AD9225ARS datasheet(14/24 Pages) AD

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AD9225

–14–

REV. A

OP AMP SELECTION GUIDE

Op amp selection for the AD9225 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 which preserves the perfor-

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

considers the AD9225’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

encouraged 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 AD9225 is also included.

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 AD9225. It provides all

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

without contributing additional noise or distortion. An RF

transformer also has the added benefit of providing electrical

isolation between the signal source and the A/D.

An improvement in THD and SFDR performance can be realized

by operating the AD9225 in the differential mode. The perfor-

mance 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).

VINA

VINB

REF

0.1 F

500

VREF

10 F

500

*OPTIONAL

AD9225

Figure 23. Direct Coupled Drive Circuit with AD8056

Dual Op Amp

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

differential dc drive to the AD9225. We have used this configu-

ration to drive the AD9225 from 2 V to 4 V spans at frequencies

approaching Nyquist with performance numbers matching those

shown on pages 2 and 3 of this data sheet (gathered 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 reference drive is required.

The driver circuit shown in Figure 23 is optimized for dc cou-

pling applications requiring optimum distortion performance.

This differential op amp driver circuit is configured to convert

and level shift a 2 V p-p single-ended, ground referenced signal

to a 4 V p-p differential signal centered at the VREF level of the

ADC. The circuit is based on two op amps which are configured

as matched unity gain difference amplifiers. The single-ended

input signal is applied to opposing inputs of the difference am-

plifiers, thus providing differential drive. The common-mode

offset voltage is applied to the noninverting resistor leg of each

difference amplifier providing the required offset voltage. The

common-mode offset can be varied over a wide span without

any serious degradation in distortion performance as shown in

Figures 25a and 25b, thus providing some flexibility in improv-

ing output compression distortion from some

±5 V op amps

with limited positive voltage swing.

To protect the AD9225 from an undervoltage fault condition

from op amps specified for

±5 V operation, two diodes to AGND

can be inserted between each op amp output and the AD9225

inputs. The AD9225 will inherently be protected against any

overvoltage condition if the op amps share the same positive

power supply (i.e., AVDD) as the AD9225. Note, the gain

accuracy and common-mode rejection of each difference ampli-

fier in this driver circuit can be enhanced by using a matched

thin-film resistor network (i.e., Ohmtek ORNA5000F) for the

op amps. Recall, the AD9225’s small signal bandwidth is

105 MHz, hence any noise falling within the baseband band-

width of the AD9225 will degrade its overall noise performance.

The noise performance of each unity gain differential driver

circuit is limited by its inherent noise gain of two. For unity gain

op amps ONLY, the noise gain can be reduced from two to one

beyond the input signals passband by adding a shunt capacitor,

CF, across each op amp’s feedback resistor. This will essentially

establish a low-pass filter, which reduces the noise gain to one

beyond the filter’s f–3 dB while simultaneously bandlimiting the

input signal to f–3 dB. Note, the pole established by this filter can

also be used as the real pole of an antialiasing filter.

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