Atmel QTAN0087 Application Note

Proximity Design Guide

1. Introduction

The ability to interact with an electronic device, while not physically making contact
with the device, has fascinated designers and users alike for years. There are a
variety of ways to implement this type of technology: IR, magnetic, optical, ultrasonic,
and capacitive. Each of these technologies comes with its own unique benefits and
trade-offs.

Capacitive sensing technologies have the general advantage of achieving very
reliable proximity detection with low power, low cost, and relatively easy design.

®

Atmel
250 mm.

has the advantage of being able to do all of these with ranges to over

Proximity
Design Guide

Application Note

Capacitive sensing generates an electric field, or E-field, as part of the sensing
process; this applies to both self-capacitance (QTouch
mutual-capacitive sensors (QMatrix
the sensitivity of the standard capacitive sensing circuitry or algorithm.

This can be applied to touch sensor designs using an Atmel application-specific
device, or an Atmel microcontroller with the appropriate QTouch Library linked to your
application code.

Adding proximity detection to a design provides many benefits:

• A more intuitive user interface

• Power savings – The ability to have an application start/stop based on a user’s
proximity to a device. The device can stay in sleep mode until a presence is
detected, reducing power consumption and extending battery life.

• Make the application interact with human presence:

– Mobile phone – use proximity sensing to reduce RF power when a mobile

phone is placed near a person's head

– Heating controller – use proximity sensing to activate control panel backlighting

when a person approaches

This application note gives advice about the current capacitive touch technologies
offered by Atmel, with respect to their usefulness as proximity sensors, and a
mechanism to contain and control the E-field generated by the sensors.

®

). Proximity detection is achieved by adjusting

®

and QTouchADC®) and

QTAN0087

10760B–AT42–03/12

2. Sensing Technology

Measuring

Circuit

Sensor

Measuring

Circuit

Sensor

2.1 Introduction

The measurement circuit uses the Atmel patented charge-transfer (QT) technology to measure
changes to a sensor as an object approaches. There are three sensing technologies available
from Atmel:

• QTouch – Self capacitance, measured using Vih of the sensor input/output (IO) pin

• QTouchADC – Self capacitance, measured using an internal Successive Approximation
Register (SAR) analog-to-digital converter (ADC)

• QMatrix – Mutual capacitance, measured using an internal counter and comparator

Figure 2-1. Self Capacitance – QTouch and QTouchADC

Figure 2-2. Mutual Capacitance – QMatrix

2

Proximity Design Guide

10760B–AT42–03/12

2.2 QTouch and QTouchADC

QTouch and QTouchADC devices charge a sense electrode of unknown capacitance to a
known potential. The electrode is typically a copper area on a printed circuit board (PCB). The
resulting charge is transferred into a measurement circuit. By measuring the charge after one or
more charge-and-transfer cycles, the capacitance of the sense plate can be determined. Placing
a finger on or near the touch surface introduces external capacitance that affects the flow of
charge at that point. This registers as a touch.

Figure 2-3. QTouch and QTouchADC E-field

Proximity Design Guide

10760B–AT42–03/12

Both QTouch and QTouchADC use the self capacitance of a sensor. The E-field associated with
self capacitance is projected away from the sensor into the air. The E-field seeks to couple to
ground or an object in close proximity to sensor E-field.

• Electric field lines are projected into the air away from the sensor in an isotropic pattern (a
field generated uniformly in all directions)

• Approximately 180 mm of proximity detection is possible with QTouch

• Greater than 250 mm of proximity detection is possible with QTouchADC

• Both parallel and sequential sensor measurements are possible

• QTouch requires two processor pins, one optional series resistor (used to reduce emissions
conducted noise) and one capacitor per sensor

• QTouchADC requires one processor pin and one optional resistor (used to reduce emissions
and conducted noise) per sensor

• Sensors are single layer and are not shape constrained

3

2.3 QMatrix

QMatrix uses the mutual capacitance of a sensor. The E-field associated with mutual
capacitance is coupled closely to the X and Y portions of the sensors. The E-field couples to an
object in close proximity to the sensor's E-field.

Figure 2-4. QMatrix E-field

QMatrix uses a pair of sensing electrodes for each channel. One is an emitting electrode into
which a charge consisting of logic pulses is driven in burst mode. The other is a receive
electrode that couples to the emitter via the overlying panel dielectric. When a finger touches the
panel the field coupling is reduced, and touch is detected.

• Field lines are tightly coupled inside the dielectric (XY sensor pattern). Not much of the field
is projected into free space

• Approximately 50 mm of proximity detection is possible

• Useful in high humidity or damp environments where water droplets and moisture may collect
over the sensors

• Sensors are arranged in a X by Y format, lowering pin count for applications that require a
large number of sensors

• Sensors are dual layer and not shape constrained. However, the gap between X and Y layers
is dependent upon the dielectric material and its thickness

4

Proximity Design Guide

10760B–AT42–03/12

3. Proximity Sensing

3.1 QTouch

Figure 3-1. QTouch Proximity Sensing

Proximity Design Guide

QTouch proximity sensors are touch keys deliberately made over-sensitive. This can be
accomplished using a combination of the following:

• Make the sensing electrode larger

• Increase the value of the sampling capacitor Cs

• Decrease the touch detection threshold

• Reduce ground loading effects

10760B–AT42–03/12

5

3.2 QTouchADC

Figure 3-2. QTouchADC Proximity Sensing

Similar to QTouch, QTouchADC proximity sensors are touch keys made over-sensitive. This can
be accomplished using a combination of the following:

• Make the sensing electrode larger

• Increase sampling resolution or over-sampling rate

• Decrease the touch detection threshold

• Reduce ground loading effects

QTouchADC is the better than QTouch for proximity sensing due to its digital oversampling
feature.

6

Proximity Design Guide

10760B–AT42–03/12