AD9225ARS datasheet(19/24 Pages) AD

19 / 24 page

AD9225

–19–

REV. A

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 trade-off exists between the complexity of this image rejection

filter and the ADC’s sample rate as well as dynamic range.

The AD9225 is well suited for various IF sampling applications.

The AD9225’s low distortion input SHA has a full-power band-

width extending beyond 130 MHz thus encompassing many

popular IF frequencies. A DNL of

±0.4 LSB (typ) combined

with low thermal input referred noise allows the AD9225 in the

2 V span to provide 69 dB of SNR for a baseband input sine

wave. Also, its low aperture jitter of 0.8 ps rms ensures mini-

mum SNR degradation at higher IF frequencies. In fact, the

AD9225 is capable of still maintaining 68 dB of SNR at an IF of

71 MHz with a 2 V input span. Note, although the AD9225 can

yield a 1 dB to 2 dB improvement in SNR when configured for

the larger 4 V span, the 2 V span achieves the optimum full-

scale distortion performance at these higher input frequencies.

Also, the 2 V span reduces the performance requirements of the

input driver circuitry (i.e., IP3) and thus may also be more

attractive from a system implementation perspective.

Figure 35 shows a simplified schematic of the AD9225 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 strategically

selected such that the bandlimited IF signal aliases back into the

center of the ADC’s baseband region (i.e., FS/4). This demodu-

lation technique typically reduces the complexity of the post

digital demodulator ASIC which follows the ADC.

OPTIONAL

BANDPASS

FILTER

SAW

FILTER

HIGH

LINEARITY

AMPLIFIER

FROM

STAGES

MIXER

200

0.1 F

MINICIRCUITS

T4-6T

RF2317

RF2312

0.1 F

10 F

VINA

VINB

CML

AD9225

VREF

SENSE

REFCOM

Figure 35. Example of AD9225 IF Sampling Circuit

To maximize its distortion performance, the AD9225 is config-

ured in the differential mode with a 2 V span using a transformer.

The center-tap of the transformer is biased at midsupply via the

CML output of the AD9225. Preceding the AD9225 and trans-

former 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 which lies within the AD9225’s

130 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 AD9225’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.

Figures 36–39 combine the dual tone SFDR as well as single

tone SFDR and SNR performances at IF frequencies of 35 MHz,

45 MHz, 70 MHz, and 85 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 AD9225 without any preceding

gain stage. The AD9225 was operated in the differential mode

(via transformer) with a 2 V span and a sample rate between

28 MSPS and 36 MSPS. The analog supply (AVDD) and the

digital supply (DRVDD) were set to +5 V and 3.3. V respectively.

AIN – dBFS

–15

–10

–5

100

SFDR

DUAL-TONE

(dBFS)

SNR

SINGLE-TONE

(dBc)

SFDR

SINGLE-TONE

(dBFS)

Figure 36. IF Undersampling at 35 MHz (F1 = 34.63 MHz,

F2 = 35.43 MHz, CLOCK = 20 MHz)

AIN – dBFS

–15

–10

–5

100

SFDR

DUAL-TONE

(dBFS)

SNR

SINGLE-TONE

(dBc)

SFDR

SINGLE-TONE

(dBFS)

Figure 37. IF Undersampling at 45 MHz (F1 = 44.81 MHz,

F2 = 45.23 MHz, CLOCK = 20 MHz)

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