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16 Document No. 001-64846 Rev. *I Getting Started with CapSense

2.4.2 Signal-to-Noise Ratio (SNR)

Signal is a generic engineering term that can have many meanings. For the capacitive sensor application under

consideration for CapSense applications, signal is defined as the change in the average sensor output between the

OFF and ON states when the rising edge of the difference count starts below the noise threshold.

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 3

 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 4

 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 X0 and X1 is called the signal, S.

S = (X1 - X0) Equation 5

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))-X0 Equation 6

Thus, CapSense Signal-to-Noise Ratio, SNR, is defined as the ratio of signal (S) to noise (N).

SNR = S:N Equation 7

For robust operation of CapSense, a minimum SNR of 5:1 is recommended.

Figure 2-9. Signal and Noise

2.4.3 Measuring SNR

SNR should be measured in the noise environment where CapSense is intended to be used. In other words, measure

the system SNR under worst-case noise conditions.

The first step in measuring SNR is to monitor the raw count for each sensor. This can be done using data logging to a

text file and plotting in a spreadsheet, or through the use of the Cypress MultiChart GUI tool and I2C-USB Bridge

(see chapter 7 - Resources for more details). Whatever the method, the raw count should be observed for SNR

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Cypress CapSense CY8C20x36 Guía de usuario Pagina 16