AS3910-BQFP AMSCO [austriamicrosystems AG], AS3910-BQFP Datasheet - Page 42

• Manufacturer Part Number: AS3910-BQFP
• Description: 13.56 MHz RFID Reader IC, ISO-14443 A/B
• Manufacturer: AMSCO [austriamicrosystems AG]

AS3910
Data Sheet - A p p l i c a t i o n I n f o r m a t i o n
8.16.1 Check Antenna Resonance
In case of the perfect 90º phase shift mentioned above, the Phase Detector output results in VSP/2 output voltage. A phase shift of 1% of the
carrier frequency period (3.6º) results in the output voltage change of 2% of VSP (1% phase shift results in 60mV change at VSP=3V).
During execution of the direct command Check Antenna Resonance the Phase Detector output is multiplexed on the input of A/ D converter
which is set in relative mode. 1 LSB of the A/D conversion output represents 0.13% of carrier frequency period (0.468º). The result of A/D
conversion is in case of the perfect tuning exactly in the middle of range (1000 0000 or 0111 1111).
Value higher then 1000 0000 means that phase detector output voltage is higher then VSP/2, which corresponds to case with resonance
frequency higher then target 13.56 MHz. In the opposite case, when the resonance frequency is lower then target, the result of A/D conversion is
lower then 0111 1111.
Execution of the command Check Antenna Resonance is fast and it can be used frequently to check whether system settings are correct.
8.16.2 Calibrate Antenna Resonance
In order to implement the antenna LC tank calibration binary weighted trimming capacitors have to be connected between the two coil terminals
to the pads TRIM1_3 to TRIM1_0 and TRIM2_3 to TRIM2_0. In case single driver is used only the pads TRIM1_3 to TRIM1_0 are used, pads
TRIM2_3 to TRIM2_0 are left open.
Figure 10
transistors to VSS. During trimming procedure the resonance frequency is adjusted by connecting some of the trimming capacitors to VSS and
leaving others floating.
The switches of the same binary weight are driven from the same source and are both on or off (the switches TRIM1_2 and TRIM2_2 are for
example both either on of off).
The breakdown voltage of the HVNMNOS switch transistors is 30V, which limits the maximum peak to peak voltage on LC tank in case trimming
is used. The on resistance of TRIM1_0 and TRIM2_0 switch transistors which are meant to be connected to LSB trimming capacitor is typ 50Ω
at 3V VSP, the on resistance of other pads is binary weighted (the on resistance of TRIM1_3 and TRIM2_3 is 6.25Ω).
Figure 10. Connection of Trimming Capacitors to the Antenna LC Tank in case of Single (left) and Differential Driving (right)
www.austriamicrosystems.com/HF_RFID_Reader/AS3910
TRIM 1_0
TRIM 1_1
TRIM 1_2
TRIM 1_3
TRIM 2_3
TRIM 2_2
TRIM 2_1
TRIM 2_0
depicts connection of the trim capacitors for both, single and differential driving. The TRIM pads contain the HVNMOS switch
RF01
RF02
RFI 1
RFI 2
Antenna
Revision 2.3
TRIM 1_0
TRIM 1_1
TRIM 1_2
TRIM 1_3
TRIM 2_3
TRIM 2_2
TRIM 2_1
TRIM 2_0
RF01
RF02
RFI 1
RFI 2
½ Antenna
½ Antenna
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