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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 depicts connection of the trim capacitors for both, single and differential driving. The TRIM pads contain the HVNMOS switch

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 50

Ω

at 3V VSP, the on resistance of other pads is binary weighted (the on resistance of TRIM1_3 and TRIM2_3 is 6.25

Ω).

Figure 10. Connection of Trimming Capacitors to the Antenna LC Tank in case of Single (left) and Differential Driving (right)

TRIM 1_0

TRIM 1_1

TRIM 1_2

TRIM 1_3

RF01

RF02

RFI 1

RFI 2

Antenna

TRIM 2_3

TRIM 2_2

TRIM 2_1

TRIM 2_0

TRIM 1_0

TRIM 1_1

TRIM 1_2

TRIM 1_3

RF01

RF02

RFI 1

RFI 2

½ Antenna

½ Antenna

TRIM 2_3

TRIM 2_2

TRIM 2_1

TRIM 2_0