CapSense Technology
AN64846 - Getting Started with CapSense
®
Doc. No. 001-64846 Rev. *X
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2.4.2.4 When Is Manual-Tuning Advantageous?
SmartSense allows a device to calibrate itself for optimal performance and complete the entire tuning process
automatically. This technology will meet the needs of most designs, but, in the case where SmartSense will not work
or there are specific SNR or power requirements, the CapSense CSD parameters can be manually adjusted to meet
system requirements. This is called manual tuning. Some advantages of manual tuning, as opposed to SmartSense
Auto-tuning are:
Strict control over parameter settings: SmartSense sets all of the parameters automatically. However, there may
be situations where you need strict control over the parameters. For example, use manual tuning if you need to
strictly control the time CSD takes to scan a group of sensors. This can be done to reduce EMI in systems.
Supports higher parasitic capacitances: SmartSense supports parasitic capacitances as high as 45 pF for a 0.2-
pF finger capacitance, and as high as 35 pF for a 0.1-pF finger capacitance. If the parasitic capacitance is higher
than the value supported by SmartSense, use manual tuning.
for the step-by-step procedure on manual tuning.
2.5 Signal-to-Noise Ratio (SNR)
Signal is a generic engineering term that can have many meanings. For CapSense applications, signal is defined as
the change in the raw count between the OFF and ON states. Signal is also called Difference Count.
Noise is another term that has many meanings. The following discussion presents a definition of CapSense noise that
uses a simple mathematical model of the sensor output over time.
When the sensor is in the OFF state, the counts, X(t), can be modeled by an average count and a noise component.
𝑋(𝑡) = 𝑋0 + 𝑁0(𝑡)
Equation 7
X0 is the average of X(t)
N0(t) is the noise component for t during the OFF state
The same model applies when the sensor is in the ON state.
𝑋(𝑡) = 𝑋1 + 𝑁1(𝑡)
Equation 8
X1 is the average of X(t)
N1(t) is the noise component for t during the ON state
X0 is called the baseline level of the raw counts. The difference between X1 and X0 is called the signal, S.
𝑆 = 𝑋1 − 𝑋0
Equation 9
The noise components N0(t) and N1(t) are similar but not identical. For example, N1(t) usually contains a higher level
of AC line noise in finger sensing applications compared to N0(t). This occurs because the human body acts as an
antenna to 50-Hz and 60-Hz line noise, and the finger contact with the sensor overlay couples the noise into the
CapSense system.
We define the noise level N as the worst case measured peak noise in the OFF state.
N = max(N0(t)) = max(X(t)) − min(X(t))
Equation 10
Thus, CapSense SNR, is defined as the ratio of signal (S) to noise (N).
𝑆𝑁𝑅 = 𝑆: 𝑁
Equation 11
Based on the experiments and knowledge from many CapSense applications, Cypress recommends a minimum SNR
of 5:1 to ensure sufficient margin between noise and signal for robust ON/OFF operation.
