ST70136 datasheet(8/24 Pages) STMICROELECTRONICS

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ST70136

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4 - FUNCTIONAL DESCRIPTION

4.1 - General

The ST70136 consists of the following functional

blocks:

– Transmit Signal Path

– Receive Signal Path

– Bias Voltage and Current Generation

– Digital Data Interface

– Control Serial Interface

– Tone Detector

– Power Down mode management

4.2 - Transmit Path Description

The transmit path contains the 14-bit digital to

analog converter (DAC) necessary to generate

the transmit signal from a 16-bit digital input word.

This transmit signal is then scaled by the on chip

programmable gain amplifier (TxPGA) from 0 to

-15dB in 1dB steps. The scaled output signal is

then driven off chip to the external filters and

power amplifier (PA) which drives the DMT signal

to the subscriber loop. The transmit path is fully

differential.

4.3 - Receive Path Description

The receive path contains first an attenuator

(which allows the selection between 4 attenuated

versions of the signal) followed by a programma-

ble gain amplifier (RxPGA), a 1st order low pass

anti-aliasing filter, and a 12-bit analog to digital

converter (ADC). The RxPGA gain is digitally pro-

grammable from 0 to 31dB in 1dB steps. The

receive path is fully differential.

4.4 - VCXO

The ST70136 contains the circuits required to

construct an internal VCXO. It is divided in a crys-

tal driver and an auxiliary 8 bits DAC for timing

recovery. The crystal driver is able to operate at

35.328MHz.

The DAC which is driven by the CTRLIN pin (the

input of the Serial Control Interface), provides a

current output with 8 bits resolution and can be

used to tune the crystal frequency with the help of

external components. A time constant between

DAC input and VCXOUT can be introduced (via

CTRLIN interface) and programmed with the help

of an external capacitor (on VCOCAP pin).

4.5 - Bias Voltage and Current Generation

The bias circuitry contains a bandgap voltage ref-

erence from which the converters references and

analog ground voltages are generated. This block

also generates an accurate bias current using an

external resistor.

4.6 - Digital Data Interface

To facilitate data transfer between the ST70136

and the digital data pump, a 4-bit wide serial inter-

face for the transmit and receive path is incorpo-

rated into the AFE.

This interface consists of four transmit pins

(TX[0:3]), four receive pins (RX[0:3]), and the nec-

essary control signals (CLKM, CLKWD) to trans-

mit and receive the required data.

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