AD9224ARS datasheet(12/24 Pages) AD

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AD9224

–12–

REV. A

DRIVING THE ANALOG INPUTS

The AD9224 has a highly flexible input structure allowing it to

interface with single-ended or differential input interface cir-

cuitry. The applications shown in Driving the Analog Inputs and

Reference Configurations sections, along with the information

presented in Input and Reference Overview of this data sheet,

give examples of both single-ended and differential operation.

Refer to Tables I and II for a list of the different possible input

and reference configurations and their associated figures in the

data sheet.

The optimum mode of operation, analog input range, and asso-

ciated interface circuitry will be determined by the particular

applications performance requirements as well as power supply

options. For example, a dc-coupled single-ended input would be

appropriate for most data acquisition and imaging applications.

Also, many communication applications that require a dc coupled

input for proper demodulation can take advantage of the

single-ended distortion performance of the AD9224. The input

span should be configured so the system’s performance objec-

tives and the headroom requirements of the driving op amp are

simultaneously met.

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

the best THD and SFDR performance over a wide frequency

range. Differential operation should be considered for the most de-

manding spectral based applications (e.g., direct IF-to-digital con-

version). See Figures 23, 24 and section on Differential Mode of

Operation. Differential input characterization was performed for

this data sheet using the configuration shown in Figure 24.

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

to the input signal source, while VINB of the AD9224 be biased

to the appropriate voltage corresponding to a midscale code transi-

tion. Note that signal inversion may be easily accomplished by

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

cations for the AD9224 were characterized using Figure 21

circuitry with input spans of 4 V and 2 V as well as VCM = 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

AD9224 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 therefore 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 18 shows a simple means of clamping

an ac 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

performance. A similar clamping circuit could also be used for

each input if a differential input signal is being applied. The

diodes might cause nonlinearity in the signal. Careful evaluation

should be performed on the diodes used.

AVDD

AD9224

RS1

VCC

VEE

OPTIONAL

AC COUPLING

CAPACITOR

RS2

Figure 18. Simple Clamping Circuit

SINGLE-ENDED MODE OF OPERATION

The AD9224 can be configured for single-ended operation

using dc or ac coupling. In either case, the input of the A/D

must be driven from an operational amplifier that will not de-

grade the A/D’s performance. Because the A/D 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 which may influence the

system design and performance.

Single-ended operation is often limited by the availability driv-

ing op amps. Very low distortion op amps that provide great

performance 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 the many high perfor-

mance amplifiers to maintain low distortions as their outputs

approach their positive output voltage limit (i.e., 1 dB compres-

sion point). For this reason, it is recommended that applications

requiring high performance dc coupling use the single-ended-to-

differential circuit shown in Figure 23.

DC COUPLING AND INTERFACE ISSUES

Many applications require the analog input signal to be dc coupled

to the AD9224. 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 A/D. The input range to the

A/D 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 capa-

bilities. The selected input range of the AD9224 should be consid-

ered with the headroom requirements of the particular op amp to

prevent clipping of the signal. Also, since the output of a dual

supply amplifier can swing below absolute minimum (–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. Ampli-

fiers like the AD8041 and AD8011 are useful for this purpose

but their low bandwidths will limit the AD9224’s performance.

High performance amplifiers (

± 5 V) such as the AD9631,

AD9632, AD8056 or AD8055 allow the AD9224 to be config-

ured for larger input spans which will improve the ADC’s noise

performance.

Op amp circuits using a noninverting and inverting topologies

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’s output and the AD9224 input to provide

a filter pole.

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