24-Bit Capacitance-to-Digital Converter
with Temperature Sensor
AD7745/AD7746
GENERAL DESCRIPTION
FEATURES
Capacitance-to-digital converter
New standard in single chip solutions
Interfaces to single or differential floating sensors
Resolution down to 4 aF (that is, up to 21 ENOB)
Accuracy: 4 fF
The AD7745/AD7746 are a high resolution, Σ-Δ capacitance-to-
digital converter (CDC). The capacitance to be measured is
connected directly to the device inputs. The architecture fea-
tures inherent high resolution (24-bit no missing codes, up to
21-bit effective resolution), high linearity (±±.±1%), and high
accuracy (±4 fꢀ factory calibrated). The AD7745/AD7746
capacitance input range is ±4 pꢀ (changing), while it can accept
up to 17 pꢀ common-mode capacitance (not changing), which
can be balanced by a programmable on-chip, digital-to-
capacitance converter (CAPDAC).
Linearity: 0.01%
Common-mode (not changing) capacitance up to 17 pF
Full-scale (changing) capacitance range: 4 pF
Tolerant of parasitic capacitance to ground up to 60 pF
Update rate: 10 Hz to 90 Hz
Simultaneous 50 Hz and 60 Hz rejection at 16 Hz
Temperature sensor on-chip
Resolution: 0.1°C, accuracy: 2°C
Voltage input channel
Internal clock oscillator
The AD7745 has one capacitance input channel, while the
AD7746 has two channels. Each channel can be configured as
single-ended or differential. The AD7745/AD7746 are designed
for floating capacitive sensors. ꢀor capacitive sensors with one
plate connected to ground, the AD7747 is recommended.
2-wire serial interface (I2C®-compatible)
Power
The parts have an on-chip temperature sensor with a resolution
of ±.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
parts have 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.
The AD7745/AD7746 have a 2-wire, I2C-compatible serial
interface. Both parts can operate with a single power supply
from 2.7 V to 5.25 V. They are specified over the automotive
temperature range of –4±°C to +125°C and are housed in a
16-lead TSSOP package.
2.7 V to 5.25 V single-supply operation
0.7 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
Level sensing
Flowmeters
Humidity sensing
Impurity detection
FUNCTIONAL BLOCK DIAGRAMS
VDD
VDD
TEMP
SENSOR
CLOCK
GENERATOR
TEMP
SENSOR
CLOCK
GENERATOR
AD7745
AD7746
VIN(+)
VIN(–)
VIN(+)
VIN(–)
2
2
SDA
SCL
SDA
SCL
I C
I C
MUX
MUX
24-BIT Σ-∆
MODULATOR
DIGITAL
FILTER
24-BIT Σ-∆
MODULATOR
DIGITAL
FILTER
CIN1(+)
CIN1(–)
CIN1(+)
CIN1(–)
SERIAL
INTERFACE
SERIAL
INTERFACE
CIN2(+)
CIN2(–)
CONTROL LOGIC
CALIBRATION
CONTROL LOGIC
CALIBRATION
RDY
RDY
CAP DAC
CAP DAC
CAP DAC
CAP DAC
VOLTAGE
VOLTAGE
REFERENCE
REFERENCE
EXCA
EXCB
EXC1
EXC2
EXCITATION
EXCITATION
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.
Specifications subject to change without notice. No license is granted by implication
or otherwise under any patent or patent rights of Analog Devices. Trademarks and
registered trademarks are the property of their respective owners.
One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A.
Tel: 781.329.4700
Fax: 781.461.3113
www.analog.com
© 2005 Analog Devices, Inc. All rights reserved.
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