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參數(shù)資料
| 型號： | AD7747 |
| 廠商： | Analog Devices, Inc. |
| 元件分類： | 溫度/濕度傳感器 |
| 英文描述： | 24-Bit Capacitance-to-Digital Converter with Temperature Sensor |
| 中文描述： | 24位的電容數(shù)字轉(zhuǎn)換器的溫度傳感器 |
| 文件頁數(shù)： | 18/20頁 |
| 文件大?。?/td> | 200K |
| 代理商： | AD7747 |
AD7747
Preliminary Technical Data
CIRCUIT DESCRIPTION
ACTIVE AC SHIELD CONCEPT
The AD7747 measures capacitance between CIN and ground.
That means any capacitance to ground on signal path between
AD7747 CIN pin(s) and sensor is included in the AD7747
conversion result.
The parasitic capacitance of the sensor connections can easily
be in the same, if not even higher order than the capacitance of
the sensor itself. If that parasitic capacitance is stable, it can be
treated as a non-changing capacitive offset. However, the
parasitic capacitance of sensor connections is often changing as
result of mechanical movement, changing ambient temperature,
ambient humidity, etc. These changes would be seen as drift in
the conversion result and may significantly compromise the
system accuracy.
TYPICAL APPLICATION DIAGRAM
Rev. PrC | Page 18 of 20
To eliminate the CIN parasitic capacitance to ground, the
AD7747 SHLD signal can be used for shielding the connection
between the sensor and CIN. The SHLD output is basically the
same signal waveform as the excitation of the CIN pin, the
SHLD is driven to the same voltage potential as the CIN pin.
Therefore, there is no AC current between CIN and SHLD pins
and any capacitance between these pins doesn't get involved in
the CIN charge transfer. Ideally, the CIN to SHLD capacitance
doesn't have any contribution to the AD7747 result.
To get the best result, locate the AD7747 as close as possible to
the capacitive sensor. Keep the connection between the sensor
and AD7747 CIN pin and also the return path between sensor
ground and the AD7747 GND pin short. Shield the PCB track
to CIN pin and connect the shielding to the AD7747 SHLD pin.
Also, if a shielded cable is used for sensor connection, the shield
should be connected to the AD7747 SHLD pin.
HOST
SYSTEM
0.1uF
10uF
+
+3V / +5V
POWER SUPPLY
SCL
SDA
VDD
GND
RDY
CLOCK
GENERATOR
TEMP
SENSOR
MUX
DIGITAL
FILTER
I2C
SERIAL
INTERFACE
CAP DAC 1
EXCITATION
AD7747
VIN(+)
VIN(
-
)
24-BIT
Σ
-
MODULATOR
CAP DAC 2
REFIN(+)
REFIN(
-
)
CONTROL LOGIC
CALIBRATION
VOLTAGE
REFERENCE
CIN1(+)
SHLD
CIN1(
-
)
Figure 12. Basic Application Diagram for a Differential Capacitive Sensor
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