AD7265 datasheet(17/29 Pages) AD

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AD7265

Data Sheet

Rev. B | Page 16 of 28

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 AD7265. The

circuit configurations illustrated in Figure 26 and Figure 27

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.

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. The AD8022 is a

suitable dual op amp that can be used in this configuration to

provide differential drive to the AD7265.

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

the required power supply and system performance objectives.

The driver circuits in Figure 26 and Figure 27 are optimized for

dc coupling applications requiring best distortion performance.

The circuit configuration shown in Figure 26 converts a

unipolar, single-ended signal into a differential signal.

The differential op amp driver circuit shown in Figure 27 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

27Ω

V+

V–

V+

V–

VREF

2.5V

3.75V

1.25V

2.5V

3.75V

1.25V

DCAPA/DCAPB

VIN+

VIN–

440Ω

220Ω

0.47µF

1ADDITIONAL PINS OMITTED FOR CLARITY.

220Ω

10kΩ

AD72651

Figure 26. Dual Op Amp Circuit to Convert a Single-Ended Unipolar Signal

into a Differential Signal

20kΩ

220kΩ

2 × VREF p-p

27Ω

V+

V–

V+

V–

GND

2.5V

3.75V

1.25V

2.5V

3.75V

1.25V

DCAPA/DCAPB

VIN+

VIN–

440Ω

220Ω

0.47µF

1ADDITIONAL PINS OMITTED FOR CLARITY.

220Ω

10kΩ

AD72651

Figure 27. Dual Op Amp Circuit to Convert a Single-Ended Bipolar Signal

into a Differential Unipolar Signal

Pseudo Differential Mode

The AD7265 can have a total of six pseudo differential pairs. In

this mode, VIN+ is connected to the signal source that must have

an amplitude of VREF (or 2 × VREF, depending on the range

chosen) to make use of the full dynamic range of the part. A dc

input is applied to the VIN− pin. The voltage applied to this input

provides an offset from ground or a pseudo ground for the VIN+

input. The benefit of pseudo differential inputs is that they

separate the analog input signal ground from the ADC’s ground

allowing dc common-mode voltages to be cancelled. The typical

voltage range for the VIN− pin, while in pseudo differential

mode, is shown in Figure 28 and Figure 29. Figure 30 shows a

connection diagram for pseudo differential mode.

VREF (V)

3.0

0.5

1.0

1.5

2.0

2.5

1.0

0.8

0.4

0.6

0.2

–0.2

–0.4

TA = 25°C

Figure 28. VIN− Input Voltage Range vs. VREF in

Pseudo Differential Mode with VDD = 3 V

VREF (V)

5.0

0.51.01.52.02.5

3.0

3.54.04.5

2.5

2.0

1.5

1.0

0.5

–0.5

TA = 25°C

Figure 29. VIN− Input Voltage Range vs. VREF in

Pseudo Differential Mode with VDD = 5 V

DC INPUT

VOLTAGE

VREF

p–p

VREF

VIN+

VIN–

0.47µF

1ADDITIONAL PINS OMITTED FOR CLARITY.

AD72651

Figure 30. Pseudo Differential Mode Connection Diagram

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