AS3909 Просмотр технического описания (PDF) - austriamicrosystems AG
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AS3909 Datasheet PDF : 77 Pages
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AS3909/AS3910 – 18
Application Information
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.
ams Datasheet, Confidential: 2013-Oct [3-02]
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