AD7450ARMZ-REEL7 查看數據表（PDF） - Analog Devices

零件编号

产品描述 (功能)

生产厂家

AD7450ARMZ-REEL7

Differential Input, 1 MSPS 12-Bit ADC in μSOIC-8 and SO-8

Analog Devices 

AD7450

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

single-ended signal into a differential signal.

VREF

GND

2 ؋ VREF p-p

390⍀

220⍀

V+

V–

220⍀

220⍀

V+

VDD

27⍀

VIN+

AD7450

VIN–

VREF

A

V–

27⍀

10k⍀

0.1␮F

EXTERNAL

VREF

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.

3.75V

2.5V

1.25V

R

R

VIN+

C

AD7450

VIN– VREF

R

3.75V

2.5V

1.25V

EXTERNAL

VREF (2.5V)

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,

the maximum analog input range (VIN max) is never greater than

VDD + 0.3 V to comply with the maximum ratings of the part. The

following two examples calculate the maximum VREF input that can be

used when operating the AD7450 at VDD of 5 V and 3.3 V, respectively.

Example 1:

VIN max = VDD + 0.3

VIN max = VREF + VREF 2

If VDD = 5V

ThenVIN max = 5.3V

Therefore 3 × VREF 2 = 5.3V

VREF max = 3.5V

Therefore, when operating at VDD = 5 V, the value of VREF can

range from 100 mV to a maximum value of 3.5 V. When VDD =

4.75 V, VREF max = 3.37 V.

Example 2:

VIN max = VDD + 0.3

VIN max = VREF + VREF 2

If VDD = 3.3V

ThenVIN max = 3.6V

Therefore 3 × VREF 2 = 3.6 V

VREF max = 2.4V

Therefore, when operating at VDD = 3.3 V, the value of VREF

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

VDD = 2.7 V, VREF max = 2 V.

These examples show that the maximum reference applied to

the AD7450 is directly dependant on the value of VDD.

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

the VREF pin to GND. Table I lists examples of suitable voltage

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

Figure 17 shows a typical connection diagram for the VREF pin.

Table I. Examples of Suitable Voltage References

Output

Reference Voltage

AD589

AD1580

REF192

REF43

AD780

1.235

1.225

2.5

2.5

2.5

Initial

Accuracy (% Max)

1.2–2.8

0.08–0.8

0.08–0.4

0.06–0.1

0.04–0.2

Operating

Current (␮A)

50

50

45

600

1000

VDD

0.1␮F

10nF

AD780

NC 1

O/P SEL 8 NC

0.1␮F

2 VIN

3 TEMP

4 GND

7 NC

VOUT 6

TRIM 5 NC

NC = NO CONNECT

VDD

AD7450*

VREF

2.5V

0.1␮F

*ADDITIONAL PINS OMITTED FOR CLARITY

Figure 17. Typical VREF Connection Diagram for VDD = 5 V

–14–

Rev. A

Share Link: