Datasheet
H(z) +
N0 ) N1 z
*1
32768 * D1 z
*1
TLV320AIC3107
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SLOS545D –NOVEMBER 2008–REVISED DECEMBER 2014
power down, the PGA soft-steps the volume to mute before shutting down. A read-only flag is set whenever the
gain applied by PGA equals the desired value set by the register. The soft-stepping control can also be disabled
by programming a register bit. When soft stepping is enabled, the audio master clock must be applied to the part
after the ADC power down register is written to ensure the soft-stepping to mute has completed. When the ADC
powerdown flag is no longer set, the audio master clock can be shut down.
10.3.3.2.1 Stereo Audio ADC High Pass Filter
Often in audio applications it is desirable to remove the DC offset from the converted audio data stream. The
TLV320AIC3107 has a programmable first order high pass filter which can be used for this purpose. The Digital
filter coefficients are in 16-bit format and therefore use two 8-bit registers for each of the three coefficients of N0,
N1, and D1. The transfer function of the digital high pass filter is of the form:
(5)
Programming the Left channel is done by writing to Page 1, Registers 65-70, and the right channel is
programmed by writing to Page 1, Registers 71-76. After the coefficients have been loaded, these ADC high
pass filter coefficients can be selected by writing to Page 0, Register 107, D7-D6, and the high pass filter can be
enabled by writing to Page 0, Register 12, bits D7-D4.
10.3.3.2.2 Automatic Gain Control (AGC)
An automatic gain control (AGC) circuit is included with the ADC and can be used to maintain nominally constant
output signal amplitude when recording speech signals (it can be fully disabled if not desired). This circuitry
automatically adjusts the PGA gain as the input signal becomes loud or weak, such as when a person speaking
into a microphone moves closer or farther from the microphone. The AGC algorithm has several programmable
settings, including target gain, attack and decay time constants, noise threshold, and maximum PGA gain
applicable that allow the algorithm to be fine tuned for any particular application. The algorithm uses the absolute
average of the signal (which is the average of the absolute value of the signal) as a measure of the nominal
amplitude of the output signal.
Note that completely independent AGC circuitry is included with each ADC channel with entirely independent
control over the algorithm from one channel to the next. This is attractive in cases where two microphones are
used in a system, but may have different placement in the end equipment and require different dynamic
performance for optimal system operation.
10.3.3.2.2.1 Target Level
Target Level represents the nominal output level at which the AGC attempts to hold the ADC output signal level.
The TLV320AIC3107 allows programming of eight different target levels, which can be programmed from –5.5
dB to –24 dB relative to a full-scale signal. Since the device reacts to the signal absolute average and not to
peak levels, it is recommended that the target level be set with enough margin to avoid clipping at the occurrence
of loud sounds.
10.3.3.2.2.2 Attack Time
Attack Time determines how quickly the AGC circuitry reduces the PGA gain when the input signal is too loud. It
can be varied from 7 ms to 1,408 ms. The extended Right Channel Attack time can be programmed by writing to
Page 0, Registers 103, and Left Channel is programmed by writing to Page 0, Register 105.
10.3.3.2.2.3 Decay Time
Decay Time determines how quickly the PGA gain is increased when the input signal is too low. It can be varied
in the range from 0.05 s to 22.4 s. The extended Right Channel Decay time can be programmed by writing to
Page 0, Registers 104, and Left Channel is programmed by writing to Page 0, Register 106.
The actual AGC decay time maximum is based on a counter length, so the maximum decay time will scale with
the clock set up that is used. The table below shows the relationship of the NADC ratio to the maximum time
available for the AGC decay. In practice, these maximum times are extremely long for audio applications and
should not limit any practical AGC decay time that is needed by the system.
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