AD9224ARS datasheet(14/24 Pages) AD

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AD9224

–14–

REV. A

Alternative AC Interface

Figure 22 shows a flexible ac-coupled circuit that can be con-

figured for different input spans. Since the common-mode

voltage of VINA and VINB are biased to midsupply (VCM)

independent of VREF, VREF can be pin strapped or reconfig-

ured to achieve input spans between 2 V and 4 V p-p. The

AD9224’s CMRR, along with the symmetrical coupling R-C

networks, will reject both power supply variations and noise.

VCM establishes the common-mode voltage. VCM’s source im-

pedance is 5 k

Ω. The capacitors, C1 and C2, are typically a

0.1

µF ceramic and 10 µF tantalum capacitor in parallel to

achieve a low cutoff frequency while maintaining a low imped-

ance over a wide frequency range. RS isolates the buffer ampli-

fier from the A/D input. The optimum performance is preserved

because VINA and VINB are driven via symmetrical R-C net-

works. The f–3 dB point can be approximated by the equation,

f –3dB

π×6K +(C1+ C2)

VINA

VINB

AD9224

VIN

0.1 F

VCM

0.1 F

10 F

0.1 F

Figure 22. AC-Coupled Input-Flexible Input Span,

VCM = 2.5 V

OP AMP SELECTION GUIDE

Op amp selection for the AD9224 is highly dependent on a

particular application. In general, the performance requirements

of any given application can be characterized by either time

domain or frequency domain parameters. In either case, one

should carefully select an op amp that preserves the perfor-

mance of the A/D. This task becomes challenging when one

considers the AD9224’s high performance capabilities coupled

with other extraneous system level requirements such as power

consumption and cost.

The ability to select the optimal op amp may be further compli-

cated by either limited power supply availability and/or limited

acceptable supplies for a desired op amp. Newer, high perfor-

mance op amps typically have input and output range limita-

tions in accordance with their lower supply voltages. As a result,

some op amps will be more appropriate in systems where ac-

coupling is allowable. When dc-coupling is required, op amps

without headroom constraints such as rail-to-rail op amps or

ones where larger supplies can be used should be considered.

The following section describes some op amps currently avail-

able from Analog Devices. The system designer is always en-

couraged to contact the factory or local sales office to be

updated on Analog Devices latest amplifier product offerings.

Highlights of the areas where the op amps excel and where they

may limit the performance of the AD9224 is also included.

When single-ended, dc coupling is needed. The use of the

AD8056 in a differential configuration (Figure 23) is highly

recommended.

AD8055: f–3 dB = 300 MHz.

Low cost. Best used for driving single-ended ac

coupled configuration.

Limit: THD is compromised when output is not

swinging about 0 V.

AD8056: Dual Version of above amp.

Perfect for single-ended to differential configuration

(see Figure 23). Harmonics cancel each other in

differential drive, making this amplifier highly recom-

mended for a single-ended input signal source. Handles

input signals past the 20 MHz Nyquist frequency.

AD9631: f–3 dB = 250 MHz.

Moderate cost.

Good for single-ended drive applications when signal

is anywhere between 0 V and 3 V.

Limits: THD is compromised above 8 MHz.

DIFFERENTIAL MODE OF OPERATION

Since not all applications have a signal preconditioned for differ-

ential operation, there is often a need to perform a single-ended-

to-differential conversion. In systems that do not need to be dc

coupled, an RF transformer with a center tap is the best method

to generate differential inputs for the AD9224. It provides all

the benefits of operating the A/D in the differential mode with-

out contributing additional noise or distortion. An RF transformer

also has the added benefit of providing electrical isolation be-

tween the signal source and the A/D.

An improvement in THD and SFDR performance can be real-

ized by operating the AD9224 in the differential mode. The

performance enhancement between the differential and single-

ended mode is most noteworthy as the input frequency approaches

and goes beyond the Nyquist frequency (i.e., fIN > FS /2).

The circuit shown in Figure 23 is an ideal method of applying a

differential dc drive to the AD9224. We have used this configu-

ration to drive the AD9224 from 2 V to 4 V spans at frequencies

approaching Nyquist, with performance numbers matching

those shown on the Specification pages of this data sheet (gath-

ered through a transformer). The dc input is shifted to a dc

point swinging symmetrically about the reference voltage. The

optional resistor will provide additional current if more refer-

ence drive is required.

VINA

VINB

CML

0.1 F

500

VREF

10 F

500

*OPTIONAL

AD9224

Figure 23. Direct Coupled Drive Circuit with AD8056 Dual

Op Amps

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