AD7450ARMZ datasheet(13/22 Pages) AD

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AD7450

–13–

Differential Amplifier

An ideal method of applying differential drive to the AD7450 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 buffer-

ing of the bipolar input signal. Figure 14 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 ADC.

The RC low-pass filter on each analog input is recommended in

ac applications to remove the high-frequency components of the

analog input. The architecture of the AD8138 results in outputs

that are highly balanced over a wide frequency range without

requiring tightly matched external components.

If the analog input source being used has zero impedance then 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 paral-

lel impedance on the input and thus ensure that both the positive

and negative analog inputs have the same gain (see Figure 14).

The outputs of the amplifier are perfectly matched, balanced

differential outputs of identical amplitude and exactly 180

o out

of phase.

The AD8138 is specified with 3 V, 5 V, and

±5 V power supplies,

but the best results are obtained when it is supplied by

±5 V.

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, is the AD8132.

Op Amp Pair

An op amp pair can be used to directly couple a differential

signal to the AD7450. The circuit configurations shown in

Figures 15a and 15b show how a dual op amp can be used to

convert a single-ended signal into a differential signal for both a

bipolar and a unipolar input signal, respectively.

The voltage applied to Point A sets up the common-mode voltage.

In both diagrams, it is connected in some way to the reference,

but any value in the common-mode range can be input here to

set up the common mode. Examples of suitable dual op amps

that could be used in this configuration to provide differential

drive to the AD7450 are the AD8042, AD8056, and AD8022.

Care must be taken when choosing the op amp, since the selec-

tion will depend on the required power supply and the system

performance objectives. The driver circuits in Figure 15a and

Figure 15b are optimized for dc coupling applications requiring

optimum distortion performance.

The differential op amp driver circuit in Figure 15a is configured

to convert and level shift a single-ended, ground referenced

(bipolar) signal to a differential signal centered at the VREF level

of the ADC.

V+

V–

V+

V–

390

220

10k

EXTERNAL

VREF

220

VDD

VIN+

VIN–

AD7450

220

20k

0.1 F

VREF

VREF p-p

GND

Figure 15a. Dual Op Amp Circuit to Convert a

Single-Ended Bipolar Input into a Differential Input

2.5V

3.75V

1.25V

Rs*

Rf2

2.5V

3.75V

1.25V

EXTERNAL

VREF (2.5V)

VREF

VIN+

AD7450

VIN–

AD8138

*MOUNT AS CLOSE TO THE AD7450

AS POSSIBLE AND ENSURE HIGH

PRECISION Rs AND Cs ARE USED

Rs – 50R; C – 1nF;

Rg1 = Rf1 = Rf2 = 499R; Rg2 = 523R

Rf1

Rg1

VOCM

51R

Rg2

GND

+2.5V

–2.5V

Figure 14. Using the AD8138 as a Single-Ended-to-Differential Amplifier

Rev. A

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