–14–

AD7450

The circuit configuration shown in Figure 15b converts a unipolar,

single-ended signal into a differential signal.

V+

V–

V+

V–

2

VREF

27

27

390

220

10k

EXTERNAL

220

AD7450

220

0.1 F

A

GND

Figure 15b. Dual Op Amp 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 transformer

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 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.

C

EXTERNAL

R

R

R

AD7450

2.5V

3.75V

1.25V

2.5V

3.75V

1.25V

Figure 16. Using an RF Transformer to Generate

Differential Inputs

REFERENCES SECTION

An external reference source is required to supply the reference to the

AD7450. 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 3.3 V power supply, the specified refer-

ence is 1.25 V and the maximum reference is 2.4 V. In both cases,

the reference is functional from 100 mV. It is important to ensure

that, when choosing the reference value for a particular application,

Example 1:

VV

IN

DD

max

.

=+ 03

VV

V

IN

REF

REF

max

=+

2

If V

V

DD

= 5

ThenV

V

.

= 53

Therefore

V

V

REF

32

5 3

×= .

VV

.

= 35

Example 2:

VV

IN

DD

max

.

=+ 03

VV

V

IN

REF

REF

max

=+

2

If V

V

DD

= 33

.

ThenV

V

.

= 36

Therefore

V

V

REF

32

3 6

×= .

VV

.

= 24

can range from 100 mV to a maximum value of 2.4 V. When

These examples show that the maximum reference applied to

The performance of the part at different reference values is shown

in TPC 8 to TPC 12 and in TPC 15. The value of the reference

sets the analog input span and the common-mode voltage range.

Errors in the reference source will result in gain errors in the

AD7450 transfer function and will add to specified full-scale errors

on the part. A capacitor of 0.1

µF should be used to decouple

references to be used that are available from Analog Devices, and

Table I. Examples of Suitable Voltage References

Output

Initial

Operating

Reference

Voltage

Accuracy (% Max)

Current ( A)

AD589

1.235

1.2–2.8

50

AD1580

1.225

0.08–0.8

50

REF192

2.5

0.08–0.4

45

REF43

2.5

0.06–0.1

600

AD780

2.5

0.04–0.2

1000

AD7450*

0.1 F

1

2

3

4

5

6

7

8

TEMP

GND

TRIM

O/P SEL

NC

NC

NC

NC

0.1 F

0.1 F

10nF

*ADDITIONAL PINS OMITTED FOR CLARITY

AD780

NC = NO CONNECT

2.5V

Rev. A