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Data Sheet

AD7265

Rev. B | Page 15 of 28

Differential Mode

The AD7265 can have a total of six differential analog

input pairs.

Differential signals have some benefits over single-ended

signals, including noise immunity based on the device’s

common-mode rejection and improvements in distortion

performance. Figure 23 defines the fully differential analog

input of the AD7265.

VIN+

VIN–

VREF p-p

COMMON

MODE

VOLTAGE

1ADDITIONAL PINS OMITTED FOR CLARITY.

AD7265

Figure 23. Differential Input Definition

The amplitude of the differential signal is the difference

between the signals applied to the VIN+ and VIN− pins in each

differential pair (VIN+ − VIN−). VIN+ and VIN− should be

simultaneously driven by two signals each of amplitude VREF (or

2 × VREF, depending on the range chosen) that are 180° out of

phase. The amplitude of the differential signal is therefore

(assuming the 0 to VREF range is selected) −VREF to +VREF peak-

to-peak (2 × VREF), regardless of the common mode (CM).

The common mode is the average of the two signals

(VIN+ + VIN−)/2

and is therefore the voltage on which the two inputs are

centered.

This results in the span of each input being CM ± VREF/2. This

voltage has to be set up externally, and its range varies with the

reference value, VREF. As the value of VREF increases, the common-

mode range decreases. When driving the inputs with an amplifier,

the actual common-mode range is determined by the amplifier’s

output voltage swing.

Figure 24 and Figure 25 show how the common-mode range

typically varies with VREF for a 5 V power supply using the 0 to

VREF range or 2 × VREF range, respectively. The common mode

must be in this range to guarantee the functionalityof theAD7265.

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. If the

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

from −2 VREF to +2 VREF after conversion.

VREF (V)

5.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

3.5

3.0

2.5

2.0

1.5

1.0

0.5

TA = 25°C

Figure 24. Input Common-Mode Range vs. VREF (0 to VREF Range, VDD = 5 V)

VREF (V)

2.5

0.5

1.0

1.5

2.0

5.0

4.0

4.5

3.0

3.5

2.0

2.5

0.5

1.0

1.5

TA = 25°C

Figure 25. 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. 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. Because not all applications have a signal preconditioned

for differential operation, there is often a need to perform

single-ended-to-differential conversion.

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