QProxTM QT160 / QT161

6 K

HARGE

-T

RANSFER

OUCH

TM S

ENSOR

APPLICATIONS

Instrument panels
Gaming machines

Access systems
Pointing devices

Appliance controls
Security systems

PC Peripherals
Backlighted buttons

QT160 / QT161 charge-transfer ("QT'") QTouch ICs are self-contained digital controllers capable of detecting near-proximity or
touch from up to 6 electrodes. They allow electrodes to project 6 independent sense fields through any dielectric like glass,
plastic, stone, ceramic, and wood. They can also make metal-bearing objects responsive to proximity or touch by turning them
into intrinsic sensors. These capabilities coupled with continuous self-calibration can lead to entirely new product concepts,
adding high value to product designs.

Each of the 6 channels operate independently of the others, and each can be tuned for a unique sensitivity level by simply
changing its sample capacitor value.

The devices are designed specifically for human interfaces, like control panels, appliances, gaming devices, lighting controls,
or anywhere a mechanical switch or button may be found; they may also be used for some material sensing and control
applications. The option-selectable toggle mode permits on/off touch control, for example for light switch replacement.

The devices require only a common inexpensive capacitor per channel in order to function. The QT160 also offers the unique
adjacent key suppression (AKSTM, patent pending) feature which suppresses touch from weaker responding keys and allows
only a dominant key to detect, for example to solve the problem of large fingers on tightly spaced keys.

In most cases the power supply need only be minimally regulated, for example by an inexpensive 3-terminal regulator.

The RISC core of these devices employ signal processing techniques pioneered by Quantum; these are specifically designed
to make the device survive real-world challenges, such as `stuck sensor' conditions and signal drift.

By using the charge transfer principle, these parts deliver a level of performance clearly superior to older technologies yet are
highly cost-effective.

QT160/161 1.06/1102

6 completely independent touch circuits

Individual logic outputs per channel (active high)

Projects prox fields through any dielectric

Only one external capacitor required per channel

Sensitivity easily adjusted on a per-channel basis

100% autocal for life - no adjustments required

3-5.5V, 5mA single supply operation

Toggle mode for on/off control (strap option)

10s, 60s, infinite auto-recal timeout (strap options)

AKSTM Adjacent Key Suppression (QT160)

Less expensive per key than many mechanical switches

Eval board with backlighting - p/n E160

QT161-AS

-40

C to +105

QT161-D

C to +70

QT160-AS

-40

C to +105

QT160-D

C to +70

DIP-28

SSOP-28

AVAILABLE OPTIONS

NOTE: Pinouts are not the same!

1 - OVERVIEW

QT160/161 is a 6-channel burst mode digital charge-transfer
(QT) sensor designed specifically for touch controls; they
include all hardware and signal processing functions
necessary to provide stable sensing under a wide variety of
conditions. Only a single low cost, non-critical capacitor per
channel is required for operation.

Figure 1-1 shows the basic circuit using the device. See
Tables 7-1 and 7-2 (page 11) for pin listings. The DIP and
SOIC pinouts are not the same and serious damage can
occur if a part is mis-wired).

1.1 BASIC OPERATION

The devices employ bursts of charge-transfer cycles to
acquire signals. Burst mode permits low power operation,
dramatically reduces RF emissions, lowers susceptibility to
RF fields, and yet permits excellent speed. Internally, signals
are digitally processed to reject impulse noise using a
'consensus' filter that requires three consecutive
confirmations of detection. Each channel is measured in
sequence starting with channel 1.

The QT switches and charge measurement hardware
functions are all internal to the device (Figure 1-2). A 14-bit
single-slope switched capacitor ADC includes both the
required QT charge and transfer switches in a configuration
that provides direct ADC conversion. The ADC is designed
to dynamically optimize the QT burst length according to the
rate of charge buildup on Cs, which in turn depends on the
values of Cs, Cx, and Vdd. Vdd is used as the charge
reference voltage. Larger values of Cx cause the charge
transferred into Cs to rise more rapidly, reducing available
resolution; as a minimum resolution is required for proper
operation, this can result in dramatically reduced apparent
gain. Conversely, larger values of Cs reduce the rise of
differential voltage across it, increasing available resolution
by permitting longer QT bursts. The value of Cs can thus be
increased to allow larger values of Cx to be tolerated. The IC
is responsive to both Cx and Cs, and changes in Cs can
result in substantial changes in sensor gain.

Option pins allow the selection of several timing features.

1.2 ELECTRODE DRIVE

The devices have 6 independent channels. The internal ADC
treats Cs on each channel as a floating transfer capacitor; as
a direct result, the sense electrode can be connected to
either SNS1A or SN1B with no performance difference. In
both cases the rule Cs >> Cx must be observed for proper
operation. The polarity of the charge buildup across Cs
during a burst is the same in either case.

It is possible to connect separate Cx and Cx' loads to
SNS1A and SNS1B simultaneously, although the result is no
different than if the loads were connected together at SNS1A
(or SNS1B). It is important to limit the amount of stray
capacitance on both terminals, especially if the load Cx is
already large, for example by minimizing trace lengths and
widths so as not to exceed the Cx load specification and to
allow for a larger sensing electrode size if so desired.

Unused channels: If a channel is not used, a dummy
nominal 1nF sense capacitor of any type must be connected
to the SNS pins ensure correct operation.

The PCB traces, wiring, and any components associated
with or in contact with SNS1A and SNS1B will become touch
sensitive and should be treated with caution to limit the touch
area to the desired location. Multiple touch electrodes can be
used, for example to create a control button on both sides of
an object, however it is impossible for the sensor to
distinguish between the two touch areas.

1.3 KEY DESIGN

1.3.1 K

EOMETRY

AND

IZE

There is no restriction on the shape of the key electrode; in
most cases common sense and a little experimentation can
result in a good electrode design. The devices will operate
equally well with long, thin keys as with round or square
ones; even random shapes are acceptable. The electrode
can also be a 3-dimensional surface or object. Sensitivity is
related to the amount of surface metallization, touch contact
area, overlying panel material and thickness, and ground

coupling quality of the sensor
circuit.

If a relatively large touch area is
desired, and if tests show that
the electrode has more
capacitance than the part can
tolerate, the electrode can be
made into a sparse mesh (Figure
1-3) having lower Cx than a solid
plane.

1.3.2 B

ACKLIGHTING

EYS

Touch pads can be
back-illuminated quite readily
using electrodes with a hole in
the middle (Figure 1-4). The
holes can be as large as 4 cm in
diameter provided that the ring of
metal is at least twice as wide as
the thickness of the overlying
panel, and the panel is greater
than 1/8 as thick as the diameter
of the hole. Thin panels do not
work well with this method they

QT160/161 1.06/1102

Figure 1-1 Recommended Basic Circuit (SSOP Package)

do not propagate fields laterally very well, and will have poor
sensitivity in the middle. Experimentation is required.

Since the channels acquire their signals in time-sequence,
any of the 6 electrodes can be placed in direct proximity to
each other if desired without cross-interference.

A good example of backlighting can be found in the E160
eval board for the QT160.

1.3.3 K

IRCHOFF

URRENT

Like all capacitance sensors, these parts rely on Kirchoff's
Current Law (Figure 1-5) to detect the change in capacitance
of the electrode. This law as applied to capacitive sensing
requires that the sensor's field current must complete a loop,
returning back to its source in order for capacitance to be
sensed. Although most designers relate to Kirchoff's law with
regard to hardwired circuits, it applies equally to capacitive
field flows. By implication it requires that the signal ground
and the target object must both be coupled together in some
manner for a capacitive sensor to operate properly. Note that
there is no need to provide actual hardwired ground
connections; capacitive coupling to ground (Cx1) is always
sufficient, even if the coupling might seem very tenuous. For
example, powering the sensor via an isolated transformer
will provide ample ground coupling, since there is
capacitance between the windings and/or the transformer
core, and from the power wiring itself directly to 'local earth'.
Even when battery powered, just the physical size of the
PCB and the object into which the electronics is embedded

will generally be enough to couple a few
picofarads back to local earth.

Electrodes connected to the IC
themselves act as coupling plates back
to local ground, since when one
channel is sensing the other channels
are clamped to circuit ground.

1.3.4 V

IRTUAL

APACITIVE

ROUNDS

When detecting human contact (e.g. a
fingertip), grounding of the person is
never required. The human body
naturally has several hundred
picofarads of `free space' capacitance
to the local environment (Cx3 in Figure
1-5), which is more than two orders of
magnitude greater than that required to
create a return path to the IC via earth.
The PCB however can be physically
quite small, so there may be little `free
space' coupling (Cx1 in Figure 1-5)
between it and the environment to

complete the return path. If the circuit ground cannot be
earth grounded by wire, for example via the supply
connections, then a `virtual capacitive ground' may be
required to increase return coupling.

A `virtual capacitive ground' can be created by connecting
the IC's own circuit ground to:

(1) A nearby piece of metal or metallized housing;
(2) A floating conductive ground plane;
(3) A nail driven into a wall;
(4) A larger electronic device (to which its output might be

connected anyway).

Free-floating ground planes such as metal foils should
maximize exposed surface area in a flat plane if possible. A
square of metal foil will have little effect if it is rolled up or
crumpled into a ball. Virtual ground planes are more effective
and can be made smaller if they are physically bonded to
other surfaces, for example a wall or floor.

1.3.5 F

IELD

HAPING

The electrode can be prevented from sensing in undesired
directions with the assistance of metal shielding connected

QT160/161 1.06/1102

Figure 1-2 Internal Switching & Timing

SNS2

SNS1

ELECTRODE

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Figure 1-3 Mesh Key Geometry

Figure 1-4 Open Electrode for Back-Illumination

to circuit ground (Figure 1-6). For example, on flat surfaces,
the field can spread laterally and create a larger touch area
than desired. To stop field spreading, it is only necessary to
surround the touch electrode on all sides with a ring of metal
connected to circuit ground; the ring can be on the same or
opposite side from the electrode. The ring will kill field
spreading from that point outwards.

If one side of the panel to which the electrode is fixed has
moving traffic near it, these objects can cause inadvertent
detections. This is called `walk-by' and is caused by the fact
that the fields radiate from either surface of the electrode
equally well. Again, shielding in the form of a metal sheet or
foil connected to circuit ground will prevent walk-by; putting a
small air gap between the grounded shield and the electrode
will keep the value of Cx lower and is encouraged. In the
case of the QT160/161, sensitivity can be high enough
(depending on Cx and Cs) that 'walk-by' signals are a
concern; if this is a problem, then some form of rear
shielding may be required.

1.3.6 S

ENSITIVITY

Sensitivity can be altered to suit various applications and
situations on a channel-by-channel basis. The easiest and
most direct way to impact sensitivity is to alter the value of
Cs. More Cs yields higher sensitivity.

1.3.6.1 Alternative Ways to Increase Sensitivity
Sensitivity can also be increased by using bigger electrodes,
reducing panel thickness, or altering panel composition.

Increasing electrode size can have diminishing returns, as
high values of Cx counteract sensor gain. Also, increasing
the electrode's surface area will not substantially increase
touch sensitivity if its diameter is already much larger in
surface area than the object being detected. The panel or
other intervening material can be made thinner, but again
there are diminishing rewards for doing so. Panel material
can also be changed to one having a higher dielectric
constant, which will help propagate the field through to the
front. Locally adding some conductive material to the panel
(conductive materials essentially have an infinite dielectric
constant) will also help; for example, adding carbon or metal
fibers to a plastic panel will greatly increase frontal field
strength, even if the fiber density is too low to make the
plastic bulk-conductive.

1.3.6.2 Decreasing Sensitivity
In some cases the QT160 may be too sensitive. In this case
gain can be lowered further by a number of strategies:
a) making the electrode smaller, b) making the electrode into
a sparse mesh using a high space-to-conductor ratio (Figure
1-3), or c) by decreasing the Cs capacitors.

2 - QT160/QT161 SPECIFICS

2.1 SIGNAL PROCESSING

The QT160 processes all signals using 16 bit math, using a
number of algorithms pioneered by Quantum. The algorithms
are specifically designed to provide for high survivability in
the face of adverse environmental changes.

2.1.1 D

RIFT

OMPENSATION

LGORITHM

Signal drift can occur because of changes in Cx, Cs, and
Vdd over time. If a low grade Cs capacitor is chosen, the
signal can drift greatly with temperature. If keys are subject
to extremes of temperature and humidity, the signal can also
shift. It is crucial that drift be compensated, else false
detections, non-detections, and sensitivity shifts will follow.

Drift compensation (Figure 2-1) is a method that makes the
reference level track the raw signal at a slow rate, only while
no detection is in effect. The rate of reference adjustment
must be performed slowly else legitimate detections can also
be ignored. The IC drift compensates each channel
independently using a slew-rate limited change to the
reference level; the threshold and hysteresis values are
slaved to this reference.

Once an object is sensed, the drift compensation
mechanism ceases since the signal is legitimately high, and
therefore should not cause the reference level to change.

The signal drift compensation is 'asymmetric'; the reference
level drift-compensates in one direction faster than it does in
the other. Specifically, it compensates faster for decreasing
signals than for increasing signals. Increasing signals should
not be compensated for quickly, since an approaching finger

QT160/161 1.06/1102

Figure 1-6 Shielding Against Fringe Fields

Sense

wire

Sense

wire

Figure 1-5 Kirchoff's Current Law

Sense E lectrode

Su rro un d ing e n v iro n m e n t

SENSOR

could be compensated for partially or entirely before even
approaching the sense electrode. However, an obstruction
over the sense pad, for which the sensor has already made
full allowance for, could suddenly be removed leaving the
sensor with an artificially elevated reference level and thus
become insensitive to touch. In this latter case, the sensor
will compensate for the object's removal very quickly, usually
in only a few seconds.

With large values of Cs and small values of Cx, drift
compensation will appear to operate more slowly than with
the converse.

2.1.2 T

HRESHOLD

ALCULATION

The internal threshold level is fixed at 6 counts for all
channels. These IC's employ a fixed hysteresis of 2 counts
below the threshold (33%).

2.1.3 M

-D

URATION

If an object or material contacts a sense pad the signal may
rise enough to trigger an output, preventing further normal
operation. To prevent this `stuck key' condition, the sensor
includes a timer on each channel to monitors detections. If a
detection exceeds the timer setting, the timer causes the
sensor to perform a full recalibration (when not set to
infinite). This is known as the Max On-Duration feature.

After the Max On-Duration interval, the sensor channel will
once again function normally, even if partially or fully
obstructed, to the best of its ability given electrode
conditions. There are three timeout durations available via
strap option: 10s, 60s, and infinite (disabled) (Table 2-1).

Max On-Duration works independently per channel; a
timeout on one channel has no effect on another channel
except when the AKS feature is impacted on an adjacent
key. Note also that the timings in Table 2-1 are dependent
on the oscillator frequency: Doubling the recommended
frequency will halve the timeouts.

Infinite timeout is useful in applications where a prolonged
detection can occur and where the output must reflect the
detection no matter how long. In infinite timeout mode, the
designer should take care to be sure that drift in Cs, Cx, and
Vdd do not cause the device to `stick on' inadvertently even
when the target object is removed from the sense field.

The delay timings for max on-duration will increase if the
total duration of all bursts is greater than 33ms, i.e. an
average of 5.5ms per channel.

2.1.4 D

ETECTION

NTEGRATOR

It is desirable to suppress detections generated by electrical
noise or from quick brushes with an object. To accomplish

this, the IC's incorporate a detect integration
counter that increments with each detection until a
limit is reached, after which the output is activated.
If no detection is sensed prior to the final count, the
counter is reset immediately to zero. In the
QT160/161, the required count is 3.

The Detection Integrator can also be viewed as a
'consensus' filter, that requires three detections in
three successive bursts to create an output.

2.1.5 F

ORCED

ENSOR

ECALIBRATION

Pin 28 is a Reset pin, active-low, which in cases
where power is clean can be simply tied to Vdd. On
power-up, the device will automatically recalibrate
all 6 channels of sensing.

Pin 28 can also be controlled by logic or a microcontroller to
force the chip to recalibrate, by toggling it low for 5�s then
raising it high again.

The option pins are read by the IC once each acquisition
cycle and can be changed during operation.

2.1.6 R

ESPONSE

IME

Response time is fixed at 99ms at a 10MHz clock. Response
time can be altered by changing the clock frequency.
Doubling the recommended clock frequency to 20MHz will
halve the response time to 49ms.

Response time will become slower if the total duration of all
bursts is greater than 33ms, i.e. an average of 5.5ms per
channel.

2.2 OUTPUT FEATURES

The ICs are designed for maximum flexibility and can
accommodate most popular sensing requirements. These
are selectable using strap options on pins OPT1 and OPT2.
All options are shown in Table 2-1.

2.2.1 DC M

ODE

UTPUT

The outputs of these ICs can respond in a DC mode, where
they are active upon detection. The output will remain active
for the duration of the detection, or until the Max On-Duration
expires (if not infinite), whichever occurs first. If a max
on-duration timeout occurs first, the sensor performs a full
recalibration and the output becomes inactive until the next
detection.

2.2.2 T

OGGLE

ODE

UTPUT

This makes the sensor respond in an on/off mode like a flip
flop. It is most useful for controlling power loads, for example
in kitchen appliances, power tools, light switches, etc.

Max On-Duration in Toggle mode is fixed at 10 seconds.
When a timeout occurs, the sensor recalibrates but leaves
the output state unchanged.

2.2.3 O

UTPUT

RIVE

The outputs are active-high and can source 1mA and sink
5mA of non-inductive current. If inductive loads are used,
such as small relays, the inductances should be diode
clamped to prevent damage. When set to operate in a
proximity mode (at high gain) Out currents should be limited
to 1mA to prevent gain shifting side effects from occurring,
which happens when the load current creates voltage drops
on the die and bonding wires; these small shifts can
materially influence the signal level to cause detection
instability as described below.

QT160/161 1.06/1102

Figure 2-1 Drift Compensation

Threshold

Signal

H ysteresis

R eference

Output

Электронный компонент: QT161