ADAV803ASTZ datasheet(16/56 Pages) AD

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ADAV803

Rev. 0 | Page 16 of 56

ALC)

n be

aximum front end gain.

olume

ADC is still present at the output of the ADC, but scaled by a

value determined by the volume control register.

The ALC block has two functions, attack mode and recovery

mode. Recovery mode consists of three settings: no recovery,

normal recovery, and limited recovery. These modes are

discussed in the following sections. Figure 26 is a flow diagram

of the ALC block. When the ALC has been enabled, any changes

made to the PGA or ALC settings are ignored. To change the

functionality of the ALC, it must first be disabled. The settings

can then be changed and the ALC re-enabled.

Attack Mode

When the absolute value of the ADC output exceeds the level

set by the attack threshold bits in ALC Control Register 2, attack

mode is initiated. The PGA gain for both channels is reduced by

one step (0.5 dB). The ALC then waits for a time determined by

the attack timer bits before sampling the ADC output value

again. If the ADC output is still above the threshold, the PGA

gain is reduced by a further step. This procedure continues until

the ADC output is below the limit set by the attack threshold

bits. The initial gains of the PGAs are defined by the ADC left

PGA gain register and the ADC right PGA gain register and

they can have different values. The ALC subtracts a common

gain offset to these values. The ALC preserves any gain

difference in dB as defined by these registers. At no time do the

PGA gains exceed their initial values. The initial gain setting,

therefore, also serves as a maximum value.

The limit detection mode bit in ALC Control Register 1

determines how the ALC responds to an ADC output that

exceeds the set limits. If this bit is a 1, then both channels must

exceed the threshold before the gain is reduced. This mode can

be used to prevent unnecessary gain reduction due to spurious

noise on a single channel. If the limit detection mode bit is a 0,

the gain is reduced when either channel exceeds the threshold.

is not recovered until the ALC has been reset, either

on is

in reduction unnecessarily.

ormal Recovery Mode

Normal recovery mode allows for the PGA gain to be recovered,

provided that the input signal meets certain criteria. First, the

ALC must not be in attack mode, that is, the PGA gain has been

reduced sufficiently such that the input signal is below the level

set by the attack threshold bits. Second, the output result from

the ADC must be below the level set by the recovery threshold

bits in the ALC control register. If both of these criteria are met,

the gain is recovered by one step (0.5 dB). The gain is

incrementally restored to its original value, assuming that the

ADC output level is below the recovery threshold at intervals

determined by the recovery time bits.

If the ADC output level exceeds the recovery threshold while

the PGA gain is being restored, the PGA gain value is held and

does not continue restoration until the ADC output level is

again below the recovery threshold. Once the PGA gain is

restored to its original value, it is not changed again unless the

ADC output value exceeds the attack threshold and the ALC

then enters attack mode. Care should be taken when using this

mode to choose values for the attack and recovery thresholds

that prevent excessive volume modulation caused by continuous

gain adjustments.

Limited Recovery Mode

Limited recovery mode offers a compromise between no recov-

ery and normal recovery modes. If the output level of the ADC

exceeds the attack threshold, then attack mode is initiated.

When attack mode has reduced the PGA gain to suitable levels,

the ALC attempts to recover the gain to its original level. If the

ADC output level exceeds the level set by the recovery threshold

bits, a counter is incremented (GAINCNTR). This counter is

incremented at intervals equal to the recovery time selection, if

the ADC has any excursion above the recovery threshold. If the

counter reaches its maximum value, determined by the

GAINCNTR bits in ALC Control Register 1, the PGA gain is

deemed suitable and no further gain recovery is attempted.

Whenever the ADC output level exceeds the attack threshold,

attack mode is reinitiated and the counter is reset.

Automatic Level Control (

The ADC record channel features a programmable automatic

level control block. This block monitors the level of the ADC

output signal and automatically reduces the gain, if the signal

the input pins causes the ADC output to exceed a preset limi

This function can be useful to maximize the signal dynamic

range when the input level is not well defined. The PGA ca

used to amplify the unknown signal, and the ALC reduces the

gain until the ADC output is within the preset limits. This

results in m

Because the ALC block monitors the output of the ADC, the

volume control function should not be used. The ADC v

control scales the results from the ADC, and any distortion

caused by the input signal exceeding the input range of the

No Recovery Mode

By default, there is no gain recovery. Once the gain has bee

reduced, it

by toggling the ALCEN bit in ALC Control Register 1 or by

writing any value to ALC Control Register 3. The latter opti

more efficient, because it requires only one write operation to

reset the ALC function. No recovery mode prevents volume

modulation of the signal caused by adjusting the gain, which

can create undesirable artifacts in the signal. The gain can be

reduced but not recovered. Therefore, care should be taken that

spurious signals do not interfere with the input signal, because

these might trigger a ga

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