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AD9225

OP AMP SELECTION GUIDE

Op amp selection for the AD9225 is highly dependent on the

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 care-

fully select an op amp that preserves the performance of the ADC.

This task becomes challenging when the AD9225’s high perfor-

mance capabilities are 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 performance

op amps typically have input and output range limitations 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 head-

room constraints such as rail-to-rail op amps or the ones where

larger supplies can be used should be considered. The following

section describes some op amps currently available from Analog

Devices, Inc. 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 12). Harmonics cancel each other in

differential drive, making this amplifier highly rec-

ommended 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

differential 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 ADC 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 ADC.

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

CML

0.1 F

500

VREF

10 F

500

OPTIONAL

AD9225

Figure 12. Direct Coupled Drive Circuit with

AD8056 Dual Op Amp

The circuit shown in Figure 12 is an ideal method of applying a

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

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

approaching Nyquist with performance numbers matching those

listed in the Specifications tables (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 12 is optimized for dc coupling

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 that are configured as matched

unity gain difference amplifiers. The single-ended input signal is

applied to opposing inputs of the difference amplifiers, thus provid-

ing differential drive. The common-mode offset voltage is applied

to the noninverting resistor leg of each difference amplifier provid-

ing the required offset voltage. The common-mode offset can be

varied over a wide span without any serious degradation in distor-

tion performance as shown in Figures 14 and 15, thus providing

some flexibility in improving 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 that the gain accuracy and com-

mon-mode rejection of each difference amplifier in this driver

circuit can be enhanced by using a matched thin-film resistor

network (i.e., Ohmtek ORNA5000F) for the op amps. Recall that

the AD9225’s small signal bandwidth is 105 MHz and therefore,

any noise falling within the baseband bandwidth 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 2. For unity gain op amps

ONLY, the noise gain can be reduced from 2 to 1 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 1 beyond the filter’s

f–3 dB while simultaneously bandlimiting the input signal to f–3 dB.

Note that the pole established by this filter can also be used as the

real pole of an antialiasing filter.

Rev. C

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