24-Bit Capacitance-to-Digital Converter

with Temperature Sensor

Preliminary Technical Data

AD7747

Rev. PrC, 28. July 2006

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© 2006 Analog Devices, Inc. All rights reserved.

FEATURES

Capacitance-to-digital converter

New standard in single chip solutions

Interfaces to single or differential grounded sensors

Resolution down to 40 aF (that is, up to 18.5-bit ENOB)

Accuracy: 8 fF

Linearity: 0.01%

Common-mode (not changing) capacitance up to 17 pF

Full scale (changing) capacitance range ±8 pF

Update rate: 5 Hz to 45 Hz

Simultaneous 50 Hz and 60 Hz rejection at 8.1 Hz update

Active shield for shielding sensor connection

Temperature sensor on-chip

Resolution: 0.1°C, accuracy: ±2°C

Voltage input channel

Internal clock oscillator

Power

2.7 V to 5.25 V single-supply operation

1 mA current consumption

Operating temperature: –40°C to +125°C

16-lead TSSOP package

APPLICATIONS

Automotive, industrial, and medical systems for

Pressure measurement

Position sensing

Proximity sensing

Level sensing

Flowmeters

Impurity detection

GENERAL DESCRIPTION

The AD7747 is a high-resolution, Σ−∆ capacitance-to-digital

converter (CDC). The capacitance to be measured is connected

directly to the device inputs. The architecture features inherent

high resolution (24-bit no missing codes, up to 18 bit effective

resolution), high linearity (±0.01%), and high accuracy (±8 fF

factory calibrated). The AD7747 capacitance input range is

±8 pF (changing), while it can accept up to 17 pF common-

mode capacitance (not changing), which can be balanced by a

programmable on-chip digital-to-capacitance converter

(CAPDAC).

The AD7747 is designed for single ended or differential

capacitive sensors with one plate connected to ground.

For floating (not grounded) capacitive sensors, the AD7745 or

AD7746 are recommended.

The part has an on-chip temperature sensor with a resolution of

0.1°C and accuracy of ±2°C. The on-chip voltage reference and

the on-chip clock generator eliminate the need for any external

components in capacitive sensor applications. The part has a

standard voltage input, which together with the differential

reference input allows easy interface to an external temperature

sensor, such as an RTD, thermistor, or diode.

part can operate with a single power supply 2.7 V to 5.25 V.

It is specified over the automotive temperature range of

–40°C to +125°C and are housed in a 16-lead TSSOP package.

FUNCTIONAL BLOCK DIAGRAMS

SCL

SDA

CLOCK

GENERATOR

TEMP

SENSOR

MUX

DIGITAL

FILTER

VDD

I2C

SERIAL

INTERFACE

CAP DAC 1

EXCITATION

AD7747

CIN1(+)

VIN(+)

VIN(-)

GND

24-BIT

MODULATOR

CAP DAC 2

SHLD

CIN1(-)

REFIN(+)

REFIN(-)

CONTROL LOGIC

CALIBRATION

RDY

VOLTAGE

REFERENCE

Figure 1.