AD9280
–16–
APPLICATIONS
DIRECT IF DOWN CONVERSION USING THE AD9280
Sampling IF signals above an ADC’s baseband region (i.e., dc
to F
S
/2) is becoming increasingly popular in communication
applications. This process is often referred to as Direct IF Down
Conversion or Undersampling. There are several potential ben-
efits in using the ADC to alias (i.e., or mix) down a narrowband
or wideband IF signal. First and foremost is the elimination of a
complete mixer stage with its associated amplifiers and filters,
reducing cost and power dissipation. Second is the ability to
apply various DSP techniques to perform such functions as
filtering, channel selection, quadrature demodulation, data
reduction, detection, etc. A detailed discussion on using this
technique in digital receivers can be found in Analog Devices
Application Notes AN-301 and AN-302.
In Direct IF Down Conversion applications, one exploits the
inherent sampling process of an ADC in which an IF signal
lying outside the baseband region can be aliased back into the
baseband region in a similar manner that a mixer will down-
convert an IF signal. Similar to the mixer topology, an image
rejection filter is required to limit other potential interfering
signals from also aliasing back into the ADC’s baseband region.
A tradeoff exists between the complexity of this image rejection
filter and the sample rate as well as dynamic range of the ADC.
The AD9280 is well suited for various narrowband IF sampling
applications. The AD9280’s low distortion input SHA has a
full-power bandwidth extending to 300 MHz thus encompassing
many popular IF frequencies. The AD9280 will typically yield
an improvement in SNR when configured for the 2 V span, the
1 V span provides the optimum full-scale distortion perfor-
mance. Furthermore, the 1 V span reduces the performance
requirements of the input driver circuitry and thus may be
more practical for system implementation purposes.
Figure 34 shows a simplified schematic of the AD9280 config-
ured in an IF sampling application. To reduce the complexity of
the digital demodulator in many quadrature demodulation ap-
plications, the IF frequency and/or sample rate are selected such
0.1mF
AIN
REFTS
AD9280
REFBS
REFSENSE
VREF
AVDD
200V
1kV
1kV
50V
93.1V
280V
50V
22.1V
200V
SAW
FILTER
OUTPUT
50V
BANDPASS
FILTER
G
1
= 20dB G
2
= 12dB L-C
MINI CIRCUITS
T4 - 6T
1:4
0.1mF1.0mF
Figure 34. Simplified AD9280 IF Sampling Circuit
that the bandlimited IF signal aliases back into the center of the
ADC’s baseband region (i.e., F
S
/4). For example, if an IF sig-
nal centered at 45 MHz is sampled at 20 MSPS, an image of
this IF signal will be aliased back to 5.0 MHz which corre-
sponds to one quarter of the sample rate (i.e., F
S
/4). This
demodulation technique typically reduces the complexity of the
post digital demodulator ASIC which follows the ADC.
To maximize its distortion performance, the AD9280 is config-
ured in the differential mode with a 1 V span using a transformer.
The center tap of the transformer is biased at midsupply via a
resistor divider. Preceding the AD9280 is a bandpass filter as
well as a 32 dB gain stage. A large gain stage may be required
to compensate for the high insertion losses of a SAW filter used
for image rejection. The gain stage will also provide adequate
isolation for the SAW filter from the charge “kick back” currents
associated with AD9280’s input stage.
The gain stage can be realized using one or two cascaded
AD8009 op amps amplifiers. The AD8009 is a low cost, 1 GHz,
current-feedback op amp having a 3rd order intercept character-
ized up to 250 MHz. A passive bandpass filter following the
AD8009 attenuates its dominant 2nd order distortion products
which would otherwise be aliased back into the AD9280’s
baseband region. Also, it reduces any out-of-band noise which
would also be aliased back due to the AD9280’s noise band-
width of 220+ MHz. Note, the bandpass filters specifications
are application dependent and will affect both the total distor-
tion and noise performance of this circuit.
The distortion and noise performance of an ADC at the given
IF frequency is of particular concern when evaluating an ADC
for a narrowband IF sampling application. Both single-tone and
dual-tone SFDR vs. amplitude are very useful in assessing an
ADC’s noise performance and noise contribution due to aper-
ture jitter. In any application, one is advised to test several units
of the same device under the same conditions to evaluate the
given applications sensitivity to that particular device.
