AS3911 datasheet(143/159 Pages) AMSCO

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ams Datasheet

Page 143

[v2-11] 2016-Jun-13

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AS3911 − Application Information

The procedure is the following:

• Write the non-modulated level in the RFO Normal Level

Definition Register (usually it is all 0 to have the lower

possible output resistance).

• Switch on the transmitter.

• Send the direct command Measure Amplitude. Read result

from the A/D Converter Output Register.

• Calculate the target modulated level from the target

modulation index and result of the previous point.

• In the following iterations content of the RFO Normal

Level Definition Register is modified, the command

Measure Amplitude executed and result compared to the

target modulated level as long as the result is not equal

or as close as possible to the target modulated level.

• At the end the content of the RFO Normal Level Definition

written in the RFO AM Modulated Level Definition Register

while the RFO Normal Level Definition Register is restored

with the non-modulated definition value.

Antenna Tuning

The AS3911 comprises the building blocks which make possible

checking and adjustment of the antenna LC tank resonance

frequency. The AS3911 Phase and Amplitude Detector block is

used for resonance frequency checking and adjustment.

In order to implement the antenna LC tank calibration tuning

capacitors have to be connected between the two coil terminals

to the pins TRIM1_3 to TRIM1_0 and TRIM2_3 to TRIM2_0. In

case single driver is used only the pins TRIM1_3 to TRIM1_0 are

used, pins TRIM2_3 to TRIM2_0 are left open. Figure 126 depicts

connection of the trim capacitors for both, single and

differential driving for the simple case where the antenna LC

tank is directly connected to RFO pins.

The TRIM pins contain the HVNMOS switching transistors to VSS.

The on resistance of TRIM1_0 and TRIM2_0 switch transistors,

which are meant to be connected to LSB tuning capacitor is 50

typ. at 3 V VSP_D, the on resistance of other pins is binary

weighted (the on resistance of TRIM1_3 and TRIM2_3 is 6.25

typ.) The breakdown voltage of the HVNMNOS switch

transistors is 25V, which limits the maximum peak to peak

voltage on LC tank in case tuning is used.

During tuning procedure the resonance frequency is adjusted

by connecting some of the tuning 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 or Off ).

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