24-Bit Capacitance-to-Digital Converter
with Temperature Sensor
AD7745/AD7746
Rev. 0
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Tel: 781.329.4700 www.analog.com
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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
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
2-wire serial interface (I2C®-compatible)
Power
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
GENERAL DESCRIPTION
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 (±0.01%), and high
accuracy (±4 fF factory calibrated). The AD7745/AD7746
capacitance input range is ±4 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 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. For capacitive sensors with one
plate connected to ground, the AD7747 is recommended.
The parts have 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
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 –40°C to +125°C and are housed in a
16-lead TSSOP package.
FUNCTIONAL BLOCK DIAGRAMS
DIGITAL
FILTER
24-BIT Σ-∆
MODULATOR
CLOCK
GENERATOR
MUX
TEMP
SENSOR
CAP DAC
CAP DAC VOLTAGE
REFERENCE
CONTROL LOGIC
CALIBRATION
AD7745
VDD
REFIN(+) REFIN(–) GND
SDA
SCL
RDY
VIN(+)
VIN(–)
CIN1(+)
CIN1(–)
EXCA
EXCB
05468-001
I2C
SERIAL
INTERFACE
EXCITATION
Figure 1.
DIGITAL
FILTER
24-BIT Σ-∆
MODULATOR
MUX
TEMP
SENSOR
CAP DAC
CAP DAC VOLTAGE
REFERENCE
CONTROL LOGIC
CALIBRATION
AD7746
VDD
REFIN(+) REFIN(–) GND
SDA
SCL
RDY
VIN(+)
VIN(–)
CIN1(+)
CIN1(–)
CIN2(+)
CIN2(–)
EXC1
EXC2
05468-002
CLOCK
GENERATOR
I2C
SERIAL
INTERFACE
EXCITATION
Figure 2.
AD7745/AD7746
Rev. 0 | Page 2 of 28
TABLE OF CONTENTS
Specifications..................................................................................... 3
Timing Specifications....................................................................... 5
Absolute Maximum Ratings............................................................ 6
Pin Configurations and Function Descriptions ........................... 7
Typical Performance Characteristics ............................................. 8
Output Noise and Resolution Specifications .............................. 11
Serial Interface ................................................................................ 12
Read Operation........................................................................... 12
Write Operation.......................................................................... 12
AD7745/AD7746 Reset ............................................................. 13
General Call................................................................................. 13
Register Descriptions ..................................................................... 14
Status Register............................................................................. 15
Cap Data Register....................................................................... 15
VT Data Register ........................................................................ 15
Cap Set-Up Register................................................................... 16
VT Set-Up Register .................................................................... 16
EXC Set-Up Register.................................................................. 17
Configuration Register .............................................................. 18
Cap DAC A Register................................................................... 19
Cap DAC B Register................................................................... 19
Cap Offset Calibration Register................................................ 19
Cap Gain Calibration Register.................................................. 19
Volt Gain Calibration Register ................................................. 19
Circuit Description......................................................................... 20
Overview ..................................................................................... 20
Capacitance-to-Digital Converter ........................................... 20
Excitation Source........................................................................ 20
CAPDAC ..................................................................................... 21
Single-Ended Capacitive Input................................................. 21
Differential Capacitive Input .................................................... 21
Parasitic Capacitance to Ground.............................................. 22
Parasitic Resistance to Ground................................................. 22
Parasitic Parallel Resistance ...................................................... 22
Parasitic Serial Resistance ......................................................... 23
Capacitive Gain Calibration ..................................................... 23
Capacitive System Offset Calibration...................................... 23
Internal Temperature Sensor .................................................... 23
External Temperature Sensor ................................................... 24
Voltage Input ............................................................................... 24
VDD Monitor ................................................................................ 24
Typical Application Diagram .................................................... 24
Outline Dimensions....................................................................... 25
Ordering Guide .......................................................................... 25
REVISION HISTORY
4/05—Revision 0: Initial Version
AD7745/AD7746
Rev. 0| Page 3 of 28
SPECIFICATIONS
VDD = 2.7 V to 3.6 V or 4.75 V to 5.25 V; GND = 0 V; EXC = 32 kHz; EXC = ±VDD/2; –40°C to +125°C, unless otherwise noted.
Table 1.
Parameter Min Typ Max Unit Test Conditions/Comments
CAPACITIVE INPUT
Conversion Input Range ±4.096 pF1Factory calibrated
Integral Nonlinearity (INL)2 ±0.01 % of FSR
No Missing Codes2 24 Bit Conversion time ≥ 62 ms
Resolution, p-p 16.5 Bit Conversion time = 62 ms, see Table 5
Resolution Effective 19 Bit Conversion time = 62 ms, see Table 5
Output Noise, rms 2 aF/√Hz See Table 5
Absolute Error3 ±4 fF
1 25°C, VDD = 5 V, after offset calibration
Offset Error2, 4 32 aF1 After system offset calibration,
Excluding effect of noise4
System Offset Calibration Range2 ±1 pF
Offset Drift vs. Temperature –1 aF/°C
Gain Error5 0.02 0.08 % of FS 25°C, VDD = 5 V
Gain Drift vs. Temperature2 –28 –26 –24 ppm of FS/°C
Allowed Capacitance to GND2 60 pF See Figure 9 and Figure 10
Power Supply Rejection 0.3 1 fF/V
Normal Mode Rejection 65 dB 50 Hz ± 1%, conversion time = 62 ms
55 dB 60 Hz ± 1%, conversion time = 62 ms
Channel-to-Channel Isolation 70 dB AD7746 only
CAPDAC
Full Range 17 21 pF
Resolution6 164 fF 7-bit CAPDAC
Drift vs. Temperature2 24 26 28 ppm of FS/°C
EXCITATION
Frequency 32 kHz
Voltage Across Capacitance ±VDD/8 V Configurable via digital interface
±VDD/4 V
±VDD × 3/8 V
±VDD/2 V
Average DC Voltage Across
Capacitance
<±40 mV
Allowed Capacitance to GND2 100 pF See Figure 11
TEMPERATURE SENSOR7 V
REF internal
Resolution 0.1 °C
Error2 ±0.5 ±2 °C Internal temperature sensor
±2 °C External sensing diode8
VOLTAGE INPUT7 V
REF internal or VREF = 2.5 V
Differential VIN Voltage Range ±VREF V
Absolute VIN Voltage2 GND − 0.03 VDD + 0.03 V
Integral Nonlinearity (INL) ±3 ±15 ppm of FS
No Missing Codes2 24 Bit Conversion time = 122.1 ms
Resolution, p-p 16 Bits Conversion time = 62 ms
See Table 6 and Table 7
Output Noise 3 µV rms Conversion time = 62 ms
See Table 6 and Table 7
Offset Error ±3 µV
Offset Drift vs. Temperature 15 nV/°C
Full-Scale Error2, 9 0.025 0.1 % of FS
AD7745/AD7746
Rev. 0 | Page 4 of 28
Parameter Min Typ Max Unit Test Conditions/Comments
Full-Scale Drift vs. Temperature 5 ppm of FS/°C Internal reference
0.5 ppm of FS/°C External reference
Average VIN Input Current 300 nA/V
Analog VIN Input Current Drift ±50 pA/V/°C
Power Supply Rejection 80 dB Internal reference, VIN = VREF/2
Power Supply Rejection 90 dB External reference, VIN = VREF/2
Normal Mode Rejection 75 dB 50 Hz ± 1%, conversion time = 122.1 ms
50 dB 60 Hz ± 1%, conversion time = 122.1 ms
Common-Mode Rejection 95 dB VIN = 1 V
INTERNAL VOLTAGE REFERENCE
Voltage 1.169 1.17 1.171 V TA = 25°C
Drift vs. Temperature 5 ppm/°C
EXTERNAL VOLTAGE REFERENCE INPUT
Differential REFIN Voltage2 0.1 2.5 VDD V
Absolute REFIN Voltage2 GND − 0.03 VDD + 0.03 V
Average REFIN Input Current 400 nA/V
Average REFIN Input Current Drift ±50 pA/V/°C
Common-Mode Rejection 80 dB
SERIAL INTERFACE LOGIC INPUTS
(SCL, SDA)
VIH Input High Voltage 2.1 V
VIL Input Low Voltage 0.8 V
Hysteresis 150 mV
Input Leakage Current (SCL) ±0.1 ±1 µA
OPEN-DRAIN OUTPUT (SDA)
VOL Output Low Voltage 0.4 V ISINK = −6.0 mA
IOH Output High Leakage Current 0.1 1 µA VOUT = VDD
LOGIC OUTPUT (RDY)
VOL Output Low Voltage 0.4 V ISINK = 1.6 mA, VDD = 5 V
VOH Output High Voltage 4.0 V ISOURCE = 200 µA, VDD = 5 V
VOL Output Low Voltage 0.4 V ISINK = 100 µA, VDD = 3 V
VOH Output High Voltage VDD – 0.6 V ISOURCE = 100 µA, VDD = 3 V
POWER REQUIREMENTS
VDD-to-GND Voltage 4.75 5.25 V VDD = 5 V, nominal
2.7 3.6 V VDD = 3.3 V, nominal
IDD Current 850 µA Digital inputs equal to VDD or GND
750 µA VDD = 5 V
700 µA VDD = 3.3 V
IDD Current Power-Down Mode 0.5 2 µA Digital inputs equal to VDD or GND
1 Capacitance units: 1 pF = 10-12 F; 1 fF = 10-15 F; 1 aF = 10-18 F.
2 Specification is not production tested, but is supported by characterization data at initial product release.
3 Factory calibrated. The absolute error includes factory gain calibration error, integral nonlinearity error, and offset error after system offset calibration, all at 25°C. At
different temperatures, compensation for gain drift over temperature is required.
4 The capacitive input offset can be eliminated using a system offset calibration. The accuracy of the system offset calibration is limited by the offset calibration register
LSB size (32 aF) or by converter + system p-p noise during the system capacitive offset calibration, whichever is greater. To minimize the effect of the converter +
system noise, longer conversion times should be used for system capacitive offset calibration. The system capacitance offset calibration range is ±1 pF, the larger
offset can be removed using CAPDACs.
5 The gain error is factory calibrated at 25°C. At different temperatures, compensation for gain drift over temperature is required.
6 The CAPDAC resolution is seven bits in the actual CAPDAC full range. Using the on-chip offset calibration or adjusting the capacitive offset calibration register can
further reduce the CIN offset or the unchanging CIN component.
7 The VTCHOP bit in the VT SETUP register must be set to 1 for the specified temperature sensor and voltage input performance.
8 Using an external temperature sensing diode 2N3906, with nonideality factor nf = 1.008, connected as in Figure 41, with total serial resistance <100 Ω.
9 Full-scale error applies to both positive and negative full scale.
AD7745/AD7746
Rev. 0| Page 5 of 28
TIMING SPECIFICATIONS
VDD = 2.7 V to 3.6 V, or 4.75 V to 5.25 V; GND = 0 V; Input Logic 0 = 0 V; Input Logic 1 = VDD; –40°C to +125°C, unless otherwise noted.
Table 2.
Parameter Min Typ Max Unit Test Conditions/Comments
SERIAL INTERFACE1, 2 See Figure 3
SCL Frequency 0 400 kHz
SCL High Pulse Width, tHIGH 0.6 µs
SCL Low Pulse Width, tLOW 1.3 µs
SCL, SDA Rise Time, tR 0.3 µs
SCL, SDA Fall Time, tF 0.3 µs
Hold Time (Start Condition), tHD;STA 0.6 µs After this period, the first clock is generated
Set-Up Time (Start Condition), tSU;STA 0.6 µs Relevant for repeated start condition
Data Set-Up Time, tSU;DAT 0.25 µs VDD ≥ 3.0 V
Data Set-Up Time, tSU;DAT 0.35 µs VDD < 3.0 V
Set-Up Time (Stop Condition), tSU;STO 0.6 µs
Data Hold Time, tHD;DAT (Master) 0 µs
Bus-Free Time (Between Stop and Start Condition, tBUF) 1.3 µs
1 Sample tested during initial release to ensure compliance.
2 All input signals are specified with input rise/fall times = 3 ns, measured between the 10% and 90% points. Timing reference points at 50% for inputs and outputs.
Output load = 10 pF.
P
S
t
LOW
t
R
t
F
t
HD:STA
t
HD:DAT
t
SU:DAT
t
SU:STA
t
HD:STA
t
SU:STO
t
HIGH
SCL
PS
SD
A
t
BUF
05468-003
Figure 3. Serial Interface Timing Diagram
AD7745/AD7746
Rev. 0 | Page 6 of 28
ABSOLUTE MAXIMUM RATINGS
TA = 25°C, unless otherwise noted.
Table 3.
Parameter Rating
Positive Supply Voltage VDD to GND −0.3 V to +6.5 V
Voltage on any Input or Output Pin to
GND
–0.3 V to VDD + 0.3 V
ESD Rating (ESD Association Human Body
Model, S5.1)
2000 V
Operating Temperature Range –40°C to +125°C
Storage Temperature Range –65°C to +150°C
Junction Temperature 150°C
TSSOP Package θJA,
(Thermal Impedance-to-Air)
128°C/W
TSSOP Package θJC,
(Thermal Impedance-to-Case)
14°C/W
Lead Temperature, Soldering
Vapor Phase (60 sec) 215°C
Infrared (15 sec) 220°C
Stresses above those listed under Absolute Maximum Ratings
may cause permanent damage to the device. This is a stress
rating only and functional operation of the device at these or
any other conditions above those indicated in the operational
section of this specification is not implied. Exposure to absolute
maximum rating conditions for extended periods may affect
device reliability.
ESD CAUTION
ESD (electrostatic discharge) sensitive device. Electrostatic charges as high as 4000 V readily accumulate on the
human body and test equipment and can discharge without detection. Although this product features
proprietary ESD protection circuitry, permanent damage may occur on devices subjected to high energy
electrostatic discharges. Therefore, proper ESD precautions are recommended to avoid performance
degradation or loss of functionality.
AD7745/AD7746
Rev. 0| Page 7 of 28
PIN CONFIGURATIONS AND FUNCTION DESCRIPTIONS
16
15
14
13
12
11
10
9
1
2
3
4
5
6
7
8
AD7745
TOP VIEW
(Not to Scale)
SCL
EXCA
EXCB
REFIN(+)
REFIN(–)
CIN1(–)
CIN1(+)
RDY
SDA
VDD
GND
VIN(–)
VIN(+)
NC
NC
NC
NC = NO CONNECT
05468-004
Figure 4. AD7745 Pin Configuration (16-Lead TSSOP)
16
15
14
13
12
11
10
9
1
2
3
4
5
6
7
8
AD7746
TOP VIEW
(Not to Scale)
SCL
EXCA
EXCB
REFIN(+)
REFIN(–)
CIN1(–)
CIN1(+)
RDY
SDA
VDD
GND
VIN(–)
VIN(+)
CIN2(–)
CIN2(+)
NC
NC = NO CONNECT
05468-005
Figure 5. AD7746 Pin Configuration (16-Lead TSSOP)
Table 4. Pin Function Descriptions
Pin No. Mnemonic Description
1 SCL Serial Interface Clock Input. Connects to the master clock line. Requires pull-up resistor if not already
provided in the system.
2 RDY Logic Output. A falling edge on this output indicates that a conversion on enabled channel(s) has been
finished and the new data is available. Alternatively, the status register can be read via the 2-wire serial
interface and the relevant bit(s) decoded to query the finished conversion. If not used, this pin should be left
as an open circuit.
3, 4 EXCA, EXCB CDC Excitation Outputs. The measured capacitance is connected between one of the EXC pins and one of the
CIN pins. If not used, these pins should be left as an open circuit.
5, 6 REFIN(+),
REFIN(–)
Differential Voltage Reference Input for the Voltage Channel (ADC). Alternatively, the on-chip internal
reference can be used for the voltage channel. These reference input pins are not used for conversion on
capacitive channel(s) (CDC). If not used, these pins can be left as an open circuit or connected to GND.
7 CIN1(–)
CDC Negative Capacitive Input in Differential Mode. This pin is internally disconnected in single-ended CDC
configuration. If not used, this pin can be left as an open circuit or connected to GND.
8 CIN1(+)
CDC Capacitive Input (in Single-Ended Mode) or Positive Capacitive Input (in Differential Mode). The
measured capacitance is connected between one of the EXC pins and one of the CIN pins. If not used, this pin
can be left as an open circuit or connected to GND.
9, 10
(AD7745)
NC Not Connected. This pin should be left as an open circuit.
9
(AD7746)
CIN2(+) CDC Second Capacitive Input (in Single-Ended Mode) or Positive Capacitive Input (in Differential Mode). If not
used, this pin can be left open circuit or connected to GND.
10
(AD7746)
CIN2(–) CDC Negative Capacitive Input in Differential Mode. This pin is internally disconnected in a single-ended CDC
configuration. If not used, this pin can be left as an open circuit or connected to GND.
11, 12 VIN(+), VIN(–) Differential Voltage Input for the Voltage Channel (ADC). These pins are also used to connect an external
temperature sensing diode. If not used, these pins can be left as an open circuit or connected to GND.
13 GND Ground Pin.
14 VDD Power Supply Voltage. This pin should be decoupled to GND, using a low impedance capacitor, for example
in combination with a 10 µF tantalum and a 0.1 µF multilayer ceramic.
15 NC Not Connected. This pin should be left as an open circuit.
16 SDA Serial Interface Bidirectional Data. Connects to the master data line. Requires a pull-up resistor if not provided
elsewhere in the system.
AD7745/AD7746
Rev. 0 | Page 8 of 28
TYPICAL PERFORMANCE CHARACTERISTICS
100
0–5 5
05468-014
INPUT CAPACITANCE (pF)
INL (ppm)
80
60
40
20
–4 –3 –2 –1 0 1 2 3 4
Figure 6. Capacitance Input Integral Nonlinearity,
VDD = 5 V, the Same Configuration as in Figure 31
2000
–3000
–50 150
05468-015
TEMPERATURE (°C)
GAIN ERROR (ppm)
1000
0
–1000
–2000
–25 0 25 50 75 100 125
GAIN TC ≈–26ppm/°C
Figure 7. Capacitance Input Offset Drift vs. Temperature,
VDD = 5 V, CIN and EXC Pins Open Circuit
100
–100
–50 150
05468-016
TEMPERATURE (°C)
OFFSET ERROR (aF)
75
50
25
0
–25
–50
–75
–25 0 25 50 75 100 125
Figure 8. Capacitance Input Gain Drift vs. Temperature,
VDD = 5 V, CIN(+) to EXC = 4 pF, the Same Configuration as in Figure 30
18
–20 500
05468-017
CAPACITANCE CIN PIN TO GND (pF)
CAPACITANCE ERROR (fF)
16
14
12
10
8
6
4
2
0
50 100 150 200 250 300 350 400 450
2.7V 3V 5V3.3V
Figure 9. Capacitance Input Error vs. Capacitance between CIN and GND.
CIN(+) to EXC = 4 pF, CIN(−) to EXC = 0 pF, VDD = 2.7 V, 3 V, 3.3 V, and 5 V,
the Same Configuration as in Figure 33
18
–20 500
05468-018
CAPACITANCE CIN PIN TO GND (pF)
CAPACITANCE ERROR (fF)
16
14
12
10
8
6
4
2
0
50 100 150 200 250 300 350 400 450
2.7V 3V 3.3V
5V
Figure 10. Capacitance Input Error vs. Capacitance between CIN and GND,
CIN(+) to EXC = 21 pF, CIN(−) to EXC = 23 pF, VDD = 2.7 V, 3 V, 3.3 V, and 5 V,
the Same Configuration as in Figure 34
5
–10 500
05468-019
CAPACITANCE EXC PIN TO GND (pF)
CAPACITANCE ERROR (fF)
4
3
2
1
0
50 100 150 200 250 300 350 400 450
2.7V 3V
5V
3.3V
Figure 11. Capacitance Input Error vs. Capacitance between EXC and GND,
CIN(+) to EXC = 21 pF, CIN(−) to EXC = 23 pF, VDD = 2.7 V, 3 V, 3.3 V, and 5 V,
the Same Configuration as in Figure 34
AD7745/AD7746
Rev. 0| Page 9 of 28
8
–12
–250 250
05468-028
CIN LEAKAGE TO GND (nA)
CAPACITANCE ERROR (fF)
2.7V
3V
6
4
2
0
–2
–4
–6
–8
–10
–200 –150 –100 –50 0 50 100 150 200
Figure 12. Capacitance Input Error vs. Leakage Current to GND,
CIN(+) to EXC = 4 pF, CIN(−) to EXC = 0 pF,
VDD = 2.7 V and 3 V
8
–12
–250 250
05468-030
CIN LEAKAGE TO GND (nA)
CAPACITANCE ERROR (fF)
6
4
2
0
–2
–4
–6
–8
–10
–200 –150 –100 –50 0 50 100 150 200
3.3V
5V
Figure 13. Capacitance Input Error vs. Leakage Current to GND,
CIN(+) to EXC =4 pF, CIN(−) to EXC = 0 pF,
VDD=3.3 V and 5 V
10
0.00011 100000
05468-029
PARALLEL RESISTANCE (MΩ)
CAPACITANCE ERROR (pF)
1
0.1
0.01
0.001
10 100 1000 10000
Figure 14. Capacitance Input Error vs. Resistance in Parallel
with Measured Capacitance
0
–1007
05468-031
SERIAL RESISTANCE (kΩ)
CAPACITANCE ERROR (fF)
–2
–4
–6
–8
123456
Figure 15. Capacitance Input Error vs. Serial Resistance,
CIN(+) to EXC = 21 pF, CIN(−) to EXC = 23 pF, VDD = 5 V,
the Same Configuration as in Figure 34.
0.2
–1.0
2.5 5.5
05468-032
V
DD
(V)
CAPACITANCE ERROR (fF)
0
–0.2
–0.4
–0.6
–0.8
3.0 3.5 4.0 4.5 5.0
Figure 16. Capacitance Input Power Supply Rejection (PSR),
CIN(+) to EXC = 4 pF, the Same Configuration as in Figure 30
0.20
–0.200 128
05468-033
CAPDAC CODE
CAPDAC CODE DNL (pF)
0.15
0.10
0.05
0
–0.05
–0.10
–0.15
16 32 48 64 80 96 112
Figure 17. CAPDAC Differential Nonlinearity (DNL)
AD7745/AD7746
Rev. 0 | Page 10 of 28
2.0
–2.0
–50 150
05468-034
TEMPERATURE (°C)
ERROR (°C)
1.5
1.0
0.5
0
–0.5
–1.0
–1.5
–25 0 25 50 75 100 125
Figure 18. Internal Temperature Sensor Error vs. Temperature
1.0
–3.0
–50 150
05468-035
TEMPERATURE (°C)
ERROR (°C)
0.5
0
–0.5
–1.0
–1.5
–2.0
–2.5
–25 0 25 50 75 100 125
Figure 19. External Temperature Sensor Error vs. Temperature
0
–1200 1000
05468-036
INPUT SIGNAL FREQUENCY (Hz)
GAIN (dB)
–20
–40
–60
–80
–100
100 200 300 400 500 600 700 800 900
Figure 20. Capacitance Channel Frequency Response,
Conversion Time = 11 ms
0
–1200 400
05468-037
INPUT SIGNAL FREQUENCY (Hz)
GAIN (dB)
–20
–40
–60
–80
–100
50 100 150 200 250 300 350
Figure 21. Capacitance Channel Frequency Response,
Conversion Time = 62 ms
0
–1200 400
05468-038
INPUT SIGNAL FREQUENCY (Hz)
GAIN (dB)
–20
–40
–60
–80
–100
50 100 150 200 250 300 350
Figure 22. Capacitance Channel Frequency Response,
Conversion Time = 109.6 ms
0
–1200 400
05468-039
INPUT SIGNAL FREQUENCY (Hz)
GAIN (dB)
–20
–40
–60
–80
–100
50 100 150 200 250 300 350
Figure 23. Voltage Channel Frequency Response,
Conversion Time = 122.1 ms
AD7745/AD7746
Rev. 0| Page 11 of 28
OUTPUT NOISE AND RESOLUTION SPECIFICATIONS
The AD7745/AD7746 resolution is limited by noise. The noise
performance varies with the selected conversion time.
Table 5 shows typical noise performance and resolution for the
capacitive channel. These numbers were generated from 1000
data samples acquired in continuous conversion mode, at an
excitation of 32 kHz, ±VDD/2, and with all CIN and EXC pins
connected only to the evaluation board (no external capacitors.)
Table 6 and Table 7 show typical noise performance and
resolution for the voltage channel. These numbers were
generated from 1000 data samples acquired in continuous
conversion mode with VIN pins shorted to ground.
RMS noise represents the standard deviation and p-p noise
represents the difference between minimum and maximum
results in the data. Effective resolution is calculated from rms
noise, and p-p resolution is calculated from p-p noise.
Table 5. Typical Capacitive Input Noise and Resolution vs. Conversion Time
Conversion
Time (ms)
Output Data
Rate (Hz)
–3dB Frequency
(Hz)
RMS Noise
(aF/√Hz)
RMS
Noise (aF)
P-P
Noise (aF)
Effective Resolution
(Bits)
P-P Resolution
(Bits)
11.0 90.9 87.2 4.3 40.0 212.4 17.6 15.2
11.9 83.8 79.0 3.1 27.3 137.7 18.2 15.9
20.0 50.0 43.6 1.8 12.2 82.5 19.4 16.6
38.0 26.3 21.8 1.6 7.3 50.3 20.1 17.3
62.0 16.1 13.8 1.5 5.4 33.7 20.5 17.9
77.0 13.0 10.5 1.5 4.9 28.3 20.7 18.1
92.0 10.9 8.9 1.5 4.4 27.8 20.8 18.2
109.6 9.1 8.0 1.5 4.2 27.3 20.9 18.2
Table 6. Typical Voltage Input Noise and Resolution vs. Conversion Time, Internal Voltage Reference
Conversion
Time (ms)
Output Data
Rate (Hz)
–3dB Frequency
(Hz)
RMS Noise
(µV)
P-P Noise
(µV)
Effective Resolution
(Bits)
P-P Resolution
(Bits)
20.1 49.8 26.4 11.4 62 17.6 15.2
32.1 31.2 15.9 7.1 42 18.3 15.7
62.1 16.1 8.0 4.0 28 19.1 16.3
122.1 8.2 4.0 3.0 20 19.5 16.8
Table 7. Typical Voltage Input Noise and Resolution vs. Conversion Time, External 2.5 V Voltage Reference
Conversion
Time (ms)
Output Data
Rate (Hz)
–3dB Frequency
(Hz)
RMS Noise
(µV)
P-P Noise
(µV)
Effective Resolution
(Bits)
P-P Resolution
(Bits)
20.1 49.8 26.4 14.9 95 18.3 15.6
32.1 31.2 15.9 6.3 42 19.6 16.8
62.1 16.1 8.0 3.3 22 20.5 17.7
122.1 8.2 4.0 2.1 15 21.1 18.3
AD7745/AD7746
Rev. 0 | Page 12 of 28
SERIAL INTERFACE
The AD7745/AD7746 supports an I2C-compatible 2-wire serial
interface. The two wires on the I2C bus are called SCL (clock)
and SDA (data). These two wires carry all addressing, control,
and data information one bit at a time over the bus to all
connected peripheral devices. The SDA wire carries the data,
while the SCL wire synchronizes the sender and receiver during
the data transfer. I2C devices are classified as either master or
slave devices. A device that initiates a data transfer message is
called a master, while a device that responds to this message is
called a slave.
To control the AD7745/AD7746 device on the bus, the
following protocol must be followed. First, the master initiates a
data transfer by establishing a start condition, defined by a
high-to-low transition on SDA while SCL remains high. This
indicates that the start byte follows. This 8-bit start byte is made
up of a 7-bit address plus an R/W bit indicator.
All peripherals connected to the bus respond to the start
condition and shift in the next 8 bits (7-bit address + R/W bit).
The bits arrive MSB first. The peripheral that recognizes the
transmitted address responds by pulling the data line low
during the ninth clock pulse. This is known as the acknowledge
bit. All other devices withdraw from the bus at this point and
maintain an idle condition. An exception to this is the general
call address, which is described later in this document. The idle
condition is where the device monitors the SDA and SCL lines
waiting for the start condition and the correct address byte. The
R/W bit determines the direction of the data transfer. A Logic 0
LSB in the start byte means that the master writes information
to the addressed peripheral. In this case the AD7745/AD7746
becomes a slave receiver. A Logic 1 LSB in the start byte means
that the master reads information from the addressed peri-
pheral. In this case, the AD7745/AD7746 becomes a slave
transmitter. In all instances, the AD7745/AD7746 acts as a
standard slave device on the I2C bus.
The start byte address for the AD7745/AD7746 is 0x90 for a
write and 0x91 for a read.
READ OPERATION
When a read is selected in the start byte, the register that is
currently addressed by the address pointer is transmitted on to
the SDA line by the AD7745/AD7746. This is then clocked out
by the master device and the AD7745/AD7746 awaits an
acknowledge from the master.
If an acknowledge is received from the master, the address auto-
incrementer automatically increments the address pointer
register and outputs the next addressed register content on to
the SDA line for transmission to the master. If no acknowledge
is received, the AD7745/AD7746 return to the idle state and the
address pointer is not incremented.
The address pointers’ auto-incrementer allow block data to be
written or read from the starting address and subsequent
incremental addresses.
In continuous conversion mode, the address pointers’ auto-
incrementer should be used for reading a conversion result.
That means, the three data bytes should be read using one
multibyte read transaction rather than three separate single byte
transactions. The single byte data read transaction may result in
the data bytes from two different results being mixed. The same
applies for six data bytes if both the capacitive and the
voltage/temperature channel are enabled.
The user can also access any unique register (address) on a one-
to-one basis without having to update all the registers. The
address pointer register contents cannot be read.
If an incorrect address pointer location is accessed or, if the user
allows the auto-incrementer to exceed the required register
address, the following applies:
• In read mode, the AD7745/AD7746 continues to output
various internal register contents until the master device
issues a no acknowledge, start, or stop condition. The
address pointers’ auto-incrementer’s contents are reset to
point to the status register at Address 0x00 when a stop
condition is received at the end of a read operation. This
allows the status register to be read (polled) continually
without having to constantly write to the address pointer.
• In write mode, the data for the invalid address is not loaded
into the AD7745/AD7746 registers but an acknowledge is
issued by the AD7745/AD7746.
WRITE OPERATION
When a write is selected, the byte following the start byte is
always the register address pointer (subaddress) byte, which
points to one of the internal registers on the AD7745/ AD7746.
The address pointer byte is automatically loaded into the
address pointer register and acknowledged by the AD7745/
AD7746. After the address pointer byte acknowledge, a stop
condition, a repeated start condition, or another data byte can
follow from the master.
A stop condition is defined by a low-to-high transition on SDA
while SCL remains high. If a stop condition is ever encountered
by the AD7745/AD7746, it returns to its idle condition and the
address pointer is reset to Address 0x00.
If a data byte is transmitted after the register address pointer
byte, the AD7745/AD7746 load this byte into the register that is
currently addressed by the address pointer register, send an
acknowledge, and the address pointer auto-incrementer auto-
matically increments the address pointer register to the next
internal register address. Thus, subsequent transmitted data
bytes are loaded into sequentially incremented addresses.
AD7745/AD7746
Rev. 0| Page 13 of 28
If a repeated start condition is encountered after the address
pointer byte, all peripherals connected to the bus respond
exactly as outlined above for a start condition, that is, a repeated
start condition is treated the same as a start condition. When a
master device issues a stop condition, it relinquishes control of
the bus, allowing another master device to take control of the
bus. Hence, a master wanting to retain control of the bus issues
successive start conditions known as repeated start conditions.
AD7745/AD7746 RESET
To reset the AD7745/AD7746 without having to reset the entire
I2C bus, an explicit reset command is provided. This uses a
particular address pointer word as a command word to reset the
part and upload all default settings. The AD7745/AD7746 do
not respond to the I2C bus commands (do not acknowledge)
during the default values upload for approximately 150 µs
(max 200 µs).
The reset command address word is 0xBF.
GENERAL CALL
When a master issues a slave address consisting of seven 0s with
the eighth bit (R/W bit) set to 0, this is known as the general call
address. The general call address is for addressing every device
connected to the I2C bus. The AD7745/AD7746 acknowledge
this address and read in the following data byte.
If the second byte is 0x06, the AD7745/AD7746 are reset,
completely uploading all default values. The AD7745/AD7746
do not respond to the I2C bus commands (do not acknowledge)
during the default values upload for approximately 150 µs (max
200 µs).
The AD7745/AD7746 do not acknowledge any other general
call commands.
1–7 8 9 1
–7 8 9 1–7 8 9 PS
START ADDR R/W ACK SUBADDRESS ACK DATA ACK STOP
SDATA
SCLOCK
05468-006
Figure 24. Bus Data Transfer
DATA A(S)S SLAVE ADDR A(S) SUB ADDR A(S)
LSB = 0 LSB = 1
DATA P
S SLAVE ADDR A(S) SUB ADDR A(S) S SLAVE ADDR A(S) DATA A(M) DATA P
WRITE
SEQUENCE
READ
SEQUENCE A(S) = NO-ACKNOWLEDGE BY SLAVE
A(M) = NO-ACKNOWLEDGE BY MASTER
A(S) = ACKNOWLEDGE BY SLAVE
A(M) = ACKNOWLEDGE BY MASTER
S = START BIT
P = STOP BIT
A(S)
A(M)
05468-007
Figure 25. Write and Read Sequences
AD7745/AD7746
Rev. 0 | Page 14 of 28
REGISTER DESCRIPTIONS
The master can write to or read from all of the AD7745/
AD7746 registers except the address pointer register, which is a
write-only register. The address pointer register determines
which register the next read or write operation accesses. All
communications with the part through the bus start with an
access to the address pointer register. After the part has been
accessed over the bus and a read/write operation is selected, the
address pointer register is set up. The address pointer register
determines from or to which register the operation takes place.
A read/write operation is performed from/to the target address,
which then increments to the next address until a stop
command on the bus is performed.
Table 8. Register Summary
Address
Pointer Bit 7 Bit 6 Bit 5 Bit 4 Bit 3 Bit 2 Bit 1 Bit 0
Register (Dec) (Hex) Dir Default Value
- - - - EXCERR RDY RDYVT RDYCAP
Status 0 0x00 R
0 0 0 0 0 1 1 1
Cap Data H 1 0x01 R Capacitive channel data—high byte, 0x00
Cap Data M 2 0x02 R Capacitive channel data—middle byte, 0x00
Cap Data L 3 0x03 R Capacitive channel data—low byte, 0x00
VT Data H 4 0x04 R Voltage/temperature channel data—high byte, 0x00
VT Data M 5 0x05 R Voltage/temperature channel data—middle byte, 0x00
VT Data L 6 0x06 R Voltage/temperature channel data—low byte, 0x00
CAPEN CIN21CAPDIFF - - - - CAPCHOP
Cap Setup 7 0x07 R/W 0 0 0 0 0 0 0 0
VTEN VTMD1 VTMD0 EXTREF - - VTSHORT VTCHOP
VT Setup 8 0x08 R/W 0 0 0 0 0 0 0 0
CLKCTRL EXCON EXCB EXCB EXCA EXCA EXCLVL1 EXCLVL0
EXC Setup 9 0x09 R/W 0 0 0 0 0 0 1 1
VTFS1 VTFS0 CAPFS2 CAPFS1 CAPFS0 MD2 MD1 MD0
Configuration 10 0x0A R/W
1 0 1 0 0 0 0 0
DACAENA DACA—7-Bit Value
Cap DAC A 11 0x0B R/W 0 0x00
DACBENB DACB—7-Bit Value
Cap DAC B 12 0x0C R/W 0 0x00
Cap Offset H 13 0x0D R/W Capacitive offset calibration—high byte, 0x80
Cap Offset L 14 0x0E R/W Capacitive offset calibration—low byte, 0x00
Cap Gain H 15 0x0F R/W Capacitive gain calibration—high byte, factory calibrated
Cap Gain L 16 0x10 R/W Capacitive gain calibration—low byte, factory calibrated
Volt Gain H 17 0x11 R/W Voltage gain calibration—high byte, factory calibrated
Volt Gain L 18 0x12 R/W Voltage gain calibration—low byte, factory calibrated
1 The CIN2 bit is relevant only for AD7746. The CIN2 bit should always be 0 on the AD7745.
AD7745/AD7746
Rev. 0| Page 15 of 28
STATUS REGISTER
Address Pointer 0x00, Read Only, Default Value 0x07
This register indicates the status of the converter. The status
register can be read via the 2-wire serial interface to query a
finished conversion.
The RDY pin reflects the status of the RDY bit. Therefore, the
RDY pin high-to-low transition can be used as an alternative
indication of the finished conversion.
Table 9. Status Register Bit Map
Bit Bit 7 Bit 6 Bit 5 Bit 4 Bit 3 Bit 2 Bit 1 Bit 0
Mnemonic - - - - EXCERR RDY RDYVT RDYCAP
Default 0 0 0 0 0 1 1 1
Table 10.
Bit Mnemonic Description
7-4 - Not used, always read 0.
3 EXCERR EXCERR = 1 indicates that the excitation output cannot be driven properly.
The possible reason can be a short circuit or too high capacitance between the excitation pin and ground.
2 RDY RDY = 0 indicates that conversion on the enabled channel(s) has been finished and new unread data is
available.
If both capacitive and voltage/temperature channels are enabled, the RDY bit is changed to 0 after conversion
on both channels is finished. The RDY bit returns to 1 either when data is read or prior to finishing the next
conversion.
If, for example, only the capacitive channel is enabled, then the RDY bit reflects the RDYCAP bit.
1 RDYVT RDYVT = 0 indicates that a conversion on the voltage/temperature channel has been finished and new unread
data is available.
0 RDYCAP RDYCAP = 0 indicates that a conversion on the capacitive channel has been finished and new unread data is
available.
CAP DATA REGISTER
24 Bits, Address Pointer 0x01, 0x02, 0x03, Read-Only,
Default Value 0x000000
Capacitive channel output data. The register is updated after
finished conversion on the capacitive channel, with one
exception: When the serial interface read operation from the
CAP DATA register is in progress, the data register is not
updated and the new capacitance conversion result is lost.
The stop condition on the serial interface is considered to be the
end of the read operation. Therefore, to prevent data corruption,
all three bytes of the data register should be read sequentially
using the register address pointer auto-increment feature of the
serial interface.
To prevent losing some of the results, the CAP DATA register
should be read before the next conversion on the capacitive
channel is finished.
The 0x000000 code represents negative full scale (–4.096 pF),
the 0x800000 code represents zero scale (0 pF), and the
0xFFFFFF code represents positive full scale (+4.096 pF).
VT DATA REGISTER
24 Bits, Address Pointer 0x04, 0x05, 0x06, Read-Only,
Default Value 0x000000
Voltage/temperature channel output data. The register is
updated after finished conversion on the voltage channel or
temperature channel, with one exception: When the serial
interface read operation from the VT DATA register is in
progress, the data register is not updated and the new
voltage/temperature conversion result is lost.
The stop condition on the serial interface is considered to be the
end of the read operation. Therefore, to prevent data corruption,
all three bytes of the data register should be read sequentially
using the register address pointer auto-increment feature of the
serial interface.
For voltage input, Code 0 represents negative full scale (–VREF),
the 0x800000 code represents zero scale (0 V), and the
0xFFFFFF code represents positive full scale (+VREF).
To prevent losing some of the results, the VT DATA register
should be read before the next conversion on the voltage/
temperature channel is finished.
For the temperature sensor, the temperature can be calculated
from code using the following equation:
Temperature (°C) = (Code/2048) − 4096
AD7745/AD7746
Rev. 0 | Page 16 of 28
CAP SET-UP REGISTER
Address Pointer 0x07, Default Value 0x00
Capacitive channel setup.
Table 11. CAP Set-Up Register Bit Map
Bit Bit 7 Bit 6 Bit 5 Bit 4 Bit 3 Bit 2 Bit 1 Bit 0
Mnemonic CAPEN CIN2 CAPDIFF - - - - CAPCHOP
Default 0 0 0 0 0 0 0 0
Table 12.
Bit Mnemonic Description
7 CAPEN CAPEN = 1 enables capacitive channel for single conversion, continuous conversion, or calibration.
6 CIN2 CIN2 = 1 switches the internal multiplexer to the second capacitive input on the AD7746.
5 CAPDIFF DIFF = 1 sets differential mode on the selected capacitive input.
4-1 - These bits must be 0 for proper operation.
0 CAPCHOP The CAPCHOP bit should be set to 0 for the specified capacitive channel performance.
CAPCHOP = 1 approximately doubles the capacitive channel conversion times and slightly improves the
capacitive channel noise performance for the longest conversion times.
VT SET-UP REGISTER
Address Pointer 0x08, Default Value 0x00
Voltage/Temperature channel setup.
Table 13. VT Set-Up Register Bit Map
Bit Bit 7 Bit 6 Bit 5 Bit 4 Bit 3 Bit 2 Bit 1 Bit 0
Mnemonic VTEN VTMD1 VTMD0 EXTREF - - VTSHORT VTCHOP
Default 0 0 0 0 0 0 0 0
Table 14.
Bit Mnemonic Description
7 VTEN VTEN = 1 enables voltage/temperature channel for single conversion, continuous conversion, or calibration.
Voltage/temperature channel input configuration.
VTMD1 VTMD0 Channel Input
0 0 Internal temperature sensor
0 1 External temperature sensor diode
1 0 VDD monitor
6
5
VTMD1
VTMD0
1 1 External voltage input (VIN)
4 EXTREF EXTREF = 1 selects an external reference voltage connected to REFIN(+), REFIN(–) for the voltage input or the
VDD monitor.
EXTREF = 0 selects the on-chip internal reference. The internal reference must be used with the internal
temperature sensor for proper operation.
3-2 - These bits must be 0 for proper operation.
1 VTSHORT VTSHORT = 1 internally shorts the voltage/temperature channel input for test purposes.
0 VTCHOP = 1 VTCHOP = 1 sets internal chopping on the voltage/temperature channel.
The VTCHOP bit must be set to 1 for the specified voltage/temperature channel performance.
AD7745/AD7746
Rev. 0| Page 17 of 28
EXC SET-UP REGISTER
Address Pointer 0x09, Default Value 0x03
Capacitive channel excitation setup.
Table 15. EXC Set-Up Bit Map
Bit Bit 7 Bit 6 Bit 5 Bit 4 Bit 3 Bit 2 Bit 1 Bit 0
Mnemonic CLKCTRL EXCON EXCB
EXCB EXCA EXCA EXCLVL1 EXCLVL0
Default 0 0 0 0 0 0 0 0
Table 16.
Bit Mnemonic Description
7 CLKCTRL The CLKCTRL bit should be set to 0 for the specified AD7745/AD7746 performance.
CLKCTRL = 1 decreases the excitation signal frequency and the modulator clock frequency by factor of 2.
This also increases the conversion time on all channels (capacitive, voltage, and temperature) by a factor of 2.
6 EXCON When EXCON = 0, the excitation signal is present on the output only during capacitance channel conversion.
When EXCON = 1, the excitation signal is present on the output during both capacitance and
voltage/temperature conversion.
5 EXCB EXCB = 1 enables EXCB pin as the excitation output.
4 EXCB EXCB = 1 enables EXCB pin as the inverted excitation output.
Only one of the EXCB or the EXCB bits should be set for proper operation.
3 EXCA EXCA = 1 enables EXCA pin as the excitation output.
2 EXCA EXCA = 1 enables EXCA pin as the inverted excitation output.
Only one of the EXCA or the EXCA bits should be set for proper operation.
Excitation Voltage Level.
EXCLVL1 EXCLVL0 Voltage on Cap EXC Pin Low Level EXC Pin High Level
0 0 ±VDD/8 VDD × 3/8 VDD × 5/8
0 1 ±VDD/4 VDD × 1/4 VDD × 3/4
1 0 ±VDD × 3/8 VDD × 1/8 VDD × 7/8
1
0
EXCLVL1,
EXCLVL0
1 1 ±VDD/2 0 VDD
AD7745/AD7746
Rev. 0 | Page 18 of 28
CONFIGURATION REGISTER
Address Pointer 0x0A, Default Value 0xA0
Converter update rate and mode of operation setup.
Table 17. Configuration Register Bit Map
Bit Bit 7 Bit 6 Bit 5 Bit 4 Bit 3 Bit 2 Bit 1 Bit 0
Mnemonic VTF1 VTF0 CAPF2 CAPF1 CAPF0 MD2 MD1 MD0
Default 0 0 0 0 0 0 0 0
Table 18.
Bit Mnemonic Description
Voltage/temperature channel digital filter setup—conversion time/update rate setup.
The conversion times in this table are valid for the CLKCTRL = 0 in the EXC SETUP register. The conversion
times are longer by a factor of two for the CLKCTRL = 1.
VTCHOP = 1
VTF1 VTF0 Conversion Time (ms) Update Rate (Hz) –3 dB Frequency (Hz)
0 0 20.1 49.8 26.4
0 1 32.1 31.2 15.9
1 0 62.1 16.1 8.0
7
6
VTF1
VTF0
1 1 122.1 8.2 4.0
Capacitive channel digital filter setup—conversion time/update rate setup.
The conversion times in this table are valid for the CLKCTRL = 0 in the EXC SETUP register.
The conversion times are longer by factor of two for the CLKCTRL = 1.
CAP CHOP = 0
CAPF2 CAPF1 CAPF0 Conversion Time (ms) Update Rate –3 dB Frequency (Hz)
0 0 0 11.0 90.9 87.2
0 0 1 11.9 83.8 79.0
0 1 0 20.0 50.0 43.6
0 1 1 38.0 26.3 21.8
1 0 0 62.0 16.1 13.1
1 0 1 77.0 13.0 10.5
1 1 0 92.0 10.9 8.9
5
4
3
CAPF2
CAPF1
CAPF0
1 1 1 109.6 9.1 8.0
Converter mode of operation setup.
MD2 MD1 MD0 Mode
0 0 0 Idle
0 0 1 Continuous conversion
0 1 0 Single conversion
0 1 1 Power-Down
1 0 0 -
1 0 1 Capacitance system offset calibration
1 1 0 Capacitance or voltage system gain calibration
2
1
0
MD2
MD1
MD0
1 1 1
AD7745/AD7746
Rev. 0| Page 19 of 28
CAP DAC A REGISTER
Address Pointer 0x0B, Default Value 0x00
Capacitive DAC setup.
Table 19. Cap DAC A Register Bit Map
Bit Bit 7 Bit 6 Bit 5 Bit 4 Bit 3 Bit 2 Bit 1 Bit 0
Mnemonic DACAENA DACA—7-Bit Value
Default 0 0x00
Table 20.
Bit Mnemonic Description
7 DACAENA DACAENA = 1 connects capacitive DACA to the positive capacitance input.
6-1 DACA DACA value, Code 0x00 ≈ 0 pF, Code 0x7F ≈ full range.
CAP DAC B REGISTER
Address Pointer 0x0C, Default Value 0x00
Capacitive DAC setup.
Table 21. Cap DAC B Register Bit Map
Bit Bit 7 Bit 6 Bit 5 Bit 4 Bit 3 Bit 2 Bit 1 Bit 0
Mnemonic DACBENB DACB—7-bit value
Default 0 0x00
Table 22.
Bit Mnemonic Description
7 DACBENB DACBENB = 1 connects capacitive DACB to the negative capacitance input.
6-1 DACB DACB value, Code 0x00 ≈ 0 pF, Code 0x7F ≈ full range.
CAP OFFSET CALIBRATION REGISTER
16 Bits, Address Pointer 0x0D, 0x0E,
Default Value 0x8000
The capacitive offset calibration register holds the capacitive
channel zero-scale calibration coefficient. The coefficient is
used to digitally remove the capacitive channel offset. The
register value is updated automatically following the execution
of a capacitance offset calibration. The capacitive offset calibra-
tion resolution (cap offset register LSB) is less than 32 aF; the
full range is 1 pF.
On the AD7746, the register is shared by the two capacitive
channels. If the capacitive channels need to be offset-calibrated
individually, the host controller software should read the
AD7746 capacitive offset calibration register values after
performing the offset calibration on individual channels and
then reload the values back to the AD7746 before executing
conversion on a different channel.
CAP GAIN CALIBRATION REGISTER
16 Bits, Address Pointer 0x0F, 0x10,
Default Value 0xXXXX
Capacitive gain calibration register. The register holds the
capacitive channel full-scale factory calibration coefficient.
On the AD7746, the register is shared by the two capacitive
channels.
VOLT GAIN CALIBRATION REGISTER
16 Bits, Address Pointer 0x11,0x12,
Default Value 0xXXXX
Voltage gain calibration register. The register holds the voltage
channel full-scale factory calibration coefficient.
AD7745/AD7746
Rev. 0 | Page 20 of 28
CIRCUIT DESCRIPTION
DIGITAL
FILTER
24-BIT Σ-∆
MODULATOR
CLOCK
GENERATOR
MUX
TEMP
SENSOR
CAP DAC
CAP DAC VOLTAGE
REFERENCE
CONTROL LOGIC
CALIBRATION
AD7745
VDD
REFIN(+) REFIN(–) GND
SDA
SCL
RDY
VIN(+)
VIN(–)
CIN1(+)
CIN1(–)
EXCA
EXCB
05468-001
I
2
C
SERIAL
INTERFACE
EXCITATION
Figure 26. AD7745 Block Diagram
DIGITAL
FILTER
24-BIT Σ-∆
MODULATOR
MUX
TEMP
SENSOR
CAP DAC
CAP DAC VOLTAGE
REFERENCE
CONTROL LOGIC
CALIBRATION
AD7746
VDD
REFIN(+) REFIN(–) GND
SDA
SCL
RDY
VIN(+)
VIN(–)
CIN1(+)
CIN1(–)
CIN2(+)
CIN2(–)
EXC1
EXC2
05468-002
CLOCK
GENERATOR
I
2
C
SERIAL
INTERFACE
EXCITATION
Figure 27. AD7746 Block Diagram
OVERVIEW
The AD7745/AD7746 core is a high precision converter con-
sisting of a second order (Σ-Δ or charge balancing) modulator
and a third order digital filter. It works as a CDC for the capa-
citive inputs and as a classic ADC for the voltage input or for
the voltage from a temperature sensor.
In addition to the converter, the AD7745/AD7746 integrates a
multiplexer, an excitation source and CAPDACs for the capa-
citive inputs, a temperature sensor, a voltage reference for the
voltage and temperature inputs, a complete clock generator, a
control and calibration logic, and an I2C-compatible serial
interface.
The AD7745 has one capacitive input, while the AD7746 has
two capacitive inputs. All other features and specifications are
identical for both parts.
CAPACITANCE-TO-DIGITAL CONVERTER
Figure 28 shows the CDC simplified functional diagram. The
measured capacitance CX is connected between the excitation
source and the Σ-Δ modulator input. A square-wave excitation
signal is applied on the CX during the conversion and the
modulator continuously samples the charge going through the
CX. The digital filter processes the modulator output, which is a
stream of 0s and 1s containing the information in 0 and 1
density. The data from the digital filter is scaled, applying the
calibration coefficients, and the final result can be read through
the serial interface.
The AD7745/AD7746 is designed for floating capacitive
sensors. Therefore, both CX plates have to be isolated from
ground.
DIGITAL
FILTER
24-BIT Σ-∆
MODULATOR
CLOCK
GENERATOR
CAPACITANCE TO DIGITAL CONVERTER
(CDC)
05468-027
EXCITATION
DATA
EXC
CIN
C
X
Figure 28. CDC Simplified Block Diagram
EXCITATION SOURCE
The two excitation pins EXCA and EXCB are independently
programmable. They are identically functional and therefore
either of them can be used for the capacitive sensor excitation.
On the 2-channel AD7746 using a separate excitation pin for
each capacitive channel is recommended.
AD7745/AD7746
Rev. 0| Page 21 of 28
CAPDAC
The AD7745/AD7746 CDC full-scale input range is ±4.096 pF.
For simplicity of calculation, however, the following text and
diagrams use ±4 pF. The part can accept a higher capacitance
on the input and the common-mode or offset (not-changing
component) capacitance can be balanced by programmable
on-chip CAPDACs.
DATA
CDC
EXC
CIN(+)
CIN(–)
C
X
C
Y
CAPDAC(+)
CAPDAC(–)
05468-010
Figure 29. Using a CAPDAC
The CAPDAC can be understood as a negative capacitance
connected internally to the CIN pin. There are two independent
CAPDACs, one connected to the CIN(+) and the second
connected to the CIN(–). The relation between the capacitance
input and output data can be expressed as
(
)
(
)
)()(
−
−−+−≈ CAPDACCCAPDACCDATA YX
The CAPDACs have a 7-bit resolution, monotonic transfer
function, are well matched to each other, and have a defined
temperature coefficient. The CAPDAC full range (absolute
value) is not factory calibrated and can vary up to ±20% with
the manufacturing process. See the Specifications section and
typical performance characteristics in Figure 17.
The CAPDACs are shared by the two capacitive channels on the
AD7746. If the CAPDACs need to be set individually, the host
controller software should reload the CAPDAC values to the
AD7746 before executing conversion on a different channel.
SINGLE-ENDED CAPACITIVE INPUT
When configured for a single-ended mode (the CAPDIFF bit in
the Cap Setup register is set to 0), the AD7745/AD7746 CIN(–)
pin is disconnected internally. The CDC (without using the
CAPDACs) can measure only positive input capacitance in the
range of 0 pF to 4 pF (see Figure 30).
0x800000 ... 0xFFFFFF
DATA
CAPDIFF = 0 0 ... 4pF
CDC
EXC
CIN(+)
CIN(–)
CX
0 ... 4pF
CAPDAC(+)
OFF
CAPDAC(–)
OFF
05468-024
Figure 30. CDC Single-Ended Input Mode
The CAPDAC can be used for programmable shifting the input
range. The example in Figure 31 shows how to use the full
±4 pF CDC span to measure capacitance between 0 pF to 8 pF.
0x000000 ... 0xFFFFFF
DATA
CAPDIFF = 0 ±4pF
CDC
EXC
CIN(+)
CIN(–)
C
X
0 ... 8pF
CAPDAC(+)
4pF
CAPDAC(–)
0pF
05468-025
Figure 31. Using CAPDAC in Single-Ended Mode
Figure 32 shows how to shift the input range further, up to
21 pF absolute value of capacitance connected to the CIN(+).
0x000000 ... 0xFFFFFF
DATA
CAPDIFF = 0 ±4pF
CDC
EXC
CIN(+)
CIN(–)
C
X
13 ... 21pF
(17 ±4pF)
CAPDAC(+)
17pF
CAPDAC(–)
0pF
05468-026
Figure 32. Using CAPDAC in Single-Ended Mode
DIFFERENTIAL CAPACITIVE INPUT
When configured for a differential mode (the CAPDIFF bit in
the Cap Setup register set to 1), the AD7745/AD7746 CDC
measures the difference between positive and negative
capacitance input.
Each of the two input capacitances CX and CY between the EXC
and CIN pins must be less than 4 pF (without using the
CAPDACs) or must be less than 21 pF and balanced by the
CAPDACs. Balancing by the CAPDACs means that both
CX–CAPDAC(+) and CY–CAPDAC(–) are less than 4 pF.
If the unbalanced capacitance between the EXC and CIN pins is
higher than 4 pF, the CDC introduces a gain error, an offset
error, and nonlinearity error.
See the examples shown in Figure 33, Figure 34, and Figure 35.
0x000000 ... 0xFFFFFF
DATA
CAPDIFF = 1 ±4pF
CDC
EXC
CIN(+)
CIN(–)
C
X
0 ... 4pF C
Y
0 ... 4pF
CAPDAC(+)
OFF
CAPDAC(–)
OFF
05468-020
Figure 33. CDC Differential Input Mode
AD7745/AD7746
Rev. 0 | Page 22 of 28
0x000000 ... 0xFFFFFF
DATA
CAPDIFF = 1 ±4pF
CDC
EXC
CIN(+)
CIN(–)
C
X
15 ... 19pF
(17 ±2pF)
C
Y
15 ... 19pF
(17 ±2pF)
CAPDAC(+)
17pF
CAPDAC(–)
17pF
05468-021
Figure 34. Using CAPDAC in Differential Mode
0x000000 ... 0xFFFFFF
DATA
CAPDIFF = 1 ±4pF
CDC
EXC
CIN(+)
CIN(–)
C
X
13 ... 21pF
(17 ±4pF)
C
Y
17pF
CAPDAC(+)
17pF
CAPDAC(–)
17pF
05468-011
Figure 35. Using CAPDAC in Differential Mode
PARASITIC CAPACITANCE TO GROUND
DATA
CDC
EXC
C
GND1
CIN
C
GND2
C
X
05468-012
Figure 36. Parasitic Capacitance to Ground
The CDC architecture used in the AD7745/AD7746 measures
the capacitance CX connected between the EXC pin and the
CIN pin. In theory, any capacitance CP to ground should not
affect the CDC result (see Figure 36).
The practical implementation of the circuitry in the chip
implies certain limits and the result is gradually affected by
capacitance to ground. See the allowed capacitance to GND in
the specification table for CIN and excitation. Also see the
typical performance characteristics shown in Figure 9, Figure
10, and Figure 11.
PARASITIC RESISTANCE TO GROUND
DATA
CDC
EXC
R
GND1
CIN
R
GND2
C
X
05468-013
Figure 37. Parasitic Resistance to Ground
The AD7745/AD7746 CDC result would be affected by a leak-
age current from the CX to ground, therefore the CX should be
isolated from the ground. The influence of the leakage current
varies with the power supply voltage. The following limits can
be used as a guideline for the allowed leakage current or the
equivalent resistance between the CX and ground (Figure 37).
VDD ≈ 5 V: IGND < 150 nA (that is, RGND > 30 MΩ)
VDD ≥ 3 V: IGND < 60 nA (that is, RGND > 50 MΩ)
VDD ≥ 2.7 V: IGND < 30 nA (that is, RGND > 100 MΩ)
A higher leakage current to ground results in a gain error, an
offset error, and a nonlinearity error. See the typical
performance characteristics shown in Figure 12 and Figure 13.
PARASITIC PARALLEL RESISTANCE
DATA
CDC
EXC
CIN
R
P
05468-022
C
X
Figure 38. Parasitic Parallel Resistance
The AD7745/AD7746 CDC measures the charge transfer
between EXC pin and CIN pin. Any resistance connected in
parallel to the measured capacitance CX (see Figure 38), such as
the parasitic resistance of the sensor, also transfers charge.
Therefore, the parallel resistor is seen as an additional
capacitance in the output data. The equivalent parallel
capacitance (or error caused by the parallel resistance) can be
approximately calculated as
4
1
××
=
EXC
P
PFR
C
Where RP is the parallel resistance and CEXC is the excitation
frequency. See the typical performance characteristics shown in
Figure 14.
AD7745/AD7746
Rev. 0| Page 23 of 28
PARASITIC SERIAL RESISTANCE
DATA
CDC
EXC
R
S1
CIN
R
S2
C
X
05468-023
Figure 39. Parasitic Serial Resistance
The AD7745/AD7746 CDC result is affected by a resistance in
series with the measured capacitance. The total serial resistance,
which refers to RS1 + RS2 on Figure 39, should be less than 1 kΩ
for the specified performance. See typical performance charac-
teristics shown in Figure 15.
CAPACITIVE GAIN CALIBRATION
The AD7745/AD7746 gain is factory calibrated for the full scale
of ±4.096 pF in the production for each part individually. The
factory gain coefficient is stored in a one-time programmable
(OTP) memory and is copied to the capacitive gain register at
power-up or after reset.
The gain can be changed by executing a capacitance gain
calibration mode, for which an external full-scale capacitance
needs to be connected to the capacitance input, or by writing a
user value to the capacitive gain register. This change would be
only temporary and the factory gain coefficient would be
reloaded back after power-up or reset. The part is tested and
specified only for use with the default factory calibration
coefficient.
CAPACITIVE SYSTEM OFFSET CALIBRATION
The capacitive offset is dominated by the parasitic offset in the
application, such as the initial capacitance of the sensor, any
parasitic capacitance of tracks on the board, and the capacitance
of any other connections between the sensor and the CDC.
Therefore, the AD7745/AD7746 are not factory calibrated for
capacitive offset. It is the user’s responsibility to calibrate the
system capacitance offset in the application.
Any offset in the capacitance input larger than ±1 pF should
first be removed using the on-chip CAPDACs. The small offset
within ±1 pF can then be removed by using the capacitance
offset calibration register.
One method of adjusting the offset is to connect a zero-scale
capacitance to the input and execute the capacitance offset
calibration mode. The calibration sets the midpoint of the
±4.096 pF range (that is, Output Code 0x800000) to that
zero-scale input.
Another method would be to calculate and write the offset cali-
bration register value, the LSB is value 31.25 aF (4.096 pF/217).
The offset calibration register is reloaded by the default value at
power-on or after reset. Therefore, if the offset calibration is not
repeated after each system power-up, the calibration coefficient
value should be stored by the host controller and reloaded as
part of the AD7745/AD7746 setup.
On the AD7746, the register is shared by the two capacitive
channels. If the capacitive channels need to be offset calibrated
individually, the host controller software should read the
AD7746 capacitive offset calibration register values after
performing the offset calibration on individual channels and
then reload the values back to the AD7746 before executing a
conversion on a different channel.
INTERNAL TEMPERATURE SENSOR
DIGITAL
FILTER
AND
SCALING
24-BIT Σ-∆
MODULATOR
CLOCK
GENERATOR
INTERNAL TEMPERATURE SENSOR
05468-040
VOLTAGE
REFERENCE
DATA
IN×I
∆V
BE
V
DD
Figure 40. Internal Temperature Sensor
The temperature sensing method used in the AD7745/AD7746
is to measure a difference in ∆VBE voltage of a transistor
operated at two different currents (see Figure 40). The ∆VBE
change with temperature is linear and can be expressed as
)ln()( N
q
KT
nV f
BE ×=∆
where:
K is Boltzmann’s constant (1.38 × 10–23).
T is the absolute temperature in Kelvin.
q is the charge on the electron (1.6 × 10–19 coulombs).
N is the ratio of the two currents.
nf is the ideality factor of the thermal diode.
The AD7745/AD7746 uses an on-chip transistor to measure the
temperature of the silicon chip inside the package. The Σ-Δ
ADC converts the ∆VBE to digital, the data are scaled using
factory calibration coefficients, thus the output code is
proportional to temperature:
()
4096
2048 −=° Code
CeTemperatur
The AD7745/AD7746 has a low power consumption resulting
in only a small effect due to the part self-heating (less than
0.5°C at VDD = 5 V).
AD7745/AD7746
Rev. 0 | Page 24 of 28
If the capacitive sensor can be considered to be at the same
temperature as the AD7745/AD7746 chip, the internal
temperature sensor can be used as a system temperature sensor.
That means the complete system temperature drift
compensation can be based on the AD7745/AD7746 internal
temperature sensor without need for any additional external
components. See the typical performance characteristics in
Figure 18.
EXTERNAL TEMPERATURE SENSOR
DIGITAL
FILTER
AND
SCALING
24-BIT Σ-∆
MODULATOR
CLOCK
GENERATOR
EXTERNAL
TEMPERATURE
SENSOR
05468-041
VOLTAGE
REFERENCE
DATA
I...N×I
∆V
BE
V
DD
VIN (–)
VIN (+)
R
S2
R
S1
2N3906
Figure 41. Transistor as an External Temperature Sensor
The AD7745/AD7746 provide the option of using an external
transistor as a temperature sensor in the system. The ∆VBE
method, which is similar to the internal temperature sensor
method, is used. However, it is modified to compensate for the
serial resistance of connections to the sensor. Total serial
resistance (RS1 + RS2 in Figure 41) up to 100 Ω is compensated.
The VIN(–) pin must be grounded for proper external
temperature sensor operation.
The AD7745/AD7746 are factory calibrated for
Transistor 2N3906 with the ideality factor nf = 1.008.
See the typical performance characteristics shown
in Figure 19.
VOLTAGE INPUT
DIGITAL
FILTER
24-BIT Σ-∆
MODULATOR
CLOCK
GENERATOR
ANALOG TO DIGITAL CONVERTER
(ADC)
05468-042
VOLTAGE
REFERENCE
DATA
VIN(–)
VIN(+)
REFIN(–)
REFIN(+)
R
REF
RTD
R
T
VDD
GND
Figure 42. Resistive Temperature Sensor Connected to the Voltage Input
The AD7745/AD7746 Σ-Δ core can work as a high resolution
(up to 21 ENOB) classic ADC with a fully differential voltage
input. The ADC can be used either with the on-chip high
precision, low drift, 1.17 V voltage reference, or an external
reference connected to the fully differential reference input pins.
The voltage and reference inputs are continuously sampled by a
Σ-Δ modulator during the conversion. Therefore, the input
source impedance should be kept low. See the application
example in Figure 42.
VDD MONITOR
Along with converting external voltages, the AD7745/AD7746
Σ-Δ ADC can be used for monitoring the VDD voltage. The
voltage from the VDD pin is internally attenuated by 6.
TYPICAL APPLICATION DIAGRAM
DIGITAL
FILTER
24-BIT Σ-∆
MODULATOR
CLOCK
GENERATOR
MUX
TEMP
SENSOR
CAP DAC
CAP DAC
EXCITATION VOLTAGE
REFERENCE
CONTROL LOGIC
CALIBRATION
AD7745
VDD
REFIN(+) REFIN(–) GND
SDA
SCL
RDY
VIN(+)
VIN(–)
CIN1(+)
CIN1(–)
EXCA
EXCB
05468-008
HOST
SYSTEM
+
0.1µF10µF
3V/5V
POWER SUPPL
Y
I
2
C
SERIAL
INTERFACE
10kΩ
10kΩ
Figure 43. Basic Application Diagram for a Differential Capacitive Sensor
AD7745/AD7746
Rev. 0 | Page 25 of 28
OUTLINE DIMENSIONS
16 9
81
PIN 1
SEATING
PLANE
8°
0°
4.50
4.40
4.30
6.40
BSC
5.10
5.00
4.90
0.65
BSC
0.15
0.05
1.20
MAX 0.20
0.09 0.75
0.60
0.45
0.30
0.19
COPLANARITY
0.10
COMPLIANT TO JEDEC STANDARDS MO-153-AB
Figure 44. 16-Lead Thin Shrink Small Outline Package [TSSOP]
(RU-16)
Dimensions shown in millimeters
ORDERING GUIDE
Model Temperature Range Package Description Package Option
AD7745ARUZ1–40°C to +125°C 16-Lead TSSOP RU-16
AD7745ARUZ-REEL1 –40°C to +125°C 16-Lead TSSOP RU-16
AD7745ARUZ-REEL71 –40°C to +125°C 16-Lead TSSOP RU-16
AD7746ARUZ1 –40°C to +125°C 16-Lead TSSOP RU-16
AD7746ARUZ-REEL1 –40°C to +125°C 16-Lead TSSOP RU-16
AD7746ARUZ-REEL71 –40°C to +125°C 16-Lead TSSOP RU-16
EVAL-AD7746EB Evaluation Board
1 Z = Pb-free part.
AD7745/AD7746
Rev. 0 | Page 26 of 28
NOTES
AD7745/AD7746
Rev. 0 | Page 27 of 28
NOTES
AD7745/AD7746
Rev. 0 | Page 28 of 28
NOTES
Purchase of licensed I2C components of Analog Devices or one of its sublicensed Associated Companies conveys a license for the purchaser under the Philips I2C
Patent Rights to use these components in an I2C system, provided that the system conforms to the I2C Standard Specification as defined by Philips.
© 2005 Analog Devices, Inc. All rights reserved. Trademarks and
registered trademarks are the property of their respective owners.
C05468-0-4/05(0)
