TDA7521 Datasheet, PDF (3/11 Pages) STMicroelectronics – Analog Front End

TDA7521
1.3 Laser driver section
The laser driver system is composed by the pick-up, the laser driver and the external PNP bipolar transis-
tor. It controls the external pick-up current level (up to 100mA) through its base current in order to maintain
a certain amount of diode power emission, independently from temperature and aging effects. This is done
in a digital way by using a 6-bit DAC to set the monitor diode analog reference voltage at a constant level
(and so the current in the laser diode). Thus, 26-1 different bias currents (with relative monitor voltage be-
tween 100 and 300mV) can be selected via UART interface. A negative feedback loop sets both the mon-
itor diode voltage and the laser diode bias current.
1.4 Preamplifier section
The goal of this section is to free the four voltage signals coming from either the CD pickup itself (voltage
inputs) or the internal current-to-voltage converters (current inputs) from their intrinsic DC component and
to amplify them to a level suitable for efficient A/D conversion. In case of current inputs, four transimped-
ance amplifiers convert the currents from AC (A+C), BD (B+D), E and F inputs into output voltages suitable
for the programmable preamplification chain; otherwise, this stage is by-passed and the voltage inputs are
directly connected to the preamplification stage. The two paths (for input current or input voltage) are dig-
itally selected via UART interface. In the same way also the gain of the path and the offset cancellation for
the preamplification chain are controlled (the gain programmability range is spanning from 6 up to 29.5dB
in 48 discrete steps of 0.5dB each, while the offset nulling circuit allows a minimum correction step of about
22mV via a 6-bit DAC). Moreover the preamplification chain generates an HF signal, which carries the en-
coded audio data and is obtained by combining the photo-detector outputs as A+B+C+D. All these signals
(AC, BD, E, F and HF), which can be evaluated by means of the monitor output, are fed to the ADC sec-
tion.
1.5 ADC section
The HF and servo (AC, BD, E and F) paths are digitized by means of two ADCs: the former (8-bit resolu-
tion, interleaved comparator two step architecture) samples the HF signal at a frequency of 38×4Fs (HFM)
or 192×Fs (LFM), the latter (6-bit resolution, interleaved comparator two step architecture) allows to mul-
tiplex the data for the servo path (AC, BD, E and F) in an only analog signal AC/BD/E/F and samples this
signal at 384×Fs (HFM) or 192×Fs (LFM); that means each servo signal is sampled at 96×Fs or 48×Fs).
Then both the bitstreams (HF and AC/BD/E/F) are digitally multiplexed in a single bitstream (AC/HF/BD/
HF/E/HF/F/HF). A SYNC signal (high during the period of HF before AC output) is provided in order to
point out the start of a new frame. It is worth noting that output data change on the falling edge of the mas-
ter clock.
The Table 1 shows the output data format for the ADC section: referring to AC/HF/BD/HF/E/ HF/F, the HF
signal have an 8-bit format which represents the digitized value of the HF analog signa,lwhile the data for
the servo path (AC, BD, E and F) have a different format:6 bit for the digitized value of the analog output
from the preamplifiers plus underflow and overflow⁽¹⁾.
Table 1. TDA7521 Output Format
LSB
D0
D1
D2
D3
D4
D5
D6/UF
Note: 1. Overflow and Underflow for the HF ADC are latched by a dedicated FSM and read via UART interface.
MSB
D7/OF
1.6 DAC section
In TDA7521 are present two 3rd order SC smoothing filters to be used in Digital-to-Analog conversion. Its
input signal is a bitstream created by a 2nd order digitalΣ∆ modulator present in TDA7522. From there
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