AD7450ARM datasheet(15/24 Pages) AD

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PRELIMINARY TECHNICAL DATA

–15–

AD7450

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.

2.5V

3.75V

1.25V

Rs*

Rf2

2.5V

3.75V

1.25V

EXTERNAL

VREF (2.5V)

VREF

VIN+

AD7450

VIN-

AD8138

*Moun t as close to the AD7450 as

possible and ensure high

precision Rs and Cs are used

Rs - 10R; 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 Differen-

tial Amplifier

Opamp Pair

An opamp pair can be used to directly couple a differential

signal to the AD7450.

The circuit configurations shown

in figures 15(a) and 15(b) show how a dual opamp can be

used to convert a single ended signal into a differential

signal for both a bipolar and a unipolar input signal re-

spectively.

The voltage applied to point A is 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 setup the common mode. Examples of

suitable dual opamps that could be used in this configura-

tion to provide differential drive to the AD7450 are the

AD8042, AD8056 and the AD8022.

Care must be taken when chosing the opamp used, as the

selection will depend on the required power supply and the

system performance objectives. The driver circuits in fig-

ures 15(a) and 15(b) are optimized for dc coupling

applications requiring optimum distortion performance.

The differential op-amp driver circuit in figure 15(a) is

configured to convert and level shift a 2.5 V p-p single

ended, ground referenced (bipolar) signal to a 5 V p-p

differential signal centered at the VREF level of the ADC.

The circuit configuration shown in figure 15(b) converts a

unipolar, single ended signal into a differential signal.

GND

VREF P-to-P

390

220

10K

EXTERNAL

VREF

220

VDD

V+

V-

VIN+

VIN- VREF

AD7450

220

20K

0.1µF

Figure 15(a). Dual Opamp Circuit to Convert a Single Ended

Bipolar Input into a Differential Input

GND

VREF P-to-P

390

220

10K

EXTERNAL

VREF

220

VDD

V+

V-

VIN+

VIN- VREF

AD7450

220

0.1µF

VREF/2

Figure 15(b). Dual Opamp Circuit to Convert a Single Ended

Unipolar Input into a Differential Input

RF Transformer

In systems that do not need to be dc-coupled, an RF trans-

former with a center tap offers a good solution for

generating differential inputs. Figure 16 shows how a

transformer is used for single ended to differential conver-

sion. It provides the benefits of operating the ADC in the

differential mode without contributing additional noise

and distortion. An RF transformer also has the benefit of

providing electrical isolation between the signal source

and the ADC. A transformer can be used for most ac ap-

plications. The center tap is used to shift the differential

signal to the common mode level required, in this case it

is connected to the reference so the common mode level is

the value of the reference.

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