AD7452BRTZ-R2 datasheet(15/25 Pages) AD

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AD7452

Data Sheet

Rev. C | Page 14 of 24

TYPICAL CONNECTION DIAGRAM

Figure 21 shows a typical connection diagram for the AD7452

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 result. For more

details on driving the differential inputs and setting up the

common mode, refer to the Driving Differential Inputs section.

SERIAL

INTERFACE

2V/2.5V

VREF

AD7452

C/P

0.1

F

3V/5V

SUPPLY

0.1

F

VDD

VIN+

VIN–

SCLK

SDATA

GND

VREF

CM*

VREF

p-p

VREF

p-p

*CM IS THE COMMON-MODE VOLTAGE.

Figure 21. Typical Connection Diagram

ANALOG INPUT

The analog input of the AD7452 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 performance, doubling of

the device’s available dynamic range, and flexibility in input

ranges and bias points. Figure 22 defines the fully differential

analog input of the AD7452.

VREF

p-p

VIN+

VIN–

AD7452

VREF

p-p

COMMON-

MODE

VOLTAGE

Figure 22. Differential Input Definition

The amplitude of the differential signal is the difference

between the signals applied to the VIN+ and VIN– pins (that is,

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 (that is, 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 upon 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 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 23 and Figure 24 show how the common-mode range

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

The common mode must be in this range to guarantee the

functionality of the AD7452.

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.

VREF (V)

2.5

0.5

1.0

1.5

2.0

2.5

2.0

1.5

1.0

0.5

3.0

3.5

3.0

3.5

4.0

1.75V

3.25V

4.5

COMMON-MODE RANGE

Figure 23. Input Common-Mode Range vs. VREF

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

VREF (V)

1.5

0.25

0.50

0.75

1.00

1.25

1.0

0.5

1.50

2.00

2.0

2.5

1.75

COMMON-MODE RANGE

Figure 24. Input Common-Mode Range vs. VREF

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

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