5

ATA5282 [Preliminary]

4694B–AUTO–06/04

AGC Amplifier

Each of the three input stages contain an AGC amplifier to amplify the input signal from

the Coil. The gain is adjusted by the automatic gain control circuit if a preamble signal is

detected. The high dynamic range of the AGC amplifier enables the IC to work with input

signals from 2.8 mV

output delivers a 125-kHz signal with an amplitude adjusted for the following evaluation

circuits (preamble detection, signal conditioner, wake-up).

Automatic Gain Control

For correct demodulation, the signal conditioner needs an appropriate internal signal

amplitude. To control the input signal, the ATA5282 has a built-in digital AGC for each

input channel. This gain control circuit regulates the internal signal amplitude to the ref-

erence level (Ref2, Figure 4 on page 6). The gain control uses the signal of the input

channel with the highest amplitude for the regulation as well as signal for the signal

conditioner.

During the preamble, each period of the carrier signal decreases the gain if the internal

signal exceeds the reference level. If the signal does not achieve the reference level,

each period increases the gain. After 192 preamble periods, the standard gain control

mode is activated. In this mode, the gain is decreased every two periods if the internal

signal exceeds the reference level and increased every eight periods if the reference

level is not achieved. These measures assure that the input signal’s envelope deforma-

tion will be minimized.

During the gaps between signal bursts, the gain control is frozen to avoid that the gain

be modified by noise signals.

The tuning range of the AGC is subdivided into 256 regulator steps. The settling time for

the full tuning range requires 320 periods (192 + (2

× 64) periods) during a preamble

phase. To accelerate the settling time, fast gain control mode can be activated via the

serial interface. In this mode, the tuning range is subdivided into 128 steps and the set-

tling time is two times faster.

In standby listen mode, the gain is reset to the maximum value. A proper carrier signal

activates the automatic gain control.

The preamble (Figure 9 on page 10) with up to 320 periods of the 125 kHz magnetic

field is used to control the gain of the input amplifiers. To detect the starting point of the

header, the start gap should not exceed 256 µs (32 periods of 125 kHz).