Tuning and Signal-to-Noise Ratio (SNR)
In practice, the raw counts (digitized capacitance values) vary due to inherent noise in the system. CapSense noise is the pk-to-pk variation in raw counts in the absence of touch. A fine-tuned CapSense system reliably discriminates between the ON and OFF states of the sensor. To achieve good performance, the CapSenes signal must be significantly larger than the CapSense noise. SNR, which is defined as the ratio of CapSense signal to CapSense noise is the most important performance parameter of a CapSense sensor.
In this example, the average level of raw count in the absence of a touch is 5925 counts. When a finger is placed on the sensor, the average raw count increases to 6060 counts, which means the signal is 6060 – 5925 = 135 counts. The minimum value of the raw count in the OFF state is 5912 and the maximum value is 5938 counts. Therefore, the CapSense noise is 5938 – 5912 = 26 counts. This results in an SNR of 135 / 26 = 5.2.
The minimum SNR recommended for a CapSense sensor is 5. This 5:1 ratio comes from best practice threshold settings, which enable enough margin between signal and noise in order to provide reliable ON/OFF operation.
SmartSense is a FW algorithm that automatically sets all CapSense tuning parameters to optimum values. Advantages of SmartSense vs. Manual Tuning are:
- Reduced Design Cycle Time: The design flow for capacitive touch applications involves tuning all of the sensors. This step can be time consuming if there are many sensors in your design. In addition, you must repeat the tuning when there is a change in the design, PCB layout, or mechanical design. Auto-tuning solves these problems by setting all of the parameters automatically.
- Performance is independent of PCB variations: the Cp of individual sensors can vary due to process variations in PCB manufacturing, or vendor-to-vendor variation in a multi-sourced supply chain. If there is significant variation in CP across product batches, the CapSense parameters must be re-tuned for each batch. SmartSense sets parameters for each device automatically, hence taking care of variations in CP.
- Ease-of-use: SmartSense is faster and easier to use because only a basic knowledge of CapSense is needed.
Cypress SmartSense technology allows a device to calibrate itself for optimal performance and complete the entire tuning process automatically. This technology will meet the needs of most designs, but in cases where SmartSense does not work or there are specific SNR or power requirements, the CapSense parameters can be adjusted to meet system requirements. This is called manual tuning.
Some advantages of manual tuning, as opposed to SmartSense auto-tuning are:
- Strict control over parameter settings: SmartSense sets all of the parameters automatically. However, there may be situations where you need to have strict control over the parameters. For example, use manual tuning if you need to strictly control the time PSoC 4 takes to scan a group of sensors or strictly control the sense clock frequency of each sensor (this can be done to reduce EMI in systems).
- Supports higher parasitic capacitances: SmartSense supports parasitic capacitances as high as 45 pF for a
0.2-pF finger capacitance, and as high as 35 pF for a 0.1-pF finger capacitance. If the parasitic capacitance is higher than the value supported by SmartSense, you should use manual tuning.
To find out more in detail, please check the PSoC 4 and PSoC 6 MCU CapSense Design Guide.