
requirements measured over the required frequency passband.
For example, the AD9220 achieves the best distortion perfor-
mance over an extended frequency range as a result of its greater
full-power bandwidth and thus would represent the best selec-
tion for an IF undersampling application at 21.4 MHz. Refer to
the Applications section of this data sheet for more detailed
information and characterization of this particular application.
15pF
33⍀
VINA
CML
49.9⍀
0.1F
AD9221/
200⍀
AD9223/
AD9220
VINB
R
C
S
15pF
S
MINI CIRCUITS
T4-1
33⍀
Figure 45. Transformer Coupled Input
Transformers with other turns ratios may also be selected to
optimize the performance of a given application. For example,
a given input signal source or amplifier may realize an improve-
ment in distortion performance at reduced output power levels
and signal swings. Hence, selecting a transformer with a higher
impedance ratio (e.g., Minicircuits T16-6T with a 1:16 imped-
ance ratio) effectively “steps up” the signal level thus further
reducing the driving requirements of the signal source.
–50
–60
Referring to Figure 45, a series resistors, RS, and shunt capaci-
tor, CS, were inserted between the AD9221/AD9223/AD9220
and the secondary of the transformer. The values of 33 Ω and
15 pF were selected to specifically optimize both the THD and
SNR performance of the A/D. RS and CS help provide some
isolation from transients at the A/D inputs reflected back
through the primary of the transformer.
–70
AD9223
AD9221
–80
AD9220
The AD9221/AD9223/AD9220 can be easily configured for
either a 2 V p-p input span or 5.0 V p-p input span by setting
the internal reference (see Table II). Other input spans can be
realized with two external gain setting resistors as shown in
Figure 49 of this data sheet. Figure 46 demonstrates how both
spans of the AD9220 achieve the high degree of linearity and
–90
1
10
100
FREQUENCY – MHz
Figure 43. AD9221/AD9223/AD9220 THD vs. Input Fre-
quency (VCM = 2.5 V, 2 V p-p Input Span, AIN = –0.5 dB)
–17–
REV. D
ADI [ ADI ]
