AS3909 datasheet(18/77 Pages) AMSCO

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AS3909/AS3910 – 18

ams Datasheet, Confidential: 2013-Oct [3-02]

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

A/D Converter

The AS3909/10 contains an 8 bit successive approximation A/D

converter. Input to A/D converter can be multiplexed from

different sources to be used in several direct commands and

adjustment procedures. The result of last A/D conversion is

stored in a register which can be read through the SPI interface

(address #0D). Typical conversion time is 12μs.

The A/D converter has two operating modes, absolute and

relative.

In absolute mode the low reference is 0V and the high reference

is 2V. This means that A/D converter input range is from 0 to 2V,

00 code means input is 0V or lower, FF means that input is 2V

or higher, LSB is 7.8125mV.

In relative mode low reference is 1/6 of VSP and high reference

is 5/6 of VSP, so the input range is from 1/6VSP to 5/6VSP.

Relative mode is only used in phase measurement in which

phase detector output is proportional to power supply. In all

other cases absolute mode is used.

The A/D converter input can also be accessed externally. When

the direct command AD Convert is sent, an A/D conversion of

voltage present on pin AD_IN is performed in absolute mode,

result is stored in A/D Converter Output Register. AD_IN pin

should be left non-connected in case A/D conversion is not

needed in application.

Phase and Amplitude Detector

Phase Detector

The phase detector is observing phase difference between the

transmitter output signals (RFO1 and RFO2) and the pad signals

RFI1 and RFI2, which are proportional to the signal on the

antenna LC tank. These signals are first passed by digitizing

comparators. Digitized signals are processed by a phase

detector. Filter characteristics of the phase antenna are adapted

to one of the two possible operation modes. For antenna tuning

check, a strong low power filter is used to get average phase

difference, for PM demodulation a low pass filter having 1MHz

corner frequency is used to pass the subcarrier frequency.

Antenna Tuning Check

The Phase Detector output reflects phase relationship between

the two inputs. The 90º phase shift (ideal antenna LC tank

tuning) results in VSP/2 output voltage. In case the antenna LC

tank is detuned, phase shift changes which results in different

phase detector output voltage. In case of command Check

Antenna Resonance phase detector output is applied to A/ D

converter in relative mode. Output of phase detector is also

observed by comparator with reference signal VSP/2. Output of

this comparator is used in execution of direct command

Calibrate Antenna.

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