U2270B 데이터 시트보기 (PDF) - Atmel Corporation

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U2270B

Read/Write Base Station

Atmel Corporation 

U2270B

3.4 Low-pass Filter (LPF)

The fully integrated low-pass filter (4th-order Butterworth) removes the remaining carrier signal

and high-frequency disturbances after demodulation. The upper cut-off frequency of the LPF

depends on the selected oscillator frequency. The typical value is fOsc / 18, and data rates up to

fOsc / 25 are possible if bi-phase or Manchester encoding is used.

A high-pass characteristic results from the capacitive coupling at the input pin 4 as shown in Fig-

ure 3-6. The input voltage swing is limited to 2 Vpp. For frequency response calculation, the

impedances of the signal source and LPF input (typical 210 kΩ) have to be considered. The rec-

ommended values of the input capacitor for selected data rates are given in Section 4.,

“Applications” , on page 10.

Note: After switching on the carrier, the DC voltage of the coupling capacitor changes rapidly. When the

antenna voltage is stable, the LPF needs approximately 2 ms to recover full sensitivity.

Figure 3-6. Equivalent Circuit of Pin Input

VBias + 0.4V

RS

Input

CIN

10 kΩ

VBias - 0.4V

210 kΩ

3.5 Amplifier (AMP)

The differential amplifier has a fixed gain, typically 30. The HIPASS pin is used for DC decou-

pling. The lower cut-off frequency of the decoupling circuit can be calculated as follows:

fcut = 2-----×------π-----×-----1C-----H---P-----×------R----i

The value of the internal resistor Ri can be assumed to be 2.5 kΩ.

Recommended values of CHP for selected data rates can be found in Section 4., “Applications” ,

on page 10.

7

4684E–RFID–02/08

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