AD9220ARSZ datasheet(17/32 Pages) AD

17 / 32 page

REV. E

AD9221/AD9223/AD9220

–17–

AD828:

Dual Version of AD818

Best Applications: Differential and/or Low Imped-

ance Input

Drivers, Low Noise, Gains

≥ +2

Limits: THD above 100 kHz

AD812:

Dual, 145 MHz Unity GBW, Single-Supply Cur-

rent Feedback, +5 V to

±15 V Supplies

Best Applications: Differential and/or Low Imped-

ance Input Drivers, Sample Rates < 7 MSPS

Limits: THD above 1 MHz

AD8011:

f–3 dB = 300 MHz, +5 V or

±5 V Supplies, Current

Feedback

Best Applications: Single-Supply, AC-DC-Coupled,

Good AC Specs, Low Noise, Low Power (5 mW)

Limits: THD above 5 MHz, Usable Input/Output

Range

AD8013:

Triple, f–3 dB = 230 MHz, +5 V or

±5 V Supplies,

Current Feedback, Disable Function

Best Applications: 3:1 Multiplexer, Good AC Specs

Limits: THD above 5 MHz, Input Range

AD9631:

220 MHz Unity GBW, 16 ns Settling to 0.01%,

± 5 V Supplies

Best Applications: Best AC Specs, Low Noise,

AC-Coupled

Limits: Usable Input/Output Range, Power

Consumption

AD8047:

130 MHz Unity GBW, 30 ns Settling to 0.01%,

±5 V Supplies

Best Applications: Good AC Specs, Low Noise,

AC-Coupled

Limits: THD > 5 MHz, Usable Input Range

AD8041:

Rail-to-Rail, 160 MHz Unity GBW, 55 ns Settling

to 0.01%, 5 V Supply, 26 mW

Best Applications: Low Power, Single-Supply

Systems, DC-Coupled, Large Input Range

Limits: Noise with 2 V Input Range

AD8042:

Dual AD8041

Best Applications: Differential and/or Low Imped-

ance Input Drivers

Limits: Noise with 2 V Input Range

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 AD9221/

AD9223/AD9220. 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 ben-

efit of providing electrical isolation between the signal source

and the A/D.

Note that although a single-ended-to-differential op amp topol-

ogy would allow dc coupling of the input signal, no significant

improvement in THD performance was realized when compared

to the dc single-ended mode of operation up to the AD9221/

AD9223/AD9220’s Nyquist frequency (i.e., fIN < fS/2). Also,

the additional op amp required in the topology tends to increase

the total system noise, power consumption, and cost. Thus, a

single-ended mode of operation is recommended for most appli-

cations requiring dc coupling.

A dramatic improvement in THD and SFDR performance can

be realized by operating the AD9221/AD9223/AD9220 in the

differential mode using a transformer. Figure 17 shows a plot of

THD versus Input Frequency for the differential transformer

coupled circuit for each A/D while Figure 18 shows a plot of

SFDR versus Input Frequency. Both figures demonstrate the

enhancement in spectral performance for the differential-mode

of operation. The performance enhancement between the differen-

tial and single-ended mode is most noteworthy as the input

frequency approaches and goes beyond the Nyquist frequency

(i.e., fIN > fS/2) corresponding to the particular A/D.

The figures are also helpful in determining the appropriate A/D

for Direct IF down conversion or undersampling applications.

Refer to Analog Devices application notes AN-301 and AN-302

for an informative discussion on undersampling. One should

select the A/D that meets or exceeds the distortion performance

requirements measured over the required frequency passband.

For example, the AD9220 achieves the best distortion perfor-

mance over an extended frequency range as a result of its greater

full-power bandwidth and thus would represent the best selec-

tion for an IF undersampling application at 21.4 MHz. Refer to

the Applications section of this data sheet for more detailed

information and characterization of this particular application.

FREQUENCY – MHz

–50

–60

–90

100

–70

–80

AD9221

AD9223

AD9220

Figure 17. AD9221/AD9223/AD9220 THD vs. Input

Frequency (VCM = 2.5 V, 2 V p-p Input Span,

AIN = –0.5 dB)

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