AD7707 datasheet(33/40 Pages) AD

33 / 40 page

REV. A

AD7707

–33–

APPLICATIONS

The AD7707 provides a low cost, high resolution analog-to-

digital function with two low level input channels and one high

level input channel. Because the analog-to-digital function is

provided by a sigma-delta architecture, it makes the part more

immune to noisy environments thus making the part ideal for

use in industrial and process control applications. It also provides

a programmable gain amplifier, a digital filter and calibration

options. Thus, it provides far more system level functionality

than off-the-shelf integrating ADCs without the disadvantage of

having to supply a high quality integrating capacitor. In addi-

tion, using the AD7707 in a system allows the system designer

to achieve a much higher level of resolution because noise per-

formance of the AD7707 is better than that of the integrating

ADCs.

The on-chip PGA allows the AD7707 to handle an analog input

voltage ranges as low as 10 mV full-scale with VREF = +1.25 V.

The pseudo-differential input capability of the low level channel

allows this analog input range to have an absolute value any-

where between AGND – 100 mV and AVDD + 30 mV when the

part is operated in unbuffered mode. It allows the user to con-

nect the transducer directly to the input of the AD7707.

In addition, the three-wire digital interface on the AD7707

allows data acquisition front ends to be isolated with just three

wires. The AD7707 can be operated from a single 3 V or 5 V

and its low power operation ensures that very little power needs

to be brought across the isolation barrier in an isolated application.

Data Acquisition

Figure 24 shows a data acquisition system showing in which the

low level input channel is used to digitize signals from a thermo-

couple and the high level input channel converts process control

signals up to

±10 V in amplitude. This application shows the

high level input channel being used to convert a number of

input signals provided through an external mux controlled by

the system microcontroller. Switching channels on the external

multiplexer is equivalent to providing a step change on the

AIN3 input. It takes 3 or 4 updates before the correct output

code corresponding to the new analog input appears at the

output. Therefore, when switching between channels on the

external mux the first three outputs should be ignored following

the channel change or the FSYNC bit in the setup register

should be used to reset the digital filter and ensure that the

DRDY is set high until a valid result is available in the output

Smart Valve/Actuator Control

Another area where the low power, single supply and high volt-

age input capability is of benefit is in smart valve and and actua-

tor control circuits. The AD7707 monitors the signal from the

control valve. The controller and the AD7707 form a closed-

loop control circuit. Figure 25 shows a block diagram of a smart

actual or control circuit which includes the AD7707. The AD7707

monitors the valve position via a high quality servo pot whose

output is

±10 V.

Similar applications for the AD7707 would be in the area of

smart transmitters. Here, the entire smart transmitter must

operate from the 4 mA to 20 mA loop.

Tolerances in the loop mean that the amount of current avail-

able to power the transmitter is as low as 3.5 mA. The AD7707

consumes only 280

µA, leaving at least 3 mA available for the

rest of the transmitter. Figure 26 shows a block diagram of a

smart transmitter which includes the AD7707.

AD7707

DVDD

+5V

AIN2

REF IN(–)

AGND

MCLK IN

MCLK OUT

REF IN(+)

8.2k

CJC

AVDD

AD590

+5V

AIN1

LOCOM

2.5V

AIN3

VBIAS

HICOM

DGND

IN1

IN2

250

OUT

IN3

IN4

IN5

250

IN6

VSS

SCLK

DIN

DOUT

+15V

–15V

10V INPUT

0V TO 10V INPUT

5V INPUT

0V TO 5V INPUT

4-20mA

0-20mA

THERMOCOUPLE

JUNCTION

MICRO-

CONTROLLER

SCLK

DOUT

DIN

ANALOG

MULTIPLEXER

VDD

Figure 24. Data Acquisition System Using the AD7707

AD7707 Datasheet(PDF) 33 Page - Analog Devices

AD7707 Datasheet(HTML) 33 Page - Analog Devices

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40

Datasheet Download

Link URL