Data Sheet

AD7440/AD7450A

Rev. D | Page 19 of 27

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

of the ADC.

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

into a Differential Signal

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.

Figure 37. Using an RF Transformer to Generate Differential Inputs

DIGITAL INPUTS

The digital inputs applied to the device are not limited by the

maximum ratings, which limit the analog limits. Instead, the

digital inputs applied, CS and SCLK, can go to 7 V and are not

The main advantage of the inputs not being restricted to the

of latch-up as there would be on the analog inputs if a signal

REFERENCE

An external reference source is required to supply the reference

to the device. This reference input can range from 100 mV to

3.5 V. With a 5 V power supply, the specified reference is 2.5 V

and the maximum reference is 3.5 V. With a 3 V power supply,

the specified reference is 2 V and the maximum reference is

2.2 V. In both cases, the reference is functional from 100 mV.

Ensure that, when choosing the reference value for a particular

ratings of the device. The following two examples calculate the

2

GND

390

Ω

220

Ω

220

Ω

220

Ω

10k

Ω

27

Ω

27

Ω

0.1

µF

V+

V–

V+

V–

A

AD7440/

AD7450A

EXTERNAL

GND

V+

V–

V+

V–

A

AD7440/

AD7450A

2

390

Ω

220

Ω

220

Ω

220

Ω

220

Ω

10k

Ω

20k

Ω

27

Ω

27

Ω

0.1

µF

EXTERNAL

R

R

C

3.75V

2.5V

1.25V

3.75V

2.5V

1.25V

R

AD7440/

AD7450A

EXTERNAL