TCM38C17IDL Datasheet, PDF (16/22 Pages) Texas Instruments – QComboE FOUR-CHANNEL QUAD PCM COMBO

TCM38C17IDL
QCombo™ FOUR-CHANNEL (QUAD) PCM COMBO
SLWS040C – JUNE 1996 – REVISED OCTOBER 1999
PRINCIPLES OF OPERATION
system reliability and design considerations (continued)
precision voltage references
It is recommended that an external 1-µF capacitor be connected between REFLTR1 and AVSS and between
REFLTR2 and AVSS to ensure clean voltage references. Voltage references that determine the gain and
dynamic range characteristics of the device are generated internally. A band-gap mechanism is used to derive
a temperature-independent and bias-stable reference voltage. These references are calibrated during the
manufacturing process. Separate references are supplied to the transmit and receive sections, and each is
calibrated independently. Each reference value is then further trimmed in the gain-setting operational amplifiers
to a final precision value. Manufacturing tolerances of typically ± 0.1 dB in absolute gain (voltage amplification)
can be achieved for each half channel, providing the user a significant margin to compensate for error in other
board components.
transmit operation
The TCM38C17IDL transmit operation is described in the following paragraphs.
transmit input amplifier
The input section provides gain adjustment in the passband by means of an on-chip uncommitted operational
amplifier. Gain for the amplifier is set using external input and feedback resistors as shown in Figure 7. This
allows maximum flexibility in presetting volume levels. Unity gain can be achieved by assigning R_I and R_F equal
values. The feedback impedance between GSX and ANLGIN should be greater than 10 kΩ in parallel with less
than 50 pF. GSX also provides a means of sampling the amplified signal.
GSX
RF
RI
AREF
External
ANLGIN –
–
+
ANLGIN +
Internal
ADC
A = – ^RF
RI
Figure 7. Transmit Path Gain Setting Circuitry
transmit filter
The transmit filters provide passband flatness and stopband 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 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.
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