Friday, August 26, 2011

Capacitive Touch Sensing Layout Guidelines --- > Part 1

Capacitive Sensing

Capacitive sensing is the art of measuring a relatively very small variation of capacitance in a noisy environment.

To illustrate the principle of capacitive sensing we will use the typical simplest button implementation below but the same basic laws apply to more complex capacitive structures like sliders or wheels.

Figure shows cut view and top view of a typical capacitive button implementation. The sensor connected is a simple round copper area on top layer of the PCB. It is usually surrounded by ground for noise immunity (see § 2.3). For obvious reasons (design, isolation, robustness …) the PCB is stacked behind an overlay which usually consists in the housing of the complete system (notebook, TV, monitor, cell phone, etc) .

When no conductive object, like a finger, is close the sensor only sees an inherent capacity value CEnv created by its electrical field’s interaction with the environment, especially with ground areas. When a conductive object, like a finger, approaches the sensor the electrical field around the sensor will be modified and the total capacitance seen by the sensor increased by the finger capacitance CFinger.

The challenge of capacitive sensing is to detect this relatively small variation of CSensor (CFinger usually contributes for a few percent only) and differentiate it from environmental noise.
For this purpose, Capacitive products integrate an auto offset compensation mechanism which dynamically removes the CEnv component to extract and process CFinger only. CFinger, like any capacitance can be expressed in the formula below :

A is the common area between the two electrodes hence the common area between the finger and the sensor, typically 1cm2 for an adult finger.

d is the distance between the two electrodes hence the overlay thickness, typically 1-3mm. Overlay thickness is a compromise between mechanical/ESD robustness (the thicker the better) and power consumption (if too thick the sensitivity may need to be increased to be able to sense CFinger properly).

εo is the free space permittivity and is equal to 8.85 10e-12 F/m (constant)

εr is the dielectric hence overlay permittivity when finger is touching. Typical permittivity of some common overlay materials is given in table below. Higher εr allows reducing power consumption and/or increasing overlay thickness.

General Guidelines

The Chip
  • Strong ground connection on bottom exposed pad and ground pin
  • VANA, VDIG, VDD decoupling capacitors must be placed as close as possible to their associated pin
  • Integration capacitor Cint (see Figure 1) must be placed as close as possible to the chip and as far as possible from noisy signals

Connections to CAPx Sensors
  • 0.2 mm trace width is recommended
  • Minimum 0.2mm clearance between CAPx traces, recommended 0.5mm or above. CAPx are sensed serially by the chip, non-sensed CAPx are internally tied to ground hence the clearance requirement.
  • Connections must be as short and direct as possible.
  • Preferably not to be routed on top layer to reduce potential finger coupling (must only be maximized on the CAP sensors, not on the traces)
  • Vias number must be reduced to the minimum.
  • CAPx signals should be routed as far as possible from noisy signals (LEDs, etc) and ideally on different layer or isolated by ground.
  • Noisy signals should not be routed below CAP sensors, if needed they must be isolated with ground layer.
Ground Considerations
  • 0.5 to 3mm recommended clearance with CAP sensors. Low values maximize noise immunity while high ones minimize power consumption.
  • Below 0.5mm clearance is possible but increases significantly ground coupling hence requiring higher sensitivity setting and higher consumption.
  • Above 3mm clearance is possible but doesn’t reduce significantly ground coupling while reducing noise immunity.
  • Both top and bottom layers should be filled with hatched ground (typ. 20 %) to improve noise immunity while keeping DC cap low.

Capacitive Touch Sensing Layout Guidelines --- > Part 2

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