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DA6181B.003

17 September, 2014

FREQUENCY SELECTION

The power down control and frequency selection

using internal antenna’s tuning capacitor switch

(RFI2) are achieved by two digital control pins

PDN1 and PDN2. The control logic is presented in

table 1.

Table 1. Frequency selection and power down control

PDN1

PDN2

RFI2 Switch

Description

High

High

Open

Power down

High

Low

Open

Antenna frequency 1

Low

High

Closed

Antenna frequency 2, RFI2 capacitor connected in parallel with antenna

Low

Low

Closed

Antenna frequency 2, RFI2 capacitor connected in parallel with antenna

If frequency 1 is selected the RFI2 switch is open

(non conductive). Antenna frequency is determined

capacitances related to antenna coil, inputs RFIP,

RFIM

and

RFI2

(see

Antenna

Tuning

Considerations below). Frequency 1 is the highest

frequency of the two selected frequencies.

If frequency 2 is selected then RFI2 switch is closed

antenna and tune it to frequency 2. Frequency 2 is

the

lowest

frequency

of

the

two

selected

frequencies.

It is recommended to switch the device to power

down for at least 50ms before switching to another

frequency. This guarantees fast startup in switching

to another frequency. During minimum 50ms power

down time the AGC capacitor voltage is completely

pulled to VDD to initialize proper startup conditions

for the AGC. Without the described proper fast

startup control the startup time can be several

minutes. With fast startup it is shortened typically to

a few seconds.

ANTENNA TUNING CONSIDERATIONS

internal

antenna

capacitor

switches

have

when switch is open. When the antenna switch is

closed the off capacitance is shorted by on

resistance of the switch and it is effectively

eliminated. Following relationships can be written

for the two tuning frequencies.

1

1

2

1

TOT

ANT

C

L

f

⋅

=

π

2

2

2

1

TOT

ANT

C

L

f

⋅

=

π