AD7450ARM datasheet(12/24 Pages) AD

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PRELIMINARY TECHNICAL DATA

–12–

AD7450

100...000

+VREF -1LSB

100...001

100...010

111...111

000...000

000...001

011...110

011...111

-VREF +1LSB

0LSB

1LSB = 2xVREF/4096

ANALOG INPUT

(VIN+-VIN-)

Figure 5. AD7450 Ideal Transfer Characteristic

TYPICAL CONNECTION DIAGRAM

Figure 6 shows a typical connection diagram for the

AD7450 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

1.25 V decoupled reference source depending on the

power supply, to set up the analog input range. The com-

mon mode voltage has to be set up externally and is the

value that the two inputs are centered on. For more details

on driving the differential inputs and setting up the com-

mon mode, see the ‘Driving Differential Inputs’ section.

The conversion result for the ADC is output in a 16-bit

word consisting of four leading zeros followed by the

MSB of the 12-bit result. For applications where power

consumption is of concern, the power-down mode should

be used between conversions or bursts of several conver-

sions to improve power performance. See ‘Modes of

Operation’ section.

CM*

* CM - COMMON MODE VOLTAGE

VIN+

VIN-

VDD

SCLK

SDATA

GND

VREF

µC/µP

SERIAL

INTERFACE

+3V/+5V

SUPPLY

1.25V/2.5V

VREF

0.1µF

10µF

AD7450

VREF

P-to-P

VREF

P-to-P

Figure 6. Typical Connection Diagram

THE ANALOG INPUT

The analog input of the AD7450 is fully differential.

Dif-

ferential signals have a number of benefits over single

ended signals including noise immunity based on the

device’s common mode rejection, improvements in distor-

tion performance, doubling of the device’s available

dynamic range and flexibility in input ranges and bias

points.

Figure 7 defines the fully differential analog input of the

AD7450.

VIN+

AD7450

VIN-

VREF

P-to-P

VREF

P-to-P

COMMON

MODE

VOLTAGE

Figure 7. Differential Input Definition

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 (i.e. 2 x VREF). This is re-

gardless of the common mode (CM). The common mode

is the average of the two signals, i.e. (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 var-

ies with VREF. As the value of VREF increases, the

common mode range decreases.

When driving the inputs

with an amplfier, the actual common mode range will be

determined by the amplifier’s output voltage swing.

Figure 8 shows how the common mode range varies with

VREF for a 5 V power supply and figure 9 shows an ex-

ample of the common mode range when using the

AD8138 differential amplifer to drive the analog inputs.

The common mode must be in this range to guarantee the

specifications.

With a 3V power supply, the Common

Mode range is TBD.

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

equal to 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

he digital codes of 0 to 4095.

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