AD9243EB datasheet(16/25 Pages) AD

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AD9243

REV. A

–15–

of the A/D is 0 V to 5 V. Other input ranges could be selected

by changing VREF but the A/D’s distortion performance will

degrade slightly as the input common-mode voltage deviates

from its optimum level of 2.5 V.

Alternative AC Interface

Figure 35 shows a flexible ac coupled circuit which can be con-

figured for different input spans. Since the common-mode

voltage of VINA and VINB are biased to midsupply indepen-

dent of VREF, VREF can be pin-strapped or reconfigured to

achieve input spans between 2 V and 5 V p-p. The AD9243’s

CMRR along with the symmetrical coupling R-C networks will

reject both power supply variations and noise. The resistors, R,

establish the common-mode voltage. They may have a high value

(e.g., 5 k

Ω) to minimize power consumption and establish a low

cutoff frequency. The capacitors, C1 and C2, are typically a

0.1

µF ceramic and 10 µF tantalum capacitor in parallel to achieve

a low cutoff frequency while maintaining a low impedance over

a wide frequency range. RS isolates the buffer amplifier from the

A/D input. The optimum performance is achieved when VINA

and VINB are driven via symmetrical networks. The high pass

f–3 dB point can be approximated by the equation,

f–3 dB = 1/(2

× π × R/2 × (C1 + C2))

VINA

VINB

AD9243

+5V

–5V

VIN

+5V

Figure 35. AC-Coupled Input-Flexible Input Span,

VCM = 2.5 V

OP AMP SELECTION GUIDE

Op amp selection for the AD9243 is highly dependent on a

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 carefully select an op amp which preserves the perfor-

mance of the A/D. This task becomes challenging when one

considers the AD9243’s high performance capabilities coupled

with other external 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

headroom constraints such as rail-to-rail op amps or ones where

larger supplies can be used should be considered. The following

section describes some op amps currently available from Analog

Devices. 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 AD9243 are also included.

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

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 Sys-

tems, 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

REFERENCE CONFIGURATIONS

The figures associated with this section on internal and external

reference operation do not show recommended matching series resistors

for VINA and VINB for the purpose of simplicity. Please refer to

section “Driving the Analog Inputs, Introduction” for a discussion of

this topic. Also, the figures do not show the decoupling network asso-

ciated with the CAPT and CAPB pins. Please refer to the section “Ref-

erence Operation” for a discussion of the internal reference circuitry

and the recommended decoupling network shown in Figure 27.

USING THE INTERNAL REFERENCE

Single-Ended Input with 0 to 2

VREF Range

Figure 36 shows how to connect the AD9243 for a 0 V to 2 V or

0 V to 5 V input range via pin strapping the SENSE pin. An

intermediate input range of 0 to 2

× VREF can be established

using the resistor programmable configuration in Figure 38 and

connecting VREF to VINB.

In either case, both the common-mode voltage and input span

are directly dependent on the value of VREF. More specifically,

the common-mode voltage is equal to VREF while the input

span is equal to 2

× VREF. Thus, the valid input range extends

from 0 to 2

× VREF. When VINA is ≤ 0 V, the digital output

will be 0000 Hex; when VINA is

≥ 2 × VREF, the digital output

will be 3FFF Hex.

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