AS3911B Datasheet, PDF (25/155 Pages) ams AG – NFC Initiator / HF Reader IC

Application Information
Figure 16:
Capacitive Sensor Block Diagram
Bit rg_nfc forces gain reduction of second and third filtering
stage to -6dB while keeping the digitizer comparator window
at maximum level.
Capacitive Sensor
The Capacitive Sensor block provides a possibility of low power
detection of tag presence.
The capacitive measurement system comprises two electrodes.
One is excitation electrode emitting electrical field of a fixed
frequency in range of few hundred kHz (CSO) and the second
one is the sensing electrode (CSI). The amount of charge gen-
erated in sensing electrode represents the capacitance be-
tween the two electrodes. Capacitive sensor electrodes are tol-
erant to parasitic capacitance to ground (up to 25 pF) and to
input leakage (up to 1 MΩ).
Since the charge on the sensing electrode is generated with the
frequency of excitation electrode, synchronous rectifier is used
to detect it. This ensures good rejection of interference and
high tolerance to parasitic capacitances (to all nodes except the
excitation electrode).
Capacitive sensor system depicted on figure below uses a syn-
chronous rectifier to convert the AC charge generated by the
excitation signal on the sensing electrode. This yields a DC out-
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).
CSO
Oscillator
A/D
Converter
CSI
Synchronous
Rectifier
ams Datasheet: 2014-Jun-12 [v1-08]
AS3911B – 25