U2270B_08 Datasheet, PDF (7/18 Pages) ATMEL Corporation – Read/Write Base Station

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 f_Osc / 18, and data rates up to
f_Osc / 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 V_pp. 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
V_Bias + 0.4V
R_S
Input
C_IN
10 kΩ
V_Bias - 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:
f_cut = ₂-----_×------_π-----_×-----^1C-----_H---_P-----_×------_R----_i
The value of the internal resistor R_i can be assumed to be 2.5 kΩ.
Recommended values of C_HP for selected data rates can be found in Section 4., “Applications” ,
on page 10.
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4684E–RFID–02/08