Datasheet

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 (C
X
) 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
C
GND1
CIN
C
GND2
C
X
07086-031
Figure 42. Parasitic Capacitance to Ground
The CDC architecture used in the AD7151 measures the
capacitance C
X
connected between the EXC pin and the CIN
pin. In theory, any capacitance C
GND
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).
See
Figure 4, Figure 5, Figure 6, and Figure 7.
PARASITIC RESISTANCE TO GROUND
DATA
CDC
EXC
R
GND1
CIN
R
GND2
C
X
07086-032
Figure 43. Parasitic Resistance to Ground
The AD7151 CDC result is affected by a leakage current from
C
X
to ground; therefore, C
X
should be isolated from the ground.
The equivalent resistance between C
X
and ground should be
maximized (see
Figure 43).
See
Figure 8, Figure 9, Figure 10, and Figure 11.
PARASITIC PARALLEL RESISTANCE
DATA
CDC
EXC
CIN
R
P
C
X
07086-033
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 C
X
(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
××
=
EXCP
P
fR
C
where R
P
is the parallel resistance and f
EXC
is the excitation
frequency.
See
Figure 13.