AD9225ARSZRL datasheet(20/26 Pages) AD

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AD9225

–19–

and detection, among other things. See Application Note AN-302

on using this technique in digital receivers.

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 interferring 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. Its low aperture jitter of 0.8 ps rms ensures minimum

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. 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 distor-

tion performance at these higher input frequencies. The 2 V

span reduces only the performance requirements of the input

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

from a system implementation perspective.

Figure 26 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

applications, the IF frequency and/or sample rate are strategi-

cally selected such that the band-limited IF signal aliases back

into the center of the ADC’s baseband region (i.e., fS/4). This

demodulation technique typically reduces the complexity of the

post digital demodulator ASIC that 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 26. Example of AD9225 IF Sampling Circuit

To maximize its distortion performance, the AD9225 is configured

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 transformer is

an optional band-pass filter as well as a gain stage. A low Q passive

band-pass filter can be inserted to reduce the out-of-band distor-

tion and noise that 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 kickback 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 versus amplitude are very useful in assessing an

ADC’s dynamic and static nonlinearities. SNR versus 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 sensi-

tivity to that particular device.

Figures 27 to 30 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 that the SFDR versus

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 27. 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 28. IF Undersampling at 45 MHz (F1 = 44.81 MHz,

F2 = 45.23 MHz, CLOCK = 20 MHz)

Rev. C

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