AD9240AS datasheet(15/24 Pages) AD

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AD9240

REV. A

–15–

If the application requires the largest single-ended input range

(i.e., 0 V to 5 V) of the AD9240, the op amp will require larger

supplies to drive it. Various high speed amplifiers in the Op

Amp Selection Guide of this data sheet can be selected to

accommodate a wide range of supply options. Once again,

clamping the output of the amplifier should be considered for

these applications. Alternatively, a single-ended to differential

op amp driver circuit using the AD8042 could be used to

achieve the 5 V input span while operating from a single +5 V

supply as discussed in the previous section.

Two dc coupled op amp circuits using a noninverting and

inverting topology are discussed below. Although not shown,

the noninverting and inverting topologies can be easily config-

ured as part of an antialiasing filter by using a Sallen-Key or

Multiple-Feedback topology, respectively. An additional R-C

network can be inserted between the op amp’s output and the

AD9240 input to provide a real pole.

Simple Op Amp Buffer

In the simplest case, the input signal to the AD9240 will already

be biased at levels in accordance with the selected input range.

It is simply necessary to provide an adequately low source im-

pedance for the VINA and VINB analog input pins of the A/D.

Figure 35 shows the recommended configuration for a single-

ended drive using an op amp. In this case, the op amp is shown

in a noninverting unity gain configuration driving the VINA pin.

The internal reference drives the VINB pin. Note that the addi-

tion of a small series resistor of 30

Ω to 50 Ω connected to VINA

and VINB will be beneficial in nearly all cases. Refer to the

Analog Input Operation section for a discussion on resistor

selection. Figure 35 shows the proper connection for a 0 V to

5 V input range. Alternative single ended input ranges of 0 V to

× VREF can also be realized with the proper configuration of

VREF (refer to the section, Using the Internal Reference).

10 F

VINA

VINB

SENSE

AD9240

0.1 F

–V

VREF

2.5V

Figure 35. Single-Ended AD9240 Op Amp Drive Circuit

Op Amp with DC Level-Shifting

Figure 36 shows a dc-coupled level-shifting circuit employing an

op amp, A1, to sum the input signal with the desired dc offset.

Configuring the op amp in the inverting mode with the given

resistor values results in an ac signal gain of –1. If the signal

inversion is undesirable, interchange the VINA and VINB con-

nections to reestablish the original signal polarity. The dc volt-

age at VREF sets the common-mode voltage of the AD9240. For

example, when VREF = 2.5 V, the output level from the op amp

will also be centered around 2.5 V. The use of ratio matched,

thin-film resistor networks will minimize gain and offset errors.

An optional pull-up resistor, RP, may also be used to reduce the

output load on VREF to

±1 mA.

0VDC

+VREF

–VREF

VINA

VINB

AD9240

0.1 F

500 *

0.1 F

500 *

+VCC

500 *

VREF

500 *

RP**

AVDD

*OPTIONAL RESISTOR NETWORK-OHMTEK ORNA500D

**OPTIONAL PULL-UP RESISTOR WHEN USING INTERNAL REFERENCE

Figure 36. Single-Ended Input With DC-Coupled Level-Shift

AC COUPLING AND INTERFACE ISSUES

For applications where ac coupling is appropriate, the op amp’s

output can be easily level-shifted to the common-mode voltage,

VCM, of the AD9240 via a coupling capacitor. This has the

advantage of allowing the op amps common-mode level to be

symmetrically biased to its midsupply level (i.e., (VCC + VEE)/

2). Op amps that operate symmetrically with respect to their

power supplies typically provide the best ac performance as well

as greatest input/output span. Hence, various high speed/

performance amplifiers that are restricted to +5 V/–5 V op-

eration and/or specified for +5 V single-supply operation can be

easily configured for the 5 V or 2 V input span of the AD9240,

respectively. The best ac distortion performance is achieved

when the A/D is configured for a 2 V input span and common-

mode voltage of 2.5 V. Note that differential transformer

coupling, which is another form of ac coupling, should be

considered for optimum ac performance.

Simple AC Interface

Figure 37 shows a typical example of an ac-coupled, single-

ended configuration. The bias voltage shifts the bipolar,

ground-referenced input signal to approximately VREF. The

value for C1 and C2 will depend on the size of the resistor, R.

The capacitors, C1 and C2, are typically a 0.1

µF ceramic and

µF tantalum capacitor in parallel to achieve a low cutoff

frequency while maintaining a low impedance over a wide fre-

quency range. The combination of the capacitor and the resistor

form a high-pass filter with a high-pass –3 dB frequency deter-

mined by the equation,

f–3 dB = 1/(2

× π × R × (C1 + C2))

VINA

VINB

SENSE

AD9240

+5V

–5V

VREF

+VREF

–VREF

VIN

Figure 37. AC-Coupled Input

The low impedance VREF voltage source biases both the VINB

input and provides the bias voltage for the VINA input. Figure

37 shows the VREF configured for 2.5 V. Thus the input range

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

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