AD7440BRT-R2 datasheet(16/28 Pages) AD

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AD7440/AD7450A

Rev. B | 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 4 leading zeros followed by the MSB of the 12-bit or 10-bit

result. The 10-bit result of the AD7440 is followed by 2 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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