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Designing capacitive sensing for a specific application (Part 3 of 3)

(Editor's note : We are pleased to present this lengthy, multipart, hands-on article about capacitive sensing, which address solid engineering and design issues. Touch sensing and touchscreens are extremely popular topics with our audience; for your convenience, you can see a linked list of all articles we have published on this topic here.)

(To read Part 1 of this article, click here; to read Part 2 of this article, click here.)

  1. Layout

Another factor which needs to be taken into account is layout. Optimizing layout is critical to ensure reduced tuning efforts based on interactions with multiple system-level noise conditions.

Another method for avoiding noise entering a capacitive sensing system is at the hardware level. Let us understand what types of noises affect capacitive sensing system in a TV/monitor application, and how they can be avoided during designing the layout.

Capacitive systems in TV/Monitors are subjected to the following types of noises caused due to the peripheral circuits in the system.

  1. Switching power-supply noises or conducted interference
  2. LCD inverter noise
  3. Radiated interference

1. Switching power-supply noises or conducted interference:

Conducted noise is the noise current often generated by high-frequency switching circuits entering the system through the power and communication lines. The following guidelines will help in preventing conducted noise from entering a capacitive sensing system:

  • Provide GND and VDD planes that reduce current loops.
  • If the capacitive sensing controller PCB is connected to the power supply by a cable, minimize the cable length and consider using a shielded cable.
  • Place a ferrite bead around power supply or communication lines to help reduce high-frequency noise.

2. LCD Inverter noise:

In TV/Monitor applications, care must be taken care to prevent noise from LCD inverters from upsetting the capacitive sensing system. LCD inverters induce a lot of noise into a capacitive sensing system and reduce SNR drastically.

A simple technique to minimize LCD inverter noise is to partition the system with noise sources from capacitive sensing inputs, as demonstrated in Figure 15 . Due to the practical limitations of product size, the noise source and the capacitive-sensing circuitry may only be separated by a few inches. This small separation, however, can provide the extra margin required for good sensor performance compared to with close proximity between noise sources and capacitive-sensing circuitry.

 

Figure 15: Separating noise source from capacitive-sensing interface

3. Radiated Interference:

Radiated electrical energy can influence system measurements and potentially influence the operation of the capacitive sensing processor core. This interference enters the capacitive sensing chip at the PCB level, through the sensor traces, and via other digital and analog inputs.

The following methods can be used to suppress the radiated interference on capacitive sensing system:

  • In general, providing a ground plane on the PCB helps to reduce the RF noise picked up by the capacitive-sensing controller.
  • Every capacitive-sensing controller pin has some parasitic capacitance, CP, associated with it. Adding an external resistor forms a low-pass RC filter that can dampen RF-noise amplitude, Figure 16.  Series resistors should be placed within 10mm of the capacitive-sensing controller pins.

 

Figure 16: RC filter

  • Long traces can pick up more noise than short traces. Long traces also add to CP. Hence, trace length should be kept to a minimum.

In the article, we have spoken about how a capacitive-sensing system works, why capacitive sensors are fast replacing mechanical buttons, and what it takes to simplify capacitive-sensing system design. Some of the common challenges encountered in design a capacitive-sensing user interface and techniques to overcome these problems have been described.

Using the auto-tuning techniques discussed in the article, we can build a robust capacitive sensing system while significantly easing design complexity. Auto-tuning itself has various characteristics; the two main ones are compensation during run time (for noise and environment) and automatic parameter setting, both of which are critical in easing design time and thus reducing the time to market of a capacitive-sensing system.

About Authors

Subbarao Lanka is a Senior Applications Engineer working at Cypress Semiconductor Corp. on Capacitive Touch Sensing applications since 2007. His responsibilities include defining technical requirements for new capacitive sensing controllers, developing new capacitive sensing controllers, conducting system analysis, debugging technical issues for customers, and technical writing. He can be reached at slan@cypress.com.

Shruti H is an Applications Engineer working at Cypress Semiconductor on Capacitive Touch Sensing applications since 2009. She works on defining debugging technical issues for customers, technical requirements for new capacitive sensing controllers, developing new capacitive sensing controllers, conducting system analysis, and technical writing. She can be reached at .

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