AD9225ARSZRL datasheet(13/26 Pages) AD

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AD9225

Differential modes of operation (ac-coupled or dc-coupled input)

provide the best THD and SFDR performance over a wide fre-

quency range. Differential operation should be considered for the

most demanding spectral based applications (e.g., direct IF-to-

digital conversion). See Figures 12 and 13 and the Differential

Mode of Operation section. Differential input characterization was

performed for this data sheet using the configuration shown in

Figure 13.

Single-ended operation requires that VINA be ac-coupled or dc-

coupled to the input signal source while VINB of the AD9225 be

biased to the appropriate voltage corresponding to a midscale code

transition. Note that signal inversion may be easily accomplished

by transposing VINA and VINB. Most of the single-ended specifi-

cations for the AD9225 are characterized using Figure 21 circuitry

with input spans of 4 V and 2 V as well as VINB = 2.5 V.

Differential operation requires that VINA and VINB be simulta-

neously driven with two equal signals that are in and out of phase

versions of the input signal. Differential operation of the AD9225

offers the following benefits: (1) Signal swings are smaller and,

therefore, linearity requirements placed on the input signal source

may be easier to achieve, (2) Signal swings are smaller and there-

fore may allow the use of op amps which may otherwise have been

constrained by headroom limitations, (3) Differential operation

minimizes even-order harmonic products, and (4) Differential

operation offers noise immunity based on the device’s common-

mode rejection.

As is typical of most IC devices, exceeding the supply limits will

turn on internal parasitic diodes resulting in transient currents

within the device. Figure 7 shows a simple means of clamping an

ac-coupled or dc-coupled single-ended input with the addition of

two series resistors and two diodes. An optional capacitor is shown

for ac-coupled applications. Note that a larger series resistor could

be used to limit the fault current through D1 and D2 but should

be evaluated since it can cause a degradation in overall perfor-

mance. A similar clamping circuit could also be used for each input

if a differential input signal is being applied. The diodes might

cause nonlinearities in the signal. Careful evaluation should be

performed on the diodes used.

AVDD

AD9225

RS1

VCC

VEE

OPTIONAL

AC COUPLING

CAPACITOR

1N4148

RS2

Figure 7. Simple Clamping Circuit

SINGLE-ENDED MODE OF OPERATION

The AD9225 can be configured for single-ended operation using

dc-coupling or ac-coupling. In either case, the input of the ADC

must be driven from an operational amplifier that will not degrade

the ADC’s performance. Because the ADC operates from a single

supply, it will be necessary to level shift ground based bipolar

signals to comply with its input requirements. Both dc and ac

coupling provide this necessary function, but each method results

in different interface issues that may influence the system design

and performance.

Single-ended operation is often limited by the availability of driving

op amps. Very low distortion op amps that provide great perfor-

mance out to the Nyquist frequency of the converter are hard to

find. Compounding the problem, for dc-coupled, single-ended

applications, is the inability of many high performance amplifiers

to maintain low distortions as their outputs approach their positive

output voltage limit (i.e., 1 dB compression point). For this reason,

it is recommended that applications requiring high performance

dc coupling use the single-ended-to-differential circuit shown in

Figure 12.

DC COUPLING AND INTERFACE ISSUES

Many applications require the analog input signal to be dc-coupled

to the AD9225. An operational amplifier can be configured to

rescale and level shift the input signal so that it is compatible with

the selected input range of the ADC. The input range to the ADC

should be selected on the basis of system performance objectives,

as well as the analog power supply availability since this will place

certain constraints on the op amp selection.

Many of the new high performance op amps are specified for only

±5 V operation and have limited input/output swing capabilities.

The selected input range of the AD9225 should be considered with

the headroom requirements of the particular op amp to prevent

clipping of the signal. Since the output of a dual supply amplifier

can swing below –0.3 V, clamping its output should be considered

in some applications.

In some applications, it may be advantageous to use an op amp

specified for single-supply +5 V operation since it will inherently

limit its output swing to within the power supply rails. Amplifiers

like the AD8041 and AD8011 are useful for this purpose but their

low bandwidths will limit the AD9225’s performance. High perfor-

mance amplifiers, such as the AD9631, AD9632, AD8056, or

AD8055, allow the AD9225 to be configured for larger input spans

which will improve the ADC’s noise performance.

Op amp circuits using a noninverting and inverting topology are

discussed in the next section. Although not shown, the non-

inverting and inverting topologies can be easily configured as part

of an antialiasing filter by using a Sallen-Key or multiple-feedback

topology. An additional R-C network can be inserted between the

op amp output and the AD9225 input to provide a filter pole.

Simple Op Amp Buffer

In the simplest case, the input signal to the AD9225 will already be

biased at levels in accordance with the selected input range. It is

simply necessary to provide an adequately low source impedance

for the VINA and VINB analog pins of the ADC. Figure 8 shows

the recommended configuration a single-ended drive using an op

amp. In this case, the op amp is shown in a noninverting unity gain

configuration driving the VINA pin. The internal reference drives

the VINB pin. Note that the addition of a small series resistor of

W to 50 W connected to VINA and VINB will be beneficial in

nearly all cases. Refer to the Analog Input Operation section on a

discussion on resistor selection. Figure 8 shows the proper connec-

tion for a 0 V to 4 V input range. Alternative single-ended ranges

of 0 V to 2

¥ VREF can also be realized with the proper configura-

tion of VREF (refer to the Using the Internal Reference section).

Headroom limitations of the op amp must always be considered.

Rev. C

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