AD9240AS datasheet(14/24 Pages) AD

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AD9240

REV. A

–14–

AC Coupling with Op Amps

As previously stated, a dual op amp differential driver may be

more suitable in applications in which the spectral content of the

input signal falls below the cutoff frequency of a suitable RF

transformer and/or the cost of an RF transformer and a low

distortion driver for the transformer is prohibitive.

The ac-coupled differential driver shown in Figure 33 is best

suited for

±5 V systems in which the input signal is ground

referenced. In this case, VCM will be 0 V. This driver circuit can

achieve performance similar to an RF transformer over the

AD9240’s full Nyquist bandwidth of 5 MHz. However, unlike

the RF transformer, the lower cutoff frequency can be arbitrarily

set low by adjusting the RC time constant formed by CC and

RB/2. CN, in combination with RS, can be used to limit the con-

tribution of op amp-generated noise at higher frequencies. Low

cost, high performance dual op amps operating from

±5 V such

as the AD8056 and AD8058, are excellent choices for this appli-

cation and are capable of maintaining 78 dB SNR and 83 dB

THD at 1 MHz (5 V span). An optional resistor RO can be

added to U1B to achieve a similar group delay as U1A, potentially

improving overall distortion performance. A resistor divider net-

work formed by RB centers the inputs of the AD9240 around

AVDD/2 to achieve its optimum distortion performance.

VCM

SOURCE

0.1 F

VCC

0.1 F

VCC

U1A

U1B

Figure 33. AC Coupling of Op Amps

DC Coupling with Op Amps

The dc-coupled differential driver in Figure 34 is best suited for

±5 V systems in which the input signal is ground referenced and

optimum distortion performance is desired. This driver circuit

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

common-mode range of the AD9240. The two op amps are

configured as matched differential amps with the input signal

applied to opposing inputs to provide the differential output.

The common-mode offset voltage is applied to the noninverting

resistor network, which provides the proper level shifting. The

AD9631 is given as the amplifier of choice in this application

due to its superior distortion performance for relatively large

output swings and wide bandwidth. If cost or space are factors,

the AD8056 dual op amp will save on both, but at the cost of

slightly increased distortion with large signal levels. Figure 34

also illustrates the use of protection diodes, which are used to

protect the AD9240 from any fault condition in which the op

amps outputs inadvertently go above VDD or below GND.

VINA

VINB

CML

AD9240

390

VIN

VCML–VIN

VCML+VIN

AVDD

390

220

390

AVDD

390

220

390

AD9631

2.5k

100

0.1 F

1 F

0.1 F

OP113

390

0.1 F

Figure 34. Differential Driver with Level-Shifting

Single Supply DC-Coupled Driver

The circuit of Figure 33 can be easily modified for a single

supply, dc-coupled application. This is done by biasing VCM to

AVDD/2, the normal common-mode level in a single supply

system. Since the outputs of the op amps are centered at

AVDD/2, the ac coupling network of CC and RB can be removed.

With this done, the differential driving pair can now be run from

a single supply.

SINGLE-ENDED MODE OF OPERATION

The AD9240 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 AD9240. An operational amplifier can be con-

figured to rescale and level-shift the input signal so it is compat-

ible with the selected input range of the A/D. The input range

to the A/D should be selected on the basis of system perfor-

mance 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 AD9240 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 inherently

limit its output swing to within the power supply rails. Rail-to-

rail output amplifiers such as the AD8041 allow the AD9240 to

be configured with larger input spans which improves the noise

performance.

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