AS3911 datasheet(25/149 Pages) AMSCO

25 / 149 page

ams Datasheet, Confidential: 2013-Oct [2-03]

AS3911 – 25

Appl icatio n I n f o rm a t i o n

put voltage, which is linearly proportional to the capacitance

between the excitation and sensing electrode. The output DC

voltage is converted by an AD converter in absolute mode. Re-

sult is stored in the A/D Converter Output Register (see also A/D

Converter).

Figure 16:

Capacitive Sensor Block Diagram

Any conductive object (human hand or tag's antenna windings)

approaching the two electrodes changes the capacitance

between the excitation and sensing electrode as it 'shortens'

the distance between the two by providing conductance on the

part of the path between the two electrodes.

Capacitance measurement is started by sending direct

command Measure Capacitance. The AS3911 can also be

configured to periodically wake-up and perform the

capacitance measurement. The result is compared to a stored

reference or to an average of previous measurements and in

case the difference is greater than a predefined value an IRQ is

triggered to wake-up the controller (see also Wake-up Mode).

Capacitor sensor gain can be adjusted by setting in Capacitive

Sensor Control Register. Default gain is 2.8V/pF typ., maximum

gain is 6.5V/pF typ. Since LSB of AD converter corresponds to

approximately 7.8mV, the default gain results in sensitivity of

2.8 fF/LSB (1.2 fF/LSB maximum).

Capacitance measurement duration is 200 μs, current

consumption during measurement is 1.1 mA typ. In case

capacitive measurement is performed every 100 ms in Wake-up

mode the resulting typical average consumption is 5.8μA

(3.6μA is standby consumption in Wake-up mode).

A/D

Converter

Synchronous

Rectifier

Oscillator

CSO

CSI

AS3911 Datasheet(PDF) 25 Page - ams AG