TCM29C13A Datasheet, PDF (23/25 Pages) Texas Instruments – COMBINED SINGLE-CHIP PCM CODEC AND FILTER

TCM29C13A, TCM29C14A, TCM29C16A, TCM29C17A,
TCM129C13A, TC,129C14A, TCM129C16A, TCM129C17A
COMBINED SINGLE-CHIP PCM CODEC AND FILTER
SCTS030E – AUGUST 1989 – REVISED OCTOBER 1996
PRINCIPLES OF OPERATION
transmit operation
transmit filter
The input section provides gain adjustment in the pass band by means of an on-chip uncommitted operational
amplifier. The load impedance to ground (ANLG GND) at the amplifier output (GSX) must be greater than
10 kΩ in parallel with less than 50 pF. The input signal on ANLG IN + can be either ac or dc coupled. The input
operational amplifier can also be used in the inverting mode or differential amplifier mode.
A low-pass antialiasing filter section is included on the device. This section provides 35-dB attenuation at the
sampling frequency. No external components are required to provide the necessary antialiasing function for the
switched-capacitor section of the transmit filter.
The pass-band section provides flatness and stop-band attenuation that fulfills the AT&T D3/D4 channel bank
transmission specification and CCITT recommendation G.712. The device specifications meet or exceed digital
class 5 central office switching-systems requirements.
A high-pass section configuration has been chosen to reject low-frequency noise from 50-Hz and 60-Hz power
lines, 17-Hz European electric railroads, ringing frequencies and their harmonics, and other low-frequency
noise. Even with the high rejection at these frequencies, the sharpness of the band edge gives low attenuation
at 200 Hz. This feature allows the use of low-cost transformer hybrids without external components.
encoding
The encoder internally samples the output of the transmit filter and holds each sample on an internal
sample-and-hold capacitor. The encoder performs an analog-to-digital conversion on a switched-capacitor
array. Digital data representing the sample is transmitted on the first eight data clock bits of the next frame.
The autozero circuit corrects for dc offset on the input signal to the encoder. The autozero circuit uses the
sign-bit-averaging technique. The sign bit from the encoder output is long-term averaged and subtracted from
the input to the encoder. All dc offset is removed from the encoder input waveform.
receive operation
decoding
The serial PCM word is received at PCM IN on the first eight data clock bits of the frame. Digital-to-analog
conversion is performed, and the corresponding analog sample is held on an internal sample-and-hold
capacitor. This sample is transferred to the receive filter.
receive filter
The receive section of the filter provides pass-band flatness and stop-band rejection that fulfills both the AT&T
D3/D4 specification and CCITT recommendation G.712. The filter contains the required compensation for the
(sin x)/x response of such decoders.
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