Datasheet

AD7156
Rev. 0 | Page 24 of 28
HARDWARE DESIGN CONSIDERATIONS
OVERVIEW
The AD7156 is an interface to capacitive sensors.
On the input side, Sensor C
X
can be connected directly between
the AD7156 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 AD7156 specified limits and informa-
tion provided in this section.
On the output side, the AD7156 can work as a standalone device,
using the power-up default register settings and flagging the
result on the digital outputs. Alternatively, the AD7156 can be
interfaced to a microcontroller via the 2-wire serial interface,
offering flexibility by overwriting the AD7156 register values
from the host with a user-specific setup.
PARASITIC CAPACITANCE TO GROUND
DATA
CDC
EXC
C
GND1
CIN
C
GND2
C
X
07726-052
Figure 42. Parasitic Capacitance to Ground
The CDC architecture used in the AD7156 measures the
capacitance, C
X
, connected between the EXC pins and the
CIN pins. 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 (for information about the allowed capacitance to
GND for CIN and information about excitation see Tabl e 1
and Figure 4 to Figure 9).
PARASITIC RESISTANCE TO GROUND
DATA
CDC
EXC
R
GND1
CIN
R
GND2
C
X
07726-053
Figure 43. Parasitic Resistance to Ground
The AD7156 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). For more information, see Figure 10
to Figure 13.
PARASITIC PARALLEL RESISTANCE
DATA
CDC
EXC
CIN
R
P
C
X
07726-054
Figure 44. Parasitic Parallel Resistance
The AD7156 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
××
=
EXC
P
P
fR
C
where:
R
P
is the parallel resistance.
f
EXC
is the excitation frequency.
For additional information, see Figure 15.