AS3909 datasheet(65/77 Pages) AMSCO

65 / 77 page

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

AS3909/AS3910 – 65

Appl icatio n I n f o rm a t i o n

Antenna Calibration Using Command Calibrate Antenna

The calibration of LC tank resonance frequency is automatically

done by running the direct command Calibrate Antenna. During

execution of this command the comparator at the output of

Phase Detector is used. In case the LC tank resonance frequency

is higher than the target 13.56MHz, the Phase Detector output

gets higher than VSP/2 and the comparator output is high. In

the opposite case, when the resonance frequency is lower, the

Phase Detector output gets lower than VSP/2 and the

comparator output is low.

At the beginning of the command Calibrate Antenna execution

the switches in all TRIM pads are turned off. As consequence all

the trimming capacitors are disconnected so in case the LC tank

dimensioning is correct the resonance frequency has to be

higher than the target and the comparator output has to be

high. In case the comparator output is low at this initial state

the resonance frequency is too low even when all the trimming

capacitor are disconnected and adjusting of the resonance

frequency is not possible. An error flag is set and execution of

the command is terminated.

In case the comparator output was high at the initial state, the

LSB switches (TRIM1_0 and TRIM2_0) are switched on and after

10μs state of the comparator output is checked again. This

procedure is repeated until the comparator output transitions

to low or until the final state with all switches turned on is

reached. The switch state at which the comparator output is

transitional is the one at which the LC tank is in resonance.

In case the state with all switched turned on was reached and

the comparator output is still high, the resonance frequency is

too high even when all the capacitors are connected and the

adjusting is not possible. The error flag is set.

The result of the direct command Calibrate Antenna can be

observed by reading the Antenna Calibration Register (#0E). This

switches when resonance was reached and the error flag.

After the execution of direct command Calibrate Antenna the

resonance can be checked by running the direct command

Check Antenna Resonance.

Antenna Calibration Using External Trim Register

There is also a possibility to control the position of the TRIM

switches by writing the External Trim Register (#0F). When the

bit trim_s of this register is set to 1 position of the trim switches

is controlled by bits tre_3 to tre_0. Using this register and the

direct command Check Antenna Resonance a trimming

procedure may be implemented from externally.

Another possibility of external trimming procedure is using this

resonance is adjusted by looking for operating point with the

maximum amplitude.

AS3909 数据表(PDF) 65 Page - ams AG

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