QPROX QT113-IS, QT113-S, QT113H-D, QT113H-IS, QT113H-S Datasheet

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APPLICATIONS -

Elevator buttons

Toys & games

Access systems

Pointing devices

Appliance control

Security systems

Light switches

Prox sensors

The QT113 charge-transfer (“QT’”) touch sensor is a self-contained digital IC capable of detecting near-proximity or touch. It will project a proxim it y sense field through air , via al m ost any dielectric, li ke glass, pl asti c, stone, cerami c, and m ost kinds of wood. It can also turn sm all m etal -bearing objects into i ntri nsic sensors, m aking them r esponsive to proximi ty or touch. Thi s capability coupled with its ability to self calibrate continuously can lead to entirely new product concepts.

It is designed specifically for human interfaces, like control panels, appliances, toys, lighting controls, or anywhere a mechanical switch or button m ay be found; it m ay also be used for some material sensing and control appl ications provided that t he presence duration of objects does not exceed the recalibration timeout interval.

The QT113 requires only a common inexpensive capacitor in order to function. Power consumption is only 600µA in most applications. I n most cases the power supply need only be minim al ly regulated, for example

by Zener diodes or an inexpensive 3-terminal regulator. The QT113’s RISC core employs 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. Even sensitivity is digitally determined and remains const ant in the face of large variations in sample capacitor C

and electrode CX. No external switches, opamps, or other

analog components aside from C

are usually required.

The option-selectable toggle mode perm its on/off touch contr ol, for example for li ght switch replacement. The Quantum-pi oneered HeartBeat™ signal is also incl uded, allowing a host m icrocontroller to m onitor the health of the QT113 continuously if desired. By using the charge transfer principle, the IC delivers a level of performance clearly superior to older technologies in a highly cost-effective package.

Quantum Research Group Ltd

R1.10/0104

! Projects a proximity field through air

! Less expensive than many mechanical switches

! Sensitivity easily adjusted via capacitor value

! Turns small objects into intrinsic touch sensors

! 100% autocal for life - no adjustments required

! 2.5 to 5V, 600µµµµA single supply operation

! Toggle mode for on/off control (strap option)

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

! Gain settings in 2 discrete levels

! HeartBeat™ health indicator on output

! Active-low (QT113) or active-high outputs (QT113H)

! Only one external part required - a 1¢ capacitor

QProx™™™™ QT113 / QT113H

HARGE-TRANSFER TOUCH SENSO

Sn s2

Vss

Sn s1

GainOpt2

Opt1

Out

Vdd 1

QT113

QT113H-IS-400C to +850C

QT113-IS-400C to +850C

QT113H-DQT113H-S0

C to +700C

QT113-DQT113-S0

C to +700C

8-PIN DIPSOICT

AVAILABLE OPTIONS

1 - OVERVIEW

The QT113 is a digital burst mode charge-transfer (QT) sensor designed specifically for touch controls ; it incl udes all hardware and signal processing functions necessary to provide stable sensing under a wide variety of changing conditions. Only a single low cost, non-critical capacitor is required for operation.

Figure 1-1 shows the basic Q T113 circuit using the device, with a conventional output drive and power supply connections.

1.1 BASIC OPERATION

The QT113 employs bursts of charge-transfer cycles to acquire its signal. Burst mode permits power consumption in the microamp range, dramatically reduces RF emissions, lowers susceptibility to EMI, and yet permits excellent response time. Internally the signals are digitally processed to reject impulse noise, using a 'consensus' filter which requires three consecutive confirmations of a detection before the output is activated.

The QT switches and charge measurement hardware functions are all i nternal to the QT113 (Figure 1-2). A 14-bi t single-slope switched capacitor ADC includes both the required QT charge and transfer switches in a configurati on 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 permitti ng longer QT bursts. The value of Cs can thus be increased to allow larger values of Cx to be tolerated (Figures 4-1, 4-2, 4-3 in Specifications, rear).

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 or alteration of several special features and sensitivity.

1.2 ELECTRODE DRIVE

The internal ADC treats Cs as a floating t r ansf er capacitor; as a direct result, the sense electrode can be connected to either SNS1 or SNS2 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 possibl e to connect separate Cx and Cx’ loads to SNS1 and SNS2 simul taneously, although the result i s no diff erent than if the loads were connected together at SNS1 (or SNS2). It is i mportant t o lim i t the amount of st r ay capacitance on both term inals, especially if t he load Cx is already large, for example by mini m izing tr ace lengths and widths so as not to exceed the Cx load specificat ion and to allow for a larger sensing electrode size if so desired.

The PCB traces, wiring, and any components associated with or in contact with SNS1 and SNS2 will become touch sensitive and should be treated with caution to li m it t he touch area to the desired location. Multiple touch electrodes can be used, for example to create a control button on bot h sides of an object, however it is impossible for the sensor to distinguish between the two touch areas.

1.3 ELECTRODE DESIGN

1.3.1 E

LECTRODE GEOMETRY AND SIZE

There is no restriction on the shape of the electrode; in most cases common sense and a little experimentation can result in a good electrode design. The QT113 will operate equally well with long, thin electrodes 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 electrode surface area, orientation with respect to the object being sensed, object composition, and the ground coupling quality of both the sensor circuit and the sensed object.

If a rel ati vel y la rge el ectr ode s urf ac e is desired, and if tests show that the electrode has more capacitance than the QT113 can tolerate, the electrode

- 2 -

Figure 1-1 Standard mode options

SENSING ELECTRODE

10nF

.5 to 5

OUT

OPT1

OPT2

GAIN

SNS1

SNS2

Vss

Vdd

OUTPUT=DC TIMEOUT=10 Secs TOGGLE=OFF GAIN=HIGH

Figure 1-2 Internal Switching & Timing

SNS2

SNS1

ELECTRODE

Single-Slope 14-bit

Switched Capacitor ADC

Charge

Amp

Burst Controller

Result

Don e

Start

can be made into a sparse mesh (Figure 1-3) havi ng lower Cx than a solid plane. Sensitivity m ay even remain the same, as the sensor will be operating in a lower region of the gain curves.

1.3.2 K

IRCHOFF’S CURRENT LAW

Like all capacitance sensors, the QT113 relies on Kirchoff’s Current Law (Figure 1-4) to detect the change in capacit ance 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 wit h regard to hardwired circuits, it applies equally to capacitive field flows. By implication it requires that the signal ground and the target object must bot h be coupled together in some manner for a capaci ti ve 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 transform er will provide ample ground coupling, since there is capacitance between the windings and/or the transform er 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.

1.3.3 V

IRTUAL CAPACITIVE GROUNDS

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 Figur e 1-4), which i s m ore than two orders of m agnitude greater than that requir ed to create a return path to the QT113 via earth. The QT113's PCB however can be physically quite sm all, so there may be little ‘free space’ coupling (Cx1 in Figure 1-4) between it and t he environment to com plete the return path. If t he QT113 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 capacit i v e ground’ can be created by connecting the QT113’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 maximi ze 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 bal l. Virt ual ground pl anes are mor e effective and can be made smaller if they are physically bonded to other surfaces, for example a wall or floor.

1.3.4 F

IELD SHAPING

The electrode can be prevented from sensing in undesired directions with the assistance of metal shielding connected to circuit gr ound ( Figure 1-5). For example, on f lat sur faces, 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 cal led ‘walk-by’ and is caused by the fact that the fields radiate from either surface of the electrode equally well. Again, shieldi ng in the form of a metal sheet or foil connected to circ uit ground will prevent walk-by; putting a small ai r gap between the grounded shield and the electrode will keep the value of Cx lower and is encouraged. In the case of the QT113, sensitivity can be high enough (depending on Cx and Cs) that 'walk-by' signals are a concern; if this is a probl em , then some form of rear shieldi ng may be required.

1.3.5 S

ENSITIVITY

The QT113 can be set for one of 2 gain levels using option pin 5 (Table 1-1). This sensiti vity change i s made by alt ering the internal num eri cal threshold level required for a detection. Note that sensitivity is also a functi on of other things: like the value of Cs, electrode size, shape, and orientation, the composition and aspect of the object to be sensed, the thickness and composi tion of any overlaying panel m aterial, and the degree of ground coupling of both sensor and object.

1.3.5.1 Increasing Sensitivity

In some cases it may be desirable to increase sensitivity further, for example when using the sensor with very thick panels having a low dielectric constant.

Sensitivity can often be increased by using a bigger electrode, reducing panel thickness, or altering panel compositi on. Increasing electrode size can have diminishi ng returns, as high values of Cx will reduce sensor gain (Figures

- 3 -

Figure 1-3 Mesh Electrode Geometry

Figure 1-4 Kirchoff's Current Law

Sense Electrode

Surround ing e nv ironm en t

SENSOR

4-1 to 4-3). The value of Cs also has a dram atic effect on sensitivity, and t his can be increased in value (up to a limit ). 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 fi eld strength, even if the fiber density is too low to make the plastic bulk-conductive.

1.3.5.2 Decreasing Sensitivity

In some cases the QT113 may be too sensitive, even on low gain. In thi s case gain can be lowered further by a num ber of strategies: making the electrode smaller, making the electrode into a sparse mesh using a high space-to-conductor ratio (Figure 1-3), or by decreasing Cs.

2 - QT113 SPECIFICS

2.1 SIGNAL PROCESSING

The QT113 processes all signals using 16 bit math, usi ng a number of algorithm s pioneered by Quantum. The algorithms are specifically designed to provide for high 'survivability' in the face of numerous adverse environmental changes.

2.1.1 D

RIFT COMPENSATION ALGORITHM

Signal drift can occur because of changes in Cx and Cs over time. It is crucial that drift be compensated for, otherwise false detections, non-detections, and sensitivity shifts will follow.

Drift com pensation (Figur e 2-1) is perform ed by making t he reference level track the raw signal at a slow rate, but only while there is no detection in effect. The rate of adjustment must be performed slowly, otherwise legitimate detections could be ignored. The QT113 drift compensates using a slew-rate limit ed 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 QT113's drift compensation is 'asymmetric': the reference level drift-com pensates 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 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 all owance 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.

Note that the positive and negative drift

compensation rates are different.

2.1.2 T

HRESHOLD CALCULATION

Unlike the QT110 device, the internal threshold level is fixed at one of two setting as determined by Table 1-1. These setting are fixed with respect to the internal reference level, which in turn can move in accordance with the drift compensation mechanism..

The QT113 employs a hysteresis dropout below the threshold level of 17% of the delta between the reference and threshold levels.

2.1.3 MAX ON-D

URATION

If an object or material obstructs the sense pad the signal may rise enough to create a detection, preventing further

- 4 -

Figure 1-5

Shielding Against Fringe Fields

Sense

wire

Sense

wire

Unshielded

Shielded

Figure 2-1 Drift Compensation

Threshold

Signal

Hysteresis

Reference

Output

Vss (Gnd)

Low - 12 counts

Vdd

High - 6 counts

Tie Pin 5 to:Gain

Table 1-1 Gain Setting Strap Options

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