REV. E
AD9221/AD9223/AD9220
–17–
AD828:
Dual Version of AD818
Best Applications: Differential and/or Low Imped-
ance Input
Drivers, Low Noise, Gains
≥ +2
Limits: THD above 100 kHz
AD812:
Dual, 145 MHz Unity GBW, Single-Supply Cur-
rent Feedback, +5 V to
±15 V Supplies
Best Applications: Differential and/or Low Imped-
ance Input Drivers, Sample Rates < 7 MSPS
Limits: THD above 1 MHz
AD8011:
f–3 dB = 300 MHz, +5 V or
±5 V Supplies, Current
Feedback
Best Applications: Single-Supply, AC-DC-Coupled,
Good AC Specs, Low Noise, Low Power (5 mW)
Limits: THD above 5 MHz, Usable Input/Output
Range
AD8013:
Triple, f–3 dB = 230 MHz, +5 V or
±5 V Supplies,
Current Feedback, Disable Function
Best Applications: 3:1 Multiplexer, Good AC Specs
Limits: THD above 5 MHz, Input Range
AD9631:
220 MHz Unity GBW, 16 ns Settling to 0.01%,
± 5 V Supplies
Best Applications: Best AC Specs, Low Noise,
AC-Coupled
Limits: Usable Input/Output Range, Power
Consumption
AD8047:
130 MHz Unity GBW, 30 ns Settling to 0.01%,
±5 V Supplies
Best Applications: Good AC Specs, Low Noise,
AC-Coupled
Limits: THD > 5 MHz, Usable Input Range
AD8041:
Rail-to-Rail, 160 MHz Unity GBW, 55 ns Settling
to 0.01%, 5 V Supply, 26 mW
Best Applications: Low Power, Single-Supply
Systems, DC-Coupled, Large Input Range
Limits: Noise with 2 V Input Range
AD8042:
Dual AD8041
Best Applications: Differential and/or Low Imped-
ance Input Drivers
Limits: Noise with 2 V Input Range
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 AD9221/
AD9223/AD9220. It provides all the benefits of operating the
A/D in the differential mode without contributing additional
noise or distortion. An RF transformer also has the added ben-
efit of providing electrical isolation between the signal source
and the A/D.
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 increase
the total system noise, power consumption, and cost. Thus, a
single-ended mode of operation is recommended for most appli-
cations requiring dc coupling.
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 17 shows a plot of
THD versus Input Frequency for the differential transformer
coupled circuit for each A/D while Figure 18 shows a plot of
SFDR versus Input Frequency. Both figures demonstrate the
enhancement in spectral performance for the differential-mode
of operation. The performance enhancement between the differen-
tial 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.
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.
THD
–
dB
FREQUENCY – MHz
–50
–60
–90
1
100
10
–70
–80
AD9221
AD9223
AD9220
Figure 17. AD9221/AD9223/AD9220 THD vs. Input
Frequency (VCM = 2.5 V, 2 V p-p Input Span,
AIN = –0.5 dB)