AS3900 datasheet(56/59 Pages) AMSCO

56 / 59 page

www.austriamicrosystems.com/RF-Transceivers/AS3900

Revision 1.00c

56 - 59

AS3900

Data Sheet - Application Information

9.2 Crystal Oscillators

Two external crystal oscillators are needed for the application, which need two external loading capacitors (C1-C4). The loading capacitor values

depend on the total load capacitance CL, specified for each crystal. The total load capacitance that is seen between the crystal terminals should

be equal to the specified CL, so that the crystal oscillates at the specified frequency. The crystal oscillators need to fulfill the accuracy

specification of ±120ppm. A small SMD crystal is used in the demo board.

Table 41. Recommended Crystal Oscillators

Crystal

Part Number

Frequency

Accuracy

Load capacitance Dimensions (L/W/T)

Manufacturer

9.3 Power Supply

Due to fast switching of internal blocks of the AS3900, the power supply pins need to be buffered by decoupling capacitors. These capacitors

guarantee a low impedance supply line over a wide frequency range. Fast transient currents are supplied by these decoupling capacitors and

avoid voltage spikes on the supply. A parallel circuit of 1µF, 100nF, 100pF capacitors is recommended on all supply pins (VDDRF, VDD,

VDD_OTP). Small-sized X5R or X7R ceramic capacitors are recommended.

Note: VDD_OTP is only needed in fuse mode, and requires 3.6V for burning the internal fuses. Otherwise VDD_OTP need not be connected.

9.4 Antenna

As at 27MHz the wavelength is several meters it would be very challenging to implement an electromagnetic antenna with acceptable

performances (2m communication range) and small dimensions (some cm), therefore the loop antenna based on magnetic coupling would be

preferred.

This design consists of the design of the following antennas:

PCB (1 or 2 layers) loop antenna: Dimensions: half credit card. Expected range >2m

Ferrite core antenna: Samples from different coil manufacturer are already available. Expected range >1m

Capacitive coupling antenna: The resonator consists of SMD discrete components, but PCB patches are used for capacitive coupling with

metal or human body. Expected range is some cm but becomes very big as long as the body or metal is in the proximity of the patches

Pseudo SMD electric antenna: A λ/4 antenna is wound to a coil but acts totally electric.

9.5 Tuning Circuit

Each antenna needs to be tuned for the 27.12MHz ISM band. Usually a parallel resonance circuit is applied. The inductance of the loop antenna

is brought into resonance by C10. R1 defines the quality factor Q. C9 and C11 are decoupling capacitors that are mandatory because in receive

mode the PA is shorted to GND.

For C9, C10 and C11 small-sized ceramic capacitors are recommended. COG ceramic capacitors are recommended as they retain capacitance

over wide range of voltage and temperature.

More information regarding the antenna design its tuning circuits are available in the antenna application note. The AS3900 demo board features

a PCB loop antenna with a parallel resonance tuning circuit.

The AS3900 requires following external components.

CC7V-TA1

32.768kHz

±20ppm

9pF

1.5x3.2x0.9mm

Micro Crystal

FA-128R

27.12MHz

±15ppm

8pF

2.0x1.6x0.5mm

Epson Toyocom

Table 42. External Components

Num

Component

Value

Description

Note

C1, C2

12pF

COG, 50V, 0603, Ceramic capacitor ±5%

Loading Capacitor for 32.768kHz Crystal

C1,C 2max = 36pF

C3, C4

12pF

COG, 50V, 0603, Ceramic capacitor ±5%

Loading Capacitor for 27.12MHz Crystal

C3, C4max = 36pF

C5, C7,C12

100nF

X7R, 25V, 0603, Ceramic capacitor ±10%

Supply Voltage de-coupling capacitor

C6, C8, C13

100pF

X7R, 25V, 0603, Ceramic capacitor ±10%

Supply Voltage de-coupling capacitor

C12

10µF

X5R, 16V, 0805, Capacitor ±10%

OTP Fuse Mode only

C9, C11

10pF to 47pF

COG, 50V, 0603, Ceramic capacitor ±5%

PA decoupling capacitor; antenna dependant

AS3900 Datasheet(PDF) 56 Page - ams AG

AS3900 Datasheet(HTML) 56 Page - ams AG

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59

Datasheet Download

Link URL