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AD7440/AD7450A

Data Sheet

Rev. D | Page 18 of 27

DRIVING DIFFERENTIAL INPUTS

Differential operation requires VIN+ and VIN– to be driven

simultaneously with two equal signals that are 180° out of

phase. The common mode must be set up externally and has a

range determined by VREF, the power supply, and the particular

amplifier used to drive the analog inputs (see Figure 28 and

Figure 29). Differential modes of operation with either an ac or

dc input provide the best THD performance over a wide

frequency range. Because not all applications have a signal

preconditioned for differential operation, there is often a need

to perform single-ended-to-differential conversion.

Differential Amplifier

An ideal method of applying differential drive to the

AD7440/AD7450A is to use a differential amplifier such as the

AD8138. This part can be used as a single-ended-to-differential

amplifier or as a differential-to-differential amplifier. In both

cases, the analog input needs to be bipolar. It also provides

common-mode level shifting and buffering of the bipolar input

signal. Figure 34 shows how the AD8138 can be used as a

single-ended-to-differential amplifier. The positive and negative

outputs of the AD8138 are connected to the respective inputs

on the ADC via a pair of series resistors to minimize the effects

of switched capacitance on the front end of the ADCs. The RC

low-pass filter on each analog input is recommended in ac

applications to remove high frequency components of the

analog input. The architecture of the AD8138 results in outputs

that are very highly balanced over a wide frequency range

without requiring tightly matched external components.

If the analog input source being used has zero impedance, all

four resistors (RG1, RG2, RF1, and RF2) should be the same. If

the source has a 50 Ω impedance and a 50 Ω termination, for

example, the value of RG2 should be increased by 25 Ω to

balance this parallel impedance on the input and thus ensure

that both the positive and negative analog inputs have the same

gain (see Figure 34). The outputs of the amplifier are perfectly

matched, balanced differential outputs of identical amplitude

and are exactly 180° out of phase.

The AD8138 is specified with +3 V, +5 V, and ±5 V power

supplies, but the best results are obtained with a ±5 V supply.

The AD8132 is a lower cost device that could also be used in

this configuration with slight differences in characteristics to

the AD8138 but with similar performance and operation.

Figure 34. Using the AD8138 as a Single-Ended-to-Differential Amplifier

+2.5V

GND

–2.5V

AD8138

RG1

RS*

RG2

RF2

VOCM

RF1

3.75V

VIN–

VIN+

VREF

2.5V

1.25V

3.75V

2.5V

1.25V

AD7440/

AD7450A

*MOUNT AS CLOSE TO THE AD7440/AD7450A AS POSSIBLE

AND ENSURE HIGH PRECISION RS AND CS ARE USED.

RS = 50Ω; C = 1nF

RG1 = RF1 = RF2 = 499Ω; RG2 = 523Ω

EXTERNAL

VREF (2.5V)

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