AD7452BRTZ-R2 datasheet(18/25 Pages) AD

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Data Sheet

AD7452

Rev. C | Page 17 of 24

Op Amp Pair

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

signal to the AD7452. The circuit configurations shown in

Figure 30 and Figure 31 show how a dual op amp can be used to

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

bipolar and 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. The AD8022 is a

suitable dual op amp that could be used in this configuration to

provide differential drive to the AD7452.

Care must be taken when choosing the op amp because the

selection depends on the required power supply and the system

performance objectives. The driver circuits in Figure 30 and

Figure 31 are optimized for dc coupling applications that

require optimum distortion performance.

The differential op amp driver circuit in Figure 30 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.

The circuit configuration shown in Figure 31 converts a

unipolar, single-ended signal into a differential signal.

GND

× V

REF p-p

EXTERNAL

VREF

VIN+

VIN–

AD7452

VREF

VDD

V+

V–

V+

V–

220

Ω

220

Ω

390

Ω

220

Ω

20k

Ω

10k

Ω

220

Ω

0.1

µF

Figure 30. Dual Op Amp Circuit to Convert a Single-Ended

Bipolar Signal into a Differential Signal

VREF

× V

REF p-p

EXTERNAL

VREF

VIN+

VIN–

AD7452

VREF

VDD

V+

V–

V+

V–

220

Ω

220

Ω

390

Ω

10k

Ω

220

Ω

GND

0.1

µF

Figure 31. Dual Op Amp Circuit to Convert a Single-Ended

Unipolar Signal into a Differential Signal

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 32 shows how a transformer is used

for single-ended-to-differential conversion. 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 applications. 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.

EXTERNAL

VREF (2.5 V)

VIN+

VIN–

AD7452

3.75V

2.5V

1.25V

3.75V

2.5V

1.25V

VREF

Figure 32. Using an RF Transformer to Generate Differential Inputs

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