24-Bit Capacitance-to-Digital Converter with Temperature Sensor AD7745/AD7746 GENERAL DESCRIPTION FEATURES Capacitance-to-digital converter The AD7745/AD7746 are a high resolution, - capacitance-to- New standard in single chip solutions digital converter (CDC). The capacitance to be measured is Interfaces to single or differential floating sensors connected directly to the device inputs. The architecture fea- Resolution down to 4 aF (that is, up to 21 ENOB) tures inherent high resolution (24-bit no missing codes, up to Accuracy: 4 fF 21-bit effective resolution), high linearity (0.01%), and high Linearity: 0.01% accuracy (4 fF factory calibrated). The AD7745/AD7746 Common-mode (not changing) capacitance up to 17 pF capacitance input range is 4 pF (changing), while it can accept Full-scale (changing) capacitance range: 4 pF up to 17 pF common-mode capacitance (not changing), which Tolerant of parasitic capacitance to ground up to 60 pF can be balanced by a programmable on-chip, digital-to- Update rate: 10 Hz to 90 Hz capacitance converter (CAPDAC). Simultaneous 50 Hz and 60 Hz rejection at 16 Hz Temperature sensor on-chip The AD7745 has one capacitance input channel, while the Resolution: 0.1C, accuracy: 2C AD7746 has two channels. Each channel can be configured as Voltage input channel single-ended or differential. The AD7745/AD7746 are designed Internal clock oscillator for floating capacitive sensors. For capacitive sensors with one 2 2-wire serial interface (I C-compatible) plate connected to ground, the AD7747 is recommended. Power The parts have an on-chip temperature sensor with a resolution 2.7 V to 5.25 V single-supply operation of 0.1C and accuracy of 2C. The on-chip voltage reference 0.7 mA current consumption Operating temperature: 40C to +125C and the on-chip clock generator eliminate the need for any 16-lead TSSOP package external components in capacitive sensor applications. The parts have a standard voltage input, which together with the APPLICATIONS differential reference input allows easy interface to an external Automotive, industrial, and medical systems for temperature sensor, such as an RTD, thermistor, or diode. Pressure measurement 2 The AD7745/AD7746 have a 2-wire, I C-compatible serial Position sensing interface. Both parts can operate with a single power supply Level sensing from 2.7 V to 5.25 V. They are specified over the automotive Flowmeters temperature range of 40C to +125C and are housed in a Humidity sensing 16-lead TSSOP package. Impurity detection FUNCTIONAL BLOCK DIAGRAMS VDD VDD TEMP CLOCK TEMP CLOCK SENSOR GENERATOR AD7745 SENSOR GENERATOR AD7746 VIN(+) VIN(+) VIN() VIN() 2 2 SDA SDA I C I C MUX MUX 24-BIT - DIGITAL 24-BIT - DIGITAL CIN1(+) CIN1(+) SERIAL SERIAL MODULATOR FILTER MODULATOR FILTER CIN1() CIN1() INTERFACE INTERFACE SCL SCL CIN2(+) CIN2() CONTROL LOGIC CONTROL LOGIC RDY RDY CAP DAC CAP DAC CALIBRATION CALIBRATION CAP DAC CAP DAC VOLTAGE VOLTAGE EXCA REFERENCE EXC1 REFERENCE EXCITATION EXCITATION EXCB EXC2 REFIN(+) REFIN() GND REFIN(+) REFIN() GND Figure 1. Figure 2. Rev. 0 Information furnished by Analog Devices is believed to be accurate and reliable. However, no responsibility is assumed by Analog Devices for its use, nor for any infringements of patents or other rights of third parties that may result from its use. One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A. Specifications subject to change without notice. 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All rights reserved. 05468-001 05468-002AD7745/AD7746 TABLE OF CONTENTS Specifications..................................................................................... 3 Cap Gain Calibration Register.................................................. 19 Timing Specifications....................................................................... 5 Volt Gain Calibration Register ................................................. 19 Absolute Maximum Ratings............................................................ 6 Circuit Description......................................................................... 20 Pin Configurations and Function Descriptions ........................... 7 Overview ..................................................................................... 20 Typical Performance Characteristics ............................................. 8 Capacitance-to-Digital Converter ........................................... 20 Output Noise and Resolution Specifications .............................. 11 Excitation Source........................................................................ 20 Serial Interface ................................................................................ 12 CAPDAC ..................................................................................... 21 Read Operation........................................................................... 12 Single-Ended Capacitive Input................................................. 21 Write Operation.......................................................................... 12 Differential Capacitive Input .................................................... 21 AD7745/AD7746 Reset ............................................................. 13 Parasitic Capacitance to Ground.............................................. 22 General Call................................................................................. 13 Parasitic Resistance to Ground................................................. 22 Register Descriptions ..................................................................... 14 Parasitic Parallel Resistance ...................................................... 22 Status Register............................................................................. 15 Parasitic Serial Resistance ......................................................... 23 Cap Data Register....................................................................... 15 Capacitive Gain Calibration ..................................................... 23 VT Data Register ........................................................................ 15 Capacitive System Offset Calibration...................................... 23 Cap Set-Up Register ................................................................... 16 Internal Temperature Sensor .................................................... 23 VT Set-Up Register .................................................................... 16 External Temperature Sensor ................................................... 24 EXC Set-Up Register.................................................................. 17 Voltage Input............................................................................... 24 Configuration Register .............................................................. 18 V Monitor ................................................................................ 24 DD Cap DAC A Register................................................................... 19 Typical Application Diagram.................................................... 24 Cap DAC B Register................................................................... 19 Outline Dimensions ....................................................................... 25 Cap Offset Calibration Register................................................ 19 Ordering Guide .......................................................................... 25 REVISION HISTORY 4/05Revision 0: Initial Version Rev. 0 Page 2 of 28