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

6.2 Count and Percent Change Analysis With 1.5-mm Overlay, Advice = GOOD

The example in Section 6.1 used the CAPTIVATE-BSWP sensing panel, which is designed for 1.5mm overlay, with a 7.5-mm overlay. This example demonstrates a poor design margin and SNR case. This SNR can be improved in two ways:

  • Reducing the parasitic electrode capacitance Cp
  • Increasing the touch capacitance Ct

Reducing the overlay thickness has the effect of increasing the touch capacitance Ct, leading to a larger percent change in capacitance due to a touch. With the standard 1.5-mm overlay, the change in counts is now 1490 – 690 = 800 counts, and the signal 'S' improves from 0.82% to a very ideal 7.8% change in capacitance (see Figure 12 and Figure 13).

D003_SLAA843.gifFigure 12. Count Analysis With 1.5-mm Overlay
D004_SLAA843.gifFigure 13. Percent Change Analysis With 1.5-mm Overlay

The SNR for this configuration can be calculated by Equation 13.

Equation 13. SNR = S N = 7 .8 % 0 .12 % =65 :1

Figure 13 shows that the Threshold (Th) = 4.5%. Based on Table 4, the minimum recommended threshold for 0°C operating temperature is 0.9% and the Threshold (Th) for this design is 5 times higher than the minimum recommendation. This means that if this example is designed to operate down to 0°C then the SNR analyze tool will give an advice as GOOD.

Equation 14 calculates the Min in this example. In this example, the Min of 4.38% is also 5 times higher than the minimum recommended margin in (Min) 0.83% as showing in Table 4. This means that if this example is designed to operate down to 0°C then the SNR analyze tool will give an advice as GOOD.

Equation 14. M in = Th - N =4 .5 % -0 .12 % =4 .38 %

Equation 15 calculates the Mout. This is the difference between the lowest percent change during a touch and the detection threshold.

Equation 15. M out = S low - Th =7 .7 % - 4.5% =3 .2 %

This shows that reducing the overlay thickness by a factor of 5 improved the SNR by a factor of almost 10.