Cypress CapSense CY8CMBR2044, Design Manual

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CapSense Technology
AN66308 - CY8CMBR2044 CapSense® Design Guide, Doc. No. 001-66308 Rev. *F 12
Figure 2-3. CSD Block Diagram
Cx
i
sensor
Sigma-Delta
Converter
Precharge
Clock
Cmod
High-Z
input
Sw1
Sw2
CY8CMBR2044
Gnd
= External Connection
AMUX
Bus
Vref
Rbus
Gnd
IDAC
In maintaining the average AMUX voltage at a steady state value (V
REF
), the sigma-delta converter matches the
average charge current (I
DAC
) to I
SENSOR
by controlling the bit stream duty cycle. The sigma-delta converter stores the
bit stream over the duration of a sensor scan, and the accumulated result is a digital output value, known as raw
count, which is directly proportional to C
X
. This raw count is interpreted by high-level algorithms to resolve the sensor
state. Figure 2-4 plots the CSD raw counts from a number of consecutive scans during which the sensor is touched
and then released by a finger. As explained in CapSense Fundamentals , the finger touch causes C
X
to increase by
C
F
, which in turn causes raw counts to increase proportionally. By comparing the shift in steady state raw count level
to a predetermined threshold, the high-level algorithms can determine whether the sensor is in an ON (Touch) or OFF
(No Touch) state.
Figure 2-4. CSD Raw Counts During a Finger Touch