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TCM37C14A Datasheet(PDF) 18 Page - Texas Instruments

Part No. TCM37C14A
Description  PCM COMBO WITH PROGRAMMABLE GAIN CONTROL
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Maker  TI [Texas Instruments]
Homepage  http://www.ti.com
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TCM37C14A Datasheet(HTML) 18 Page - Texas Instruments

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TCM37C14A, TCM37C15A
PCM COMBO WITH PROGRAMMABLE GAIN CONTROL
SLWS018B – JUNE 1996 – REVISED MAY 1998
18
POST OFFICE BOX 655303
DALLAS, TEXAS 75265
PRINCIPLES OF OPERATION
transmit operation
The transmit operation is described in the following paragraphs.
transmit filter
The input section provides gain adjustment in the passband by means of an on-chip uncommitted operational
amplifier. The load impedance to ground (AGND) at the amplifier output must be greater than 10 k
Ω in parallel
with less than 50 pF.
A low-pass antialiasing 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 band-pass section provides passband flatness and stopband attenuation that fulfills the AT&T D3/D4
channel bank transmission specification and CCITT recommendation G.712. Device specifications meet or
exceed digital class-5 central office switching systems requirements for input signals greater than –55 dBm0.
A high-pass section configuration was chosen to reject low-frequency noise from 50- 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 to be used
in systems.
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 A/D conversion on a switched-capacitor array. Digital
data representing the sample is then transmitted on the first eight data clocks bits of the next frame.
The autozero circuit corrects for dc offset on the input signal to the encoder, using the sign-bit-averaging
technique. The sign bit from the encoder output is long-term averaged and subtracted from the input to the
encoder, removing all dc offset from the encoder input waveform.
receive operation
The receive operation is described in the following paragraphs.
decoding
The serial PCM word is received at the PCMIN terminal on the first eight data clock bits of the frame. D/A
conversion is performed and the corresponding analog sample is held on an internal sample-and-hold capacitor.
The sample voltage is then transferred to the receive filter.
receive filter
The receive filter provides passband flatness and stopband 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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