AD9203 datasheet(19/29 Pages) AD

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AD9203

Rev. B | Page 18 of 28

APPLICATIONS

DIRECT IF DOWN CONVERSION

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

benefits in using the ADC to alias (or mix) down a narrow band

or wide band 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 manner similar to a mixer downconverting

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 trade-off exists between the complexity of this image

rejection filter and the ADC’s sample rate and dynamic range.

The AD9203 is well suited for various IF sampling applications.

Its low distortion input SHA has a full-power bandwidth

extending to 130 MHz, thus encompassing many popular IF

frequencies. Only the 2 V span should be used for

undersampling beyond 20 MHz. A DNL of ±0.25 LSB

combined with low thermal input referred noise allows the

AD9203 in the 2 V span to provide >59 dB of SNR for a

baseband input sine wave. Also, its low aperture jitter of 1.2 ps

rms ensures minimum SNR degradation at higher IF

frequencies. In fact, the AD9203 is capable of still maintaining

58 dB of SNR at an IF of 70 MHz with a 2 V input span.

To maximize its distortion performance, the AD9203 should be

configured in the differential mode with a 2 V span using a

transformer. The center-tap of the transformer is biased to the

reference output of the AD9203. Preceding the AD9203 and

transformer is an optional bandpass filter as well as a gain stage.

A low Q passive bandpass filter can be inserted to reduce out of

band distortion and noise that lies within the AD9203’s 390

MHz bandwidth. A large gain stage(s) is often required to

compensate for the high insertion losses of a SAW filter used for

channel selection and image rejection. The gain stage will also

provide adequate isolation for the SAW filter from the charge

kick back currents associated with the AD9203’s switched

capacitor input stage.

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 dynamic and static nonlinearities. SNR vs. amplitude

performance at the given IF is useful in assessing the ADC’s

noise performance and noise contribution due to aperture 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. Figure 34 and

Figure 35 combine the dual tone SFDR as well as single tone

SFDR and SNR performances at IF frequencies of 70 MHz, and

130 MHz. Note, the SFDR vs. amplitude data is referenced to

dBFS while the single tone SNR data is referenced to dBc. The

performance characteristics in these figures are representative

of the AD9203 without any preceding gain stage. The AD9203

was operated in the differential mode (via transformer) with a

2 V span and a sample rate of 40 MSPS. The analog supply

(AVDD) and the digital supply (DRVDD) were set to 3.0 V.

INPUT POWER LEVEL (dB FULL SCALE)

SFDR 2 TONE

SFDR 1 TONE

SNR

Figure 34. SNR/SFDR for IF @ 70 MHz (Clock = 40 MSPS)

INPUT POWER LEVEL (dB FULL SCALE)

SFDR 2 TONE

SFDR 1 TONE

SNR

Figure 35. SNR/SFDR for IF @ 130 MHz (Clock = 40 MSPS)

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