TCM29C13 Datasheet, PDF (21/25 Pages) Texas Instruments – COMBINED SINGLE-CHIP PCM CODEC AND FILTER

TCM29C13, TCM29C14, TCM29C16, TCM29C17,
TCM129C13, TCM129C14, TCM129C16, TCM129C17
COMBINED SINGLE-CHIP PCM CODEC AND FILTER
SCTS011H – APRIL 1986 – REVISED JULY 1996
PRINCIPLES OF OPERATION
analog loopback
A distinctive feature of the TCM29C14 and TCM129C14 is the analog loopback capability. With this feature, the
user can test the line circuit remotely by comparing the signals sent into the receive channel (PCM IN) with those
generated on the transmit channel (PCM OUT). The test is accomplished by sending a control signal that
internally connects the analog input and output ports. When ANLG LOOP is TTL high, the receive output
(PWRO+) is internally connected to ANLG IN +, GSR is internally connected to PWRO –, and ANLG IN – is
internally connected to GSX (see Figure 9).
ANLG IN – GSX
ANLG LOOP
Transmit
Voice
+
_
ANLG
IN +
A /D
PCM OUT
Digitized PCM
Loopback
Response
PWRO +
PWRO –
+
_
GSR
D/A
PCM IN
Digitized PCM
Test Tone
Figure 9. TCM129C14 and TCM29C14 Analog Loopback Configuration
Due to the difference in the transmit and receive transmission levels, a 0-dBm0 code into PCM IN emerges from
PCM OUT as a 3-dBm0 code, an implicit gain of 3 dB. Because of this, the maximum signal that can be tested
by analog loopback is 0 dBm0.
precision voltage references
Voltage references that determine the gain dynamic range characteristics of the device are generated internally.
No external components are required to provide the voltage references. A difference in subsurface charge
density between two suitably implanted MOS device is used to derive a temperature- and bias-stable reference
voltage, which is 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 ± 0.04 dB can
be achieved in absolute gain for each half channel, providing the user a significant margin to compensate for
error in other system components.
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