Cypress CY8C20xx7/S CapSense Series, Design Manual

Page: 14 / 75

CapSense Technology
CY8C20xx7/S CapSense
®
Design Guide Doc. No. 001-78329 Rev. *E 14
Figure 2-5. Switched-Capacitor Input Sinking Current from AMUXBUS
C
X
R
S
AMUXBUS
Sw
1
Sw
2
AMUXBUS
I
SENSOR
I
SW
I
SENSOR
The sigma delta converter maintains the voltage of AMUXBUS at a constant V
REF
(this process is explained in Sigma
Delta Converter ). Figure 2-6 shows the voltage waveform across the sensor capacitance. Driving switches Sw1 and
Sw2 through non-overlapping precharge clocks thus results in an average current sink (I
SENSOR
) from the AMUXBUS
as Equation 2-5 shows. The magnitude of I
SENSOR
is directly proportional to the magnitude of C
X.
I
SENSOR
= V
REF
R
S
= C
X
F
SW
V
REF
Equation 2-5
Figure 2-6. Voltage across Sensor Capacitance (C
X
)
V
t
V
REF
(1V)
0
T
SW
= 1/F
SW
Sw2 CLOSED
Sw1 OPEN
Sw2 OPEN
Sw1 CLOSED
2.2.1.2 Sigma Delta Converter
The sigma delta converter converts the input current to a corresponding digital count. It consists of a sigma delta
modulator and one current-sourcing digital-to-analog converter (I
DAC
), as Figure 2-3 on page 13 shows.
The sigma delta modulator controls the 8-bit I
DAC
current in an on/off manner. This I
DAC
is known as the modulation
I
DAC
and is referred as “I
DAC
” or “modulation I
DAC
”
in this document. The sigma delta converter also requires an
external integrating capacitor C
MOD
, as Figure 2-3 on page 13 shows. The recommended value of C
MOD
is 2.2 nF.
The sigma delta modulator switches the modulation I
DAC
ON or OFF corresponding to the small voltage variations
across C
MOD
, to maintain the C
MOD
voltage at V
REF
.
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 modulation 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 proportional to C
X
.
The sigma delta converter can operate from 9-bit to 16-
bit resolutions. If „N‟ is the resolution of the sigma delta
converter and I
DAC
is the value of the modulation I
DAC
current, the approximate equation for the raw count is
raw count = (2
N
− 1)
V
REF
F
SW
C
X
I
DAC
Equation 2-6
The raw count is interpreted by high-level algorithms to resolve the sensor state and detect touches. Figure 2-7 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