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

3.4.2 Detection Margin (Mout)

Detection margin is the opposite of false detection margin. Detection margin assumes a touched condition and is the difference between the lowest signal value Slow as showing in Figure 8 (perhaps due to noise that is present when a touch or proximity is applied) and the detection threshold (see Equation 4 and Equation 5 and Figure 8). This margin shows how stable the system is configured from getting out from the detection, in other words the system should have enough margin from the signal and the threshold so when a user touch is detected the noise will not push the signal out of detect.

Self Mode

Equation 4. M out = S low - Th

Mutual Mode

Equation 5. M out = Th - S low

For example, consider the case in which noise is present during a touch, the touch signal 'S' is not constant but has a minimum value of 1.3%, and the detection threshold is set to 1% just like the previous example. The Min in this case would be 1.3% – 1.0% = 0.3%. This parameter gives an idea of how stable the touch or proximity detection state is. If the detection threshold is set too high for a given signal, then the Min is reduced. If the detection threshold is reduced too much, it will be difficult to reliably detect touches without transitioning in and out of detection during a touch when additional noise is present on the signal.

Figure 8 shows the self mode button measurement result in terms of percent change in capacitance for both touched and untouched condition and the Margin In and the Margin Out are also defined as the percent change in capacitance as the blue area are showing in the figure.

S_T_N_1.pngFigure 8. Design Margin Terminology