AD7156
Rev. 0 | Page 11 of 28
THEORY OF OPERATION
DIGITAL
FILTER
POWER-DOWN
TIMER
CLOCK
GENERATOR
SERIAL
INTERFACE
CIN1
C
X1
C
X2
EXC1
- CDC
CIN2
EXC2
MUX
EXCITATION
CAPDAC
THRESHOLD
THRESHOLD
SCL
VDD
GND
AD7156
SDA
PROGRAMMING
INTERFACE
DIGITAL
OUTPUTS
OUT1
OUT2
3.3
7726-030
Figure 22. AD7156 Block Diagram
The AD7156 core is a high performance capacitance-to-digital
converter (CDC) that allows the part to be interfaced directly
to a capacitive sensor.
The comparators compare the CDC results with thresholds, either
fixed or dynamically adjusted by the on-chip adaptive threshold
algorithm engine. Thus, the outputs indicate a defined change in
the input sensor capacitance.
The AD7156 also integrates an excitation source, CAPDAC
for the capacitive inputs, an input multiplexer, a complete clock
generator, a power-down timer, a power supply monitor, control
logic, and an I
2
C®-compatible serial interface for configuring the
part and accessing the internal CDC data and status, if required
in the system (see Figure 22).
CAPACITANCE-TO-DIGITAL CONVERTER
Figure 23 shows the CDC simplified functional diagram. The
converter consists of a second-order Σ-Δ charge balancing
modulator and a third-order digital filter. The measured
capacitance C
X
is connected between an excitation source
and the Σ-Δ modulator input. The excitation signal is applied
on the C
X
capacitor during the conversion, and the modulator
continuously samples the charge going through the C
X
. 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 is processed by the adaptive threshold engine and output
comparators; the data can also be read through the serial interface.
The AD7156 is designed for floating capacitive sensors.
Therefore, both C
X
plates have to be isolated from ground
or any other fixed potential node in the system.
The AD7156 features slew rate limiting on the excitation voltage
output, which decreases the energy of higher harmonics on the
excitation signal and dramatically improves the system electro-
magnetic compatibility (EMC).
DIGITAL
FILTER
0x0000 TO 0xFFF0
DATA
CLOCK
GENERATOR
CAPACITANCE-TO-DIGITAL CONVERTER
(CDC)
CIN
C
X
0pF TO 4pF
EXC
EXCITATION
-
MODULATOR
07726-031
Figure 23. CDC Simplified Block Diagram
CAPDAC
The AD7156 CDC core maximum full-scale input range is 0 pF
to 4 pF. However, the part can accept a higher input capacitance,
caused, for example, by a nonchanging offset capacitance of up to
10 pF. This offset capacitance can be compensated for by using
the programmable on-chip CAPDAC.
0x0000 TO 0xFFF0
DATA
CIN
EXC
CAPDAC
10pF
0pF TO 4pF
C
X
10pF TO 14pF
07726-032
Figure 24. Using a CAPDAC
The CAPDAC can be understood as a negative capacitance
connected internally to a CIN pin. The CAPDAC has a 6-bit
resolution and a monotonic transfer function. Figure 24 shows
how to use the CAPDAC to shift the CDC 0 pF to 4 pF input
range to measure capacitance between 10 pF and 14 pF.
