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AD9280

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APPLICATIONS

DIRECT IF DOWN CONVERSION USING THE AD9280

Sampling IF signals above an ADC’s baseband region (i.e., dc

to FS/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

0.1 F

AIN

REFTS

AD9280

REFBS

REFSENSE

VREF

AVDD

200

93.1

280

22.1

200

SAW

FILTER

OUTPUT

BANDPASS

FILTER

G1 = 20dB

G2 = 12dB

L-C

MINI CIRCUITS

T4 - 6T

1:4

0.1 F

1.0 F

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., FS/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., FS/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.

REV. E

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