AD7294 datasheet(14/45 Pages) AD

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AD7294

Preliminary Technical Data

Rev. PrB | Page 14 of 45

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 4,096. If the

2 × VREF range is used, the input signal amplitude extends from

−2 VREF (VIN0 = 0 V , VIN1 = VREF) to +2 VREF (VIN1 = 0 V ,

VIN0= VREF).

Driving Differential Inputs

The differential modes available on Channels 1 to 4 in Table 7

requires that VIN+ and VIN− be driven simultaneously with two

equal signals that are 180° out of phase. The common mode must

be set up externally. The common-mode range 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 precon-

ditioned for differential operation, there is often a need to perform

single-ended-to-differential conversion.

Using an Op Amp Pair

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

to one of the analog input pairs of the AD7294. The circuit con-

figurations illustrated in Figure 14 and Figure 15 show how a dual

op amp can be used to convert a single-ended signal into a differen-

tial 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, Point A is connected 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 can

be used in this configuration to provide differential drive to the

AD7294.

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

the required power supply and system performance objectives.

The driver circuits in Figure 14 and Figure 15 are optimized for

dc coupling applications requiring best distortion performance.

The circuit configuration shown in Figure 14 converts a unipolar,

single-ended signal into a differential signal. The differential op

amp driver circuit shown in Figure 15 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.

GND

2× VREF p–p

Ω

V+

V–

V+

V–

VREF

2.5V

3.75V

1.25V

2.5V

3.75V

1.25V

DCAPA/DCAPB

VIN+

AD7294

VIN–

440

Ω

220

Ω

0.47

µF

1ADDITIONAL PINS OMITTED FOR CLARITY.

220

Ω

220

Ω

10k

Ω

Figure 14. Dual Op Amp Circuit to Convert a Single-Ended Unipolar Signal

into a Differential Signal

20k

Ω

220k

Ω

2×VREF p–p

Ω

V+

V–

V+

V–

GND

2.5V

3.75V

1.25V

2.5V

3.75V

1.25V

DCAPA/DCAPB

VIN+

AD7294

VIN–

440

Ω

220

Ω

0.47

µF

1ADDITIONAL PINS OMITTED FOR CLARITY.

220

Ω

220

Ω

10k

Ω

Figure 15. Dual Op Amp Circuit to Convert a Single-Ended Bipolar Signal

into a Differential Unipolar Signal

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