AD7939BCP datasheet(21/32 Pages) AD

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Preliminary Technical Data

AD7938/AD7939

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 4096 for the

AD7938 and 0 to 1024 for the AD7939. If the 2 × VREF range was

used then the input signal amplitude would extend from −2VREF

to +2VREF after conversion.

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

signal to one of the analog input pairs of the AD7938/AD7939.

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 Unipolar 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 AD7938/AD7939 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.

The circuit configuration shown in

converts a

unipolar, single-ended signal into a differential signal.

Figure 28. Input Common-Mode Range vs. VREF (0 to VREF Range, VDD = 5 V)

The differential op amp driver circuit 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 (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

× 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

AD7938/

AD7939

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 has

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

VREF

GND

390

Ω

220

Ω

220

Ω

20k

Ω

220

Ω

10k

Ω

V+

V–

V+

V–

VIN+

VIN–

VREF

AD7938/

AD7939

0.47

µF

3.75V

2.5V

1.25V

3.75V

2.5V

1.25V

Rev. PrN | Page 21 of 32

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