AD9240AS datasheet(16/24 Pages) AD

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AD9240

REV. A

–16–

degrade slightly as the input common-mode voltage deviates

from its optimum level of 2.5 V.

Alternative AC Interface

Figure 38 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 AD9240’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 imped-

ance over a wide frequency range. RS isolates the buffer ampli-

fier 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

AD9240

+5V

–5V

VIN

+5V

Figure 38. AC-Coupled Input-Flexible Input Span,

VCM = 2.5 V

OP AMP SELECTION GUIDE

Op amp selection for the AD9240 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 that preserves the perfor-

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

considers the AD9240’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 limited power supply availability and/or limited accept-

able supplies for a desired op amp. Newer, high performance op

amps typically have input and output range limitations in accor-

dance 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 De-

vices’ latest amplifier product offerings. Highlights of the areas

where the op amps excel and where they may limit the perfor-

mance of the AD9240 are also included.

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

AD8042:

Dual AD8041

Best Applications: Differential and/or Low Imped-

ance Input Drivers

Limits: Noise with 2 V Input Range

REFERENCE CONFIGURATIONS

For the purpose of simplicity, the figures associated with this

section on internal and external reference operation do not

show recommended matching series resistors for VINA and

VINB. Please refer to section Driving the Analog Inputs, Intro-

duction, for a discussion of this topic. The figures do not show

the decoupling network associated with the CAPT and CAPB

pins. Please refer to the Reference Operation section for a discus-

sion of the internal reference circuitry and the recommended

decoupling network shown in Figure 30.

USING THE INTERNAL REFERENCE

Single-Ended Input with 0 to 2

VREF Range

Figure 39 shows how to connect the AD9240 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 41 and

connecting VREF to VINB.

10 F

VINA

VREF

AD9240

0.1 F

VINB

2xVREF

SHORT FOR 0 TO 2V

INPUT SPAN

SENSE

SHORT FOR 0 TO 5V

INPUT SPAN

REFCOM

Figure 39. Internal Reference (2 V p-p Input Span,

VCM = 1 V, or 5 V p-p Input Span, VCM = 2.5 V)

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.

Shorting the VREF pin directly to the SENSE pin places the

internal reference amplifier in unity-gain mode and the result-

ant VREF output is 1 V. The valid input range is, therefore, 0 V

to 2 V. Shorting the SENSE pin directly to the REFCOM pin

configures the internal reference amplifier for a gain of 2.5 and

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