CapSense Technology
AN66308 - CY8CMBR2044 CapSense® Design Guide, Doc. No. 001-66308 Rev. *F
11
Figure 2-2. Section of typical CapSense PCB with the Sensor being activated by a Finger
In addition to the parallel plate capacitance, a finger in contact with the overlay causes electric field fringing between
itself and other conductors in the immediate vicinity. Typically, the effect of these fringing fields is minor, and it can
usually be ignored.
Even without a finger touching the overlay, the sensor input pin has some parasitic capacitance (C
P
). C
P
results from
the combination of the CapSense controller internal parasitic and electric field coupling among the sensor pad,
traces, and vias, and other conductors in the system, such as ground plane, other traces, any metal in the product’s
chassis or enclosure, and so on. The CapSense controller measures the total capacitance (C
X
) connected to a
sensor pin.
When a finger is not touching a sensor, use this equation:
Equation 2
With a finger on the sensor, C
X
equals the sum of C
P
and C
F
:
Equation 3
In general, C
P
is an order of magnitude greater than C
F
. C
P
usually ranges from 10 pF to 20 pF, but in extreme cases
it can be as high as 40 pF. C
F
usually ranges from 0.1 pF to 0.4 pF.
Capacitive Sensing Method
CY8CMBR2044 device supports the CapSense Sigma Delta (CSD) with SmartSense™ Auto-Tuning for converting
sensor capacitance (C
X
) into digital counts. The CSD method is described in the following sections.
CapSense Sigma-Delta (CSD)
The CSD method in the CY8CMBR2044 device incorporates C
X
into a switched capacitor circuit, as shown in
The sensor (C
X
) is alternatively connected to GND and the Analog MUX (AMUX) bus by the non-
overlapping switches Sw1 and Sw2, respectively. Sw1 and Sw2 are driven by a precharge clock to bleed a current
(I
SENSOR
) from the AMUX bus. The magnitude of I
SENSOR
is directly proportional to the magnitude of C
X
. The sigma-
delta converter samples AMUX bus voltage and generates a modulating bit stream that controls the constant current
source (I
DAC
), which charges AMUX such that the average AMUX bus voltage is maintained at V
REF
. The sensor
bleeds off the charge I
SENSOR
from the modulating capacitor (C
MOD
). C
MOD,
in combination with Rbus, forms a low-pass
filter that attenuates precharge switching transients at the sigma-delta converter input.
