Cypress CY8C20xx7/S CapSense Series, Design Manual

Page: 34 / 75

CapSense Performance Tuning with User Modules
CY8C20xx7/S CapSense
®
Design Guide Doc. No. 001-78329 Rev. *E 34
4.1.1.2 CapSense Noise
CapSense noise is the peak-to-peak variation in sensor response when a finger is not present, as demonstrated in
Figure 4-1 . In this example, the output waveform without a finger is bounded by a minimum of 5912 counts and a
maximum of 5938 counts. Because the noise is the difference between the minimum and the maximum values of this
waveform, Noise = 5938
– 5912 = 26 counts.
4.1.1.3 CapSense SNR
CapSense SNR is the simple ratio of signal and noise. Continuing with the example, if the signal is 135 counts and
noise is 26 counts, the SNR is 135:26, which reduces to an SNR of 5.2:1. The minimum recommended SNR for
CapSense is 5:1, which means the signal is five times larger than the noise. Filters are commonly implemented in
firmware to reduce noise. See
Software Filtering for more information.
4.1.2 Charge/Discharge Rate
To achieve maximum sensitivity in the tuning process, the sensor capacitor must be fully charged and discharged
during each cycle. The charge/discharge path switches between two states at a rate set by a user module parameter
called Precharge Clock in the CSDPLUS User Module.
The charge/discharge path includes series resistance that slows down the transfer of charge. The rate of change for
this charge transfer is characterized by an RC time constant involving the sensor capacitor (C) and series resistance
(R), as shown in
Figure 4-2 .
Figure 4-2. Charge/Discharge Waveforms
V
s
V
x
Vref
t
t
R
x
C
x
V
s
V
x
5*R
x
*C
x
5*R
x
*C
x
Ts
min
>= 10*R
x
*C
x
Vref
Set the charge/discharge rate to a level that is compatible with this RC time constant. In general, you should allow a
period of 5 RC for each transition, with two transitions per period (one charge, one discharge). The equations for
minimum time and maximum frequency are:
𝑇𝑠
𝑚𝑖𝑛
= 10 × 𝑅
𝑋
𝐶
𝑋
Equation 4-1
𝑓𝑠
𝑚𝑎𝑥
=
1
10×𝑅
𝑋
𝐶
𝑋
Equation 4-2
For example, assume the series resistor includes a 560-
Ω external resistor and up to 800 Ω of internal resistance,
and the sensor capacitance is typical:
R
X
= 1.4 k
Ω
C
X
= 24 pF
The value of the time constant and maximum front-end switching frequency in this example is:
Ts
min
= 0.34 µs