SLAA843A August   2018  – March 2019 MSP430FR2512 , MSP430FR2512 , MSP430FR2522 , MSP430FR2522 , MSP430FR2532 , MSP430FR2532 , MSP430FR2533 , MSP430FR2533 , MSP430FR2632 , MSP430FR2632 , MSP430FR2633 , MSP430FR2633

 

  1.   Sensitivity, SNR, and design margin in capacitive touch applications
    1.     Trademarks
    2. 1 Overview
      1. 1.1 Design Objectives
        1. 1.1.1 Reliability
        2. 1.1.2 Robustness
      2. 1.2 The Designer's Dilemma
    3. 2 Recommended Actions for Developers
      1. 2.1 Run SNR and Design Margin Tests
    4. 3 Terminology
      1. 3.1 Signal (S)
      2. 3.2 Noise (N)
      3. 3.3 Threshold (Sensitivity) (Th)
      4. 3.4 Design Margin
        1. 3.4.1 False Detection Margin (Min)
        2. 3.4.2 Detection Margin (Mout)
      5. 3.5 Signal-to-Noise Ratio (SNR)
      6. 3.6 Advice
    5. 4 CapTIvate Device Performance
      1. 4.1 Minimum Recommended Values
      2. 4.2 CapTIvate Device SNR
    6. 5 Interpreting the Results
      1. 5.1 Interpreting the Advice
      2. 5.2 Check Other Results
    7. 6 Application of Terms
      1. 6.1 Count and Percent Change Analysis With 7.5-mm Overlay, Advice = POOR
      2. 6.2 Count and Percent Change Analysis With 1.5-mm Overlay, Advice = GOOD
      3. 6.3 Count and Percent Change Analysis (1.5-mm Overlay vs 7.5-mm Overlay)
      4. 6.4 Effect of Post-Processing and Sampling Rate
    8. 7 Summary
  2.   Revision History

4.2 CapTIvate Device SNR

Table 5 lists typical and minimum SNR values for CapTIvate devices for an example use case with a 0.5-pF touch capacitance and a 20-pF electrode parasitic capacitance. The equivalent signal 'S' is 0.5 pF divided by 20 pF, which equals 2.5% change in capacitance.

Table 5. CapTIvate Device SNR

CapTIvate Technology Touch Capacitance Electrode (Parasitic) Capacitance Signal Ambient Temperature Typical SNR Expected Minimum SNR
First generation 0.5 pF 20 pF 2.5% 25°C 36:1 5:1
0.5 pF 20 pF 2.5% 0°C 28:1 3.1:1
0.5 pF 20 pF 2.5% -40°C 19:1 1.9:1
Second generation 0.5 pF 20 pF 2.5% 25°C 42:1 13:1
0.5 pF 20 pF 2.5% -40°C 28:1 13:1

NOTE

SNR values, particularly expected minimum SNR values, are provided for informational purposes to assist in application development. Refer to the device-specific data sheet for specified SNR values.

The SNR values for CapTIvate devices in Table 5 are characterized and tested using a count filter beta of 3, an LTA filter beta of 7, and a scan rate of 167 sps.

It is possible to derive the equivalent typical and maximum device noise floor (a useful parameter for margin analysis) from the SNR parameters by backing out the test case (2.5% signal). Divide the signal by the SNR value to obtain the noise floor. These values are obtained by solving the SNR equation for noise (see Equation 8).

Equation 8. N = S SNR