Piezo Driver Note

The piezo acoustic driver has been removed, as the duration required to operate the beeper would interfere with

the sensing interval and slow down the device.

The gain pin adjustment is via fixed thresholds slaved to the reference level. Likewise, gain strapping is

different from the QT110: instead of connecting the Gain pin to SNS1 or SNS2, Gain must be connected to

either Vdd (+) or Gnd as shown in Table 1-1 below. The E110 board does not directly support this and a wire

should be run from the centerpost of the Gain jumper strip to either switched-Vdd or Gnd, as desired.

The sensitivity of the circuit is governed also by the relative sizes of Cs and Cx. A detection is made if the

signal rises by 6 or 12 counts from the reference level depending on gain pin strapping; this amount, unlike the

QT110, is not ratiometric to the signal level and therefore the sensitivity can be altered by simply changing Cs

(and by changing gain strapping). To provide a consistent level of sensitivity, only stable types of capacitors are

recommended for Cs, such as NPO, C0G, PPS film, and certain types of polycarbonate when used over normal

room temperature ranges.

Larger values of Cs will make the sensor more sensitive, while larger amounts of Cx will desensitize it (see

Charts 1, 2). Minimizing stray Cx is crucial if high levels of sensitivity are desired. By using values of Cs around

0.47uF (470nF), proximity distances of several centimeters can easily be obtained from small electrodes.

NOTE: It is extremely important to maintain stable levels of Vdd, as the supply is used as a

reference. Minor fluctuations in Vdd WILL cause false triggers or rapid swings in gain. DO NOT use

bench power supplies or supply circuits shared with other digital functions. Ordinary 78L05 class

regulators are fine in almost all cases. The QT113 is an extremely sensitive device. Do not take

power supply issues lightly.

The QT113 has different output mode strap options from the QT110 as shown:

Vdd

Gnd

10s

Gnd

60s

Gnd

Vdd

10s

Vdd

The only change is that 'Pulse Mode' has been replaced by a DC Out mode having no recalibration timeout

(max on-duration set to infinite).

All other operating modes, specifications, and wiring should be taken from the QT110 data sheet.

Calibration and drift compensation operate in the same manner as in the QT110. 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.