AD7440BRT-R2 datasheet(16/28 Pages) AD

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AD7440/AD7450A

Rev. C | Page 16 of 28

TYPICAL CONNECTION DIAGRAM

Figure 26 shows a typical connection diagram for the

AD7440/AD7450A for both 5 V and 3 V supplies. In this setup,

the GND pin is connected to the analog ground plane of the

system. The VREF pin is connected to either a 2.5 V or a 2 V

decoupled reference source, depending on the power supply, to

set up the analog input range. The common-mode voltage has

to be set up externally and is the value on which the two inputs

are centered. The conversion result is output in a 16-bit word

with four leading zeros followed by the MSB of the 12-bit or

10-bit result. The 10-bit result of the AD7440 is followed by two

trailing zeros. For more details on driving the differential inputs

and setting up the common mode, refer to the Driving

Differential Inputs section.

AD7440/

AD7450A

0.1

μF

0.1

μF

VREF

VDD

VIN+

SCLK

3V/5V

SUPPLY

SERIAL

INTERFACE

μC/μP

SDATA

GND

VIN–

2V/2.5V

VREF

*CM IS THE COMMON-MODE VOLTAGE.

CM*

VREF

p-p

CM*

VREF

p-p

Figure 26. Typical Connection Diagram

ANALOG INPUT

The analog input of the AD7440/AD7450A is fully differential.

Differential signals have a number of benefits over single-

ended signals, including noise immunity based on the device’s

common-mode rejection, improvements in distortion perfor-

mance, doubling of the device’s available dynamic range, and

flexibility in input ranges and bias points. Figure 27 defines the

fully differential analog input of the AD7440/AD7450A.

VREF

p-p

VIN+

VIN–

VREF

p-p

AD7440/

AD7450A

COMMON-MODE

VOLTAGE

Figure 27. Differential Input Definitions

The amplitude of the differential signal is the difference

between the signals applied to the VIN+ and VIN– pins

(i.e., VIN+ – VIN–). VIN+ and VIN– are simultaneously driven by

two signals each of amplitude VREF that are 180° out of phase.

The amplitude of the differential signal is therefore –VREF to

+VREF peak-to-peak (2 × VREF). This is true regardless of the

common mode (CM).

The common mode is the average of the two signals, that is,

(VIN+ + VIN–)/2 and is therefore the voltage that the two inputs

are centered on. 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 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 28 and Figure 29 show how the common-mode range

typically varies with VREF for both a 5 V and a 3 V power supply.

The common mode must be in this range to guarantee the

functionality of the AD7440/AD7450A.

For ease of use, the common mode can be set up to equal VREF,

resulting in the differential signal being ±VREF centered on VREF.

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 in the

case of the AD7450A and 0 to 1024 in the AD7440.

4.5

4.0

3.5

3.0

2.5

2.0

1.5

1.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

VREF (V)

1.75V

3.25V

COMMON-MODE RANGE

Figure 28. Input Common-Mode Range vs. VREF

(VDD = 5 V and VREF (Max) = 3.5 V)

2.5

0.5

1.0

1.5

2.0

0.25

0.50

0.75

1.00

1.25

1.50

1.75

2.00

VREF (V)

COMMON-MODE RANGE

Figure 29. Input Common-Mode Range vs. VREF

(VDD = 3 V and VREF (Max) =2V)

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