AD9243AS datasheet(13/24 Pages) AD

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AD9243

REV. A

–13–

DC Coupling with Op Amps

Applications that require dc coupling can also benefit by driving

the AD9243 differentially. Since the signal swing requirements

of each input is reduced by a factor of two in the differential

mode, the AD9243 can be configured for a 5 V input span in a

+5 V or

±5 V system. This allows various high performance op

amps specified for +5 V and

±5 V operation to be configured in

various differential driver topologies. The optimum op amp

driver topology depends on whether the common-mode voltage

of the single-ended-input signal requires level-shifting.

Figure 30 shows a cross-coupled differential driver circuit best

suited for systems in which the common-mode signal of the

input is already biased to approximately midsupply (i.e., 2.5 V).

The common-mode voltage of the differential output is set by

the voltage applied to the “+” input of A2. The closed loop

gain of this symmetrical driver can be easily set by RIN and RF.

For more insight into the operation of this cross-coupled driver,

please refer to the AD8042 data sheet.

VINA

VINB

CML

AD9243

0.1 F

RIN

VIN

VCML–VIN

AVDD/2

VCML+VIN

AD8042

CF*

*OPTIONAL NOISE/BAND LIMITING CAPACITOR

Figure 30. Cross-Coupled Differential Driver

The driver circuit shown in Figure 31 is best suited for systems

in which the bipolar input signal is referenced to AGND and

requires proper level shifting. This driver circuit provides the

ability to level-shift the input signal to within the common-

mode range of the AD9243. The two op amps are configured as

matched difference amplifiers with the input signal applied to

opposing inputs to provide the differential output. The common-

mode offset voltage is applied to the noninverting resistor net-

work which provides the proper level shifting. The circuit also

employs optional diodes and pull-up resistors which may help

improve the op amps’ distortion performance by reducing their

headroom requirements. Rail-to-rail output amplifiers like the

AD8042 have sufficient headroom and thus do not require

these optional components.

VINA

VINB

CML

AD9243

390

VIN

VCML–VIN

VCML+VIN

AVDD

390

AVDD

390

AD8047

2.5k

100

0.1 F

1 F

0.1 F

OP113

390

Figure 31. Differential Driver with Level-Shifting

SINGLE-ENDED MODE OF OPERATION

The AD9243 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.

DC COUPLING AND INTERFACE ISSUES

Many applications require the analog input signal to be dc

coupled to the AD9243. An operational amplifier can be con-

figured 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. Hence, the selected input range of the AD9243 should

be sensitive to 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 –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 inher-

ently limit its output swing to within the power supply rails.

Rail-to-rail output amplifiers such as the AD8041 allow the

AD9243 to be configured with larger input spans which im-

proves the noise performance.

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