Cypress PSoC 4000 Series - Page 130

130 PSoC 4000 Family: PSoC 4 Architecture TRM, Document No. 001-89309 Rev. *D
CapSense
Figure 17-5. Sourcing Current to AMUXBUS A
Two non-overlapping, out of phase clocks of frequency F
SW
(see Figure 17-3) control the switches SW
2
and SW
3
. The contin-
uous switching of SW
2
and SW
3
forms an equivalent resistance R
S
, as Figure 17-5 shows. The value of the equivalent resis-
tance R
S
is:
Equation 17-1
Where:
C
S
= Sensor capacitance
F
SW
= Frequency of the switching clock
The sigma delta converter maintains the voltage of AMUXBUS A at a constant V
REF
(this process is explained in Sigma Delta
Converter on page 131). Figure 17-6 shows the voltage waveform across the sensor capacitance.
Figure 17-6. Voltage Across Sensor Capacitance
Equation 23-3 gives the value of average current supplied to AMUXBUS A.
Equation 17-2
Figure 17-7 shows the switched capacitance configuration for sinking current from AMUXBUS A. Figure 17-8 shows the
resulting voltage waveform across C
S
.
C
S
R
S
AMUXBUS A
V
DDD
V
DDD
SW
2
SW
3
AMUXBUS A
I
SW
I
SW
I
SW
R
S
1
C
S
F
SW
------------------=
V
t
V
REF
(1.2 V)
0
T
SW
= 1/F
SW
V
DDD
SW
2
Closed
SW
3
Open
SW
2
Open
SW
3
Closed
I
S
C
S
F
SW
V
DDD
V–
REF
=