QT118H-IS View Datasheet(PDF) - Quantum Research Group

Part Name

Description

Manufacturer

QT118H-IS

CHARGE-TRANSFER TOUCH SENSOR

Quantum Research Group 

1 - OVERVIEW

The QT118H is a digital burst mode charge-transfer (QT)

sensor designed specifically for touch controls; it includes 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 QT118H circuit using the device,

with a conventional output drive and power supply

connections. Figure 1-2 shows a second configuration using

a common power/signal rail which can be a long twisted pair

from a controller; this configuration uses the built-in pulse

mode to transmit the output state to the host controller.

1.1 BASIC OPERATION

The QT118H employs short, ultra-low duty cycle bursts of

charge-transfer cycles to acquire its signal. Burst mode

permits power consumption in the low 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 four consecutive

confirmations of a detection before the output is activated.

The QT switches and charge measurement hardware

functions are all internal to the QT118H (Figure 1-3). 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

(Figures 4-1, 4-2, 4-3 in Specifications, rear).

Figure 1-1 Standard mode options

+2.5 to 5

1

2

Vdd

7

OUT

SNS2

3

OPT1

5

GAIN

4

OPT2

6

SNS1

OUTPUT = DC

Vss

TIMEOUT = 10 Secs

TOGGLE = OFF

8

GAIN = HIGH

SENSING

ELECTRODE

Cs

2nF - 500nF

Cx

Cs is thus non-critical; as it drifts with temperature, the

threshold algorithm compensates for the drift automatically.

A simple circuit variation is to replace Cs with a bare piezo

sounder (Section 2), which is merely another type of

capacitor, albeit with a large thermal drift coefficient. If Cpiezo

is in the proper range, no other external component is

required. If Cpiezo is too small, it can simply be ‘topped up’

with an inexpensive ceramic capacitor connected in parallel

with it. The QT118H drives a 4kHz signal across SNS1 and

SNS2 to make the piezo (if installed) sound a short tone for

75ms immediately after detection, to act as an audible

confirmation.

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 transfer 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.

The IC is highly tolerant of changes in Cs since it computes It is possible to connect separate Cx and Cx’ loads to SNS1

the threshold level ratiometrically with respect to absolute and SNS2 simultaneously, although the result is no different

load, and does so dynamically at all times.

than if the loads were connected together at SNS1 (or

SNS2). It is important to limit the

Figure 1-2 2-wire operation, self-powered

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.

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 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

lq

1

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