1 - OVERVIEW

The QT118HA is intended to replace the QT118H as a lower

cost alternative. This device functions identically to the QT118H,

except that it is more sensitive. To compensate for the sensitivity

increase, it is required to do either of these two things:

1. Increase the Cx loading to ground on SNS2 by 10pF

2. Decrease the Cs value

Option 1 is very simple and guarantees that the sensitivity of the

QT118HA is identical to the older device. Option 2 requires

some trial and error to test the sensitivity of the touch pad or

prox field, so that it is about the same as before. Cs changes

ranging from 10 - 60% may be required depending on the circuit

layout and electrode design.

All other aspects of this datasheet are identical to the QT118H

datasheet except for this section and specifications on pages 9

and 10, and the part marking.

1.1 BASIC OPERATION

The QT118HA employs short, low duty cycle bursts of QT cycles

to acquire capacitance. 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 QT118HA (Figure 1-3). A single-slope

switched capacitor ADC includes both the required QT charge

and transfer switches in a configuration that provides direct ADC

conversion. The sensitivity depends on the values of Cs, Cx,

and to a smaller degree, Vdd. Vdd is used as the charge

reference voltage.

Higher values of Cs increase gain; higher values of Cx load

reduce it. The value of Cs can thus be increased to allow larger

values of Cx to be tolerated (Figures 4-1 and 4-2, page 10).

Piezo sounder drive:

The QT118HA can drive a piezo sounder

after a detection for feedback. The piezo sounder replaces or

augments the Cs capacitor; this works since piezo sounders are

also capacitors, albeit with a large thermal drift coefficient. If

too small, it can simply be ‘topped up’ with a ceramic capacitor

in parallel. The QT118HA 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 in theory be connected to

either SNS1 or SNS2 with no performance difference. However,

the noise immunity of the device is improved by connecting the

electrode to SNS2, preferably via a series resistor Re (Figure

1-1) to roll off higher harmonic frequencies, both outbound and

inbound.

In order to reduce power consumption and to assist in

discharging Cs between acquisition bursts, a 470K series

resistor Rs should always be connected across Cs (Figure 1-1).

The rule Cs >> Cx must be observed for proper operation.

Normally Cx is on the order of 10pF or so, while Cs might be

10nF (10,000pF), or a ratio of about 1:1000.

It is important to minimize the amount of unnecessary stray

capacitance Cx, for example by minimizing trace lengths and

widths and backing off adjacent ground traces and planes so as

keep gain high for a given value of Cs, 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

.

l

l

l

q

q

q

2

QT118HA_AR1.02_0408

Figure 1-1 Standard mode options

SENSING

ELECTRODE

Cs

Rs

2nF - 500nF

3

4

6

5

1

+2.5 ~ +5

7

2

OUT

OPT1

OPT2

GAIN

SNS1

SNS2

Vss

Vdd

OUTPUT = DC

TIMEOUT = 10 Secs

TOGGLE = OFF

GAIN = HIGH

8

+2 ~ +5

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

+

10µF

1N4148

n-ch Mosfet

CMOS

LOGIC

3.5 - 5.5V

1K

Twisted

pair

8

OUT

OPT1

OPT2

GAIN

SNS1

SNS2

Vss

Vdd

3

4

6

5

1

7

2

SENSING

ELECTRODE