AD9220ARSZ datasheet(14/32 Pages) AD

14 / 32 page

REV. E

–14–

AD9221/AD9223/AD9220

DRIVING THE ANALOG INPUTS

Introduction

The AD9221/AD9223/AD9220 has a highly flexible input

structure, allowing it to interface with single-ended or differen-

tial input interface circuitry. The applications shown in sections

Driving the Analog Inputs and Reference Configurations, along

with the information presented in Input and Reference Over-

view 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

application’s 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 excel-

lent single-ended distortion performance of the AD9221/AD9223/

AD9220. The input span should be configured such that the

system’s performance objectives and the headroom requirements

of the driving op amp are simultaneously met.

Alternatively, the differential mode of operation with a transformer

coupled input provides the best THD and SFDR performance

over a wide frequency range. This mode of operation should be

considered for the most demanding spectral based applications

that allow ac coupling (e.g., Direct IF to Digital Conversion).

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

to the input signal source while VINB of the AD9221/AD9223/

AD9220 can 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. The rated

specifications for the AD9221/AD9223/AD9220 are character-

ized using single-ended circuitry with input spans of 5 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

AD9221/AD9223/AD9220 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 that may otherwise have been constrained by headroom

FREQUENCY– MHz

0.1

100

AD9221

AD9223

AD9220

Figure 11. AD9221/AD9223/AD9220 Input CMR vs.

Input Frequency

limitations, (3) Differential operation minimizes even-order

harmonic products, and (4) Differential operation offers noise

immunity based on the device’s common-mode rejection.

Figure 11 depicts the common-mode rejection of the three devices.

As is typical of most CMOS devices, exceeding the supply limits

will turn on internal parasitic diodes, resulting in transient cur-

rents within the device. Figure 12 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 capaci-

tor 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 degrada-

tion in overall performance. A similar clamping circuit could also

be used for each input if a differential input signal is being applied.

Table II. Reference Configuration Summary

Reference

Input Span (VINA–VINB)

Operating Mode

(V p-p)

Required VREF (V)

Connect

INTERNAL

SENSE

VREF

INTERNAL

2.5

SENSE

REFCOM

INTERNAL

≤ SPAN ≤ 5 and

≤ VREF ≤ 2.5 and

VREF and SENSE

SPAN = 2

× VREF

VREF = (1 + R1/R2)

SENSE and REFCOM

EXTERNAL

≤ SPAN ≤ 51 ≤ VREF ≤ 2.5

SENSE

AVDD

(Nondynamic)

VREF

EXT. REF.

EXTERNAL

≤ SPAN ≤ 5

CAPT and CAPB

SENSE

AVDD

(Dynamic)

Externally Driven

VREF

REFCOM

EXT. REF.

CAPT

EXT. REF.

CAPB

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