AS3911 datasheet(24/149 Pages) AMSCO

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AS3911 – 24

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

A p pl i c at io n I n f o r m at i o n

RSSI

The receiver also performs the RSSI (Received Signal Strength

Indicator) measurement of both channels. RSSI measurement

is started after rising edge of rx_on. It stays active while signal

rx_on is high; while rx_on is low it is frozen. It is a peak hold

system; the value can only increase from initial zero value. Every

time the AGC reduces the gain the RSSI measurement is reset

and starts from zero. Result of RSSI measurements is 4-bit value

which can be observed by reading the RSSI Display Register.

The LSB step is 2.8 dB, the maximum code is Dh (13d).

Since the RSSI measurement is of peak hold type the RSSI

measurement result does not follow any variations in the signal

strength (the highest value will be kept). In order to follow RSSI

variation it is possible to reset RSSI bits and restart the

measurement by sending direct command Clear RSSI.

Receiver in NFCIP-1 Active Communication Mode

There are several features built in receiver to enable reliable

reception of active NFCIP-1 communication. All these settings

are automatically preset by sending direct command Analog

Preset after the NFCIP-1 mode has been configured. In addition

to filtering options there are two NFC specific configuration bits

stored in the Receiver Configuration Register 3.

Bit lim enables clipping circuits which are positioned after first

and second gain stages. The intention of clipping circuits is to

limit the signal level for the following filtering stage (in case the

NFC peer is close the input signal level can be quite high).

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 M

Ω).

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-

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