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AD7440BRT-REEL7
(Rev.:2004)

Differential Input, 1 MSPS 10-Bit and 12-Bit ADCs in an 8-Lead SOT-23

Analog Devices 

Op Amp Pair

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

signal to the AD7440/AD7450A. The circuit configurations

shown in Figure 35 and Figure 36 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 AD7440/AD7450A.

Take care when choosing the op amp; the selection depends on

the required power supply and system performance objectives.

The driver circuits in Figure 35 and Figure 36 are optimized for

dc coupling applications requiring best distortion performance.

The circuit configuration shown in Figure 35 converts a

unipolar, single-ended signal into a differential signal.

The differential op amp driver circuit in Figure 36 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.

2 × VREF p-p

VREF

GND

220Ω

390Ω

V+

V–

220Ω

220Ω

V+

27Ω

VDD

VIN+ AD7440/

AD7450A

VIN–

VREF

A

27Ω

V–

0.1µF

10kΩ

EXTERNAL

VREF

Figure 35. Dual Op Amp Circuit to Convert a Single-Ended Unipolar Signal

into a Differential Signal

AD7440/AD7450A

2 × VREF p-p

220Ω

GND

390Ω

V+

27Ω

VDD

220Ω

V–

220Ω

220Ω

V+

VIN+ AD7440/

AD7450A

VIN–

VREF

A

20kΩ

27Ω

V–

10kΩ

0.1µF

EXTERNAL

VREF

Figure 36. Dual Op Amp Circuit to Convert a Single-Ended Bipolar Signal into

a Differential Signal

RF Transformer

An RF transformer with a center tap offers a good solution for

generating differential inputs in systems that do not need to

be dc-coupled. Figure 37 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 contri-

buting 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.

3.75V

2.5V

1.25V

R

R

VIN+

C

R

AD7440/

AD7450A

VIN–

VREF

3.75V

2.5V

1.25V

EXTERNAL

VREF

Figure 37. Using an RF Transformer to Generate Differential Inputs

Rev. B | Page 19 of 28

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