at42qt113b ATMEL Corporation, at42qt113b Datasheet - Page 4

• Manufacturer Part Number: at42qt113b
• Manufacturer: ATMEL Corporation

2.2
4
Basic Operation
AT42QT113B
The QT113B is a direct replacement for the QT113. The only circuit change required in most
cases is the reduction of the value of the Cs capacitor to compensate for the fact that the
QT113B is slightly more sensitive than the older part. A reduction by a factor of 2x is often
required, but some experimentation is necessary to ascertain the correct value of Cs.
The QT113B 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 internal to the QT113B
(Figure 2-1 on page
(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 (see
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.
Figure 2-1.
Result
Start
Internal Switching and Timing
4). A 14-bit single-slope switched capacitor analog-to-digital converter
Done
Charge
Amp
Appendix A on page
SNS2
SNS1
20).
C
s
ELECTRODE
9525B–AT42–09/10
C
x