参数资料
型号: AD7151BRMZ-REEL
厂商: Analog Devices Inc
文件页数: 16/28页
文件大小: 0K
描述: IC CAP CONV 1CH ULT LP 10MSOP
标准包装: 3,000
类型: 电容数字转换器
输入类型: 电压
输出类型: 数字
接口: 2 线串口
电流 - 电源: 120µA
安装类型: 表面贴装
封装/外壳: 10-TFSOP,10-MSOP(0.118",3.00mm 宽)
供应商设备封装: 10-MSOP
包装: 带卷 (TR)
AD7151
Rev. 0 | Page 23 of 28
HARDWARE DESIGN CONSIDERATIONS
OVERVIEW
The AD7151 is an interface to capacitive sensors.
On the input side, the sensor (CX) can be connected directly
between the AD7151 EXC and CIN pins. The way it is
connected and the electrical parameters of the sensor
connection, such as parasitic resistance or capacitance, can
affect the system performance. Therefore, any circuit with
additional components in the capacitive front end, such as
overvoltage protection, has to be carefully designed considering
the AD7151 specified limits and information provided in this
section.
On the output side, the AD7151 can work as a standalone
device, using the power-up default register settings and flagging
the result on digital outputs. Alternatively, the AD7151 can be
interfaced to a microcontroller via the 2-wire serial interface,
offering flexibility by overwriting the AD7151 register values
from the host with a user-specific setup.
PARASITIC CAPACITANCE TO GROUND
DATA
CDC
EXC
CGND1
CIN
CGND2
CX
07
08
6-
03
1
Figure 42. Parasitic Capacitance to Ground
The CDC architecture used in the AD7151 measures the
capacitance CX connected between the EXC pin and the CIN
pin. In theory, any capacitance CGND to ground should not affect
the CDC result (see Figure 42).
The practical implementation of the circuitry in the chip
implies certain limits, and the result is gradually affected by
capacitance to ground (see Table 1 for information about the
allowed capacitance to GND for CIN and information about
excitation).
PARASITIC RESISTANCE TO GROUND
DATA
CDC
EXC
RGND1
CIN
RGND2
CX
07
08
6-
0
32
Figure 43. Parasitic Resistance to Ground
The AD7151 CDC result is affected by a leakage current from
CX to ground; therefore, CX should be isolated from the ground.
The equivalent resistance between CX and ground should be
maximized (see Figure 43).
PARASITIC PARALLEL RESISTANCE
DATA
CDC
EXC
CIN
RP
CX
07
08
6-
03
3
Figure 44. Parasitic Parallel Resistance
The AD7151 CDC measures the charge transfer between the
EXC and CIN pins. Any resistance connected in parallel to the
measured capacitance CX (see Figure 44), 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
f
R
C
where RP is the parallel resistance and fEXC is the excitation
frequency.
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