AS3911 datasheet(133/149 Pages) AMSCO

133 / 149 page

ams Datasheet, Confidential: 2013-Oct [2-03]

AS3911 – 133

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

Definition of Modulation Depth Using Bits mod5 to mod0:

The RFID standard documents usually define the AM

modulation level in form of the modulation index. The

modulation index is defined by formula (a-b)/(a+b) where a is

amplitude of the non-modulated carrier and b is the amplitude

of the modulated carrier.

The modulation index specification is different for different

standards. The ISO14443B modulation index is typically 10%

with allowed range from 8% to 14%, range from 10 to 30% is

defined in the ISO15693 and 8% to 30% in the FeliCa™ and

NFCIP-1 212 kb/s and 424 kb/s.

The bits mod5 to mod0 are used to calculate the amplitude of

the modulated level. The non-modulated level which was

before measured by the A/D converter and stored in an 8 bit

The bits mod5 to mod0 define binary decimals of this number.

Example:

In case of the modulation index 10% the modulated level

amplitude is 1.2222 times lower than the non-modulated level.

1.2222 converted to binary and truncated to 6 decimals is

1.001110. So in order to define the modulation index 10% the

bits mod5 to mod0 have to be set to 001110.

The table below depicts setting of the mod bits for some often

used modulation indexes.

Figure 122:

Setting of the mod Bits for some often used Modulation Indexes

Execution of Direct Command ‘Calibrate Modulation Depth’:

The modulation level is adjusted by increasing the RFO1 and

RFO2 driver output resistance. The RFO drivers are composed

of 8 binary weighted segments. Usually all these segments are

turned on to define the normal, non-modulated level, there is

also a possibility to increase the output resistance of the

non-modulated state by writing the RFO Normal Level

Definition Register.

Modulation Index

[%]

a/b

[dec]

a/b

[bin]

mod5………mod0

1.1739

1.001011

001011

1.2222

1.001110

001110

1.3256

1.010100

010100

1.5000

1.100000

100000

1.8571

1.110111

110111

1.9843

1.111111

111111

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