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

4.1 Minimum Recommended Values

Table 4 shows the Minimum Recommended Threshold and Minimum Recommended Margin In for both First-generation and Second-generation CapTIvate technology devices under different operating temperatures and we assume the user tune the system at room temperature.

Table 3 shows the formulas on how to calculate these parameters for different operating temperature and characterization process temperature. Minimum Recommended Threshold is calculated by adding the expected maximum device noise floor with a predefined safety factor. Minimum Recommended Margin In value is calculated by looking at the difference between the minimum recommended threshold and the typical noise floor values.

Table 3. Minimum Recommendation Parameters Formulas

Minimum recommended threshold = Expected maximum device noise floor
at operating temperature		 + Safety factor
Minimum recommended margin in = Expected maximum device noise floor
at operating temperature		 – Typical device noise floor
at characterization temperature		 + Safety factor

Example: A product uses MSP430FR2633 (first generation) and it is designed to operate down to 0°C and the characterization process is performed under normal room temperature at 25°C.

Minimum recommended threshold = 0.8% + 0.1% = 0.9%

Minimum recommended margin in = 0.8% – 0.07% + 0.1% = 0.83%

Table 4. CapTIvate Minimum Recommended Design Margins

CapTIvate Technology Ambient Temperature Typical Device Noise Floor Expected Maximum Device Noise Floor Safety Factor Minimum Recommended Threshold Minimum Recommended Margin In
First generation 25°C 0.07% 0.5% 0.1% 0.6% 0.53%
0°C 0.09% 0.8% 0.9% 0.83%
-40°C 0.13% 1.3% 1.4% 1.33%
Second generation 25°C 0.06% 0.2% 0.1% 0.3% 0.24%
-40°C 0.09% 0.2% 0.3% 0.24%

The device noise floor can vary from unit to unit, as indicated by the typical and minimum SNR range and the typical and maximum noise floor. TI recommends considering the worst-case performance (minimum SNR and maximum noise floor) when analyzing an application. The SNR tool incorporates device variation analysis when it provides advice. The device noise floor also has a temperature dependency. Applications that must operate in cold environments should be designed with higher Margin In values to accommodate higher noise levels at colder temperatures.