AD7933 datasheet(21/32 Pages) AD

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Preliminary Technical Data

AD7933/AD7934

Using an Op Amp Pair

When a conversion takes place, the common mode is rejected

resulting in a virtually noise free signal of amplitude −VREF to

+VREF corresponding to the digital codes of 0 to 1024 for the

AD7933 and 0 to 4096 for the AD7934.

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

signal to one of the analog input pairs of the AD7933/AD7934.

The circuit configurations shown in

and

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.

Figure 30

Figure 30. Dual Op Amp Circuit to Convert a

Single-Ended Unipolar Signal into a Differential Signal

Figure 31

Figure 31. Dual Op Amp Circuit to Convert a

Single-Ended Bipolar Signal into a Differential Signal

VREF (V)

3.5

3.0

2.0

1.5

2.5

1.0

0.5

1.5

1.0

2.0

2.5

3.0

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. A suitable dual op amp

that could be used in this configuration to provide differential

drive to the AD7933/AD7934 is the AD8022.

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

the required power supply and system performance objectives.

The driver circuits in

and

are optimized for

dc coupling applications requiring best distortion performance.

Figure 28. Input Common-Mode Range vs.

VREF (0 to VREF Range, VDD = 5 V)

The circuit configuration shown in

converts a

unipolar, single-ended signal into a differential signal.

VREF (V)

4.5

4.0

3.0

1.5

2.0

2.5

3.5

1.0

0.5

0.1

0.6

1.6

1.1

2.1

2.6

The circuit configuration in

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.

× VREF p-p

VREF

GND

390

Ω

220

Ω

220

Ω

20k

Ω

220

Ω

10k

Ω

V+

V–

V+

V–

VIN+

VIN–

VREF

AD7933/

AD7934

0.47

µF

3.75V

2.5V

1.25V

3.75V

2.5V

1.25V

Figure 29. Input Common-Mode Range vs.

VREF (2 × VREF Range, VDD = 5 V)

Driving Differential Inputs

Differential operation requires that VIN+ and VIN− be

simultaneously driven with two equal signals that are 180° out

of phase. The common mode must be set up externally and

have a range that is determined by VREF, the power supply, and

the particular amplifier used to drive the analog inputs.

Differential modes of operation with either an ac or dc input

provide the best THD performance over a wide frequency

range. Since not all applications have a signal preconditioned for

differential operation, there is often a need to perform single-

ended-to-differential conversion.

× VREF p-p

GND

390

Ω

220

Ω

220

Ω

220

Ω

20k

Ω

220

Ω

10k

Ω

V+

V–

V+

V–

VIN+

VIN–

VREF

AD7933/

AD7934

0.47

µF

3.75V

2.5V

1.25V

3.75V

2.5V

1.25V

Rev. PrG | Page 21 of 32

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