TCM29C13, TCM29C14, TCM29C16, TCM29C17,

TCM129C13, TCM129C14, TCM129C16, TCM129C17

COMBINED SINGLE-CHIP PCM CODEC AND FILTER

SCTS011H – APRIL 1986 – REVISED JULY 1996

21

POST OFFICE BOX 655303

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).

_

+

_

+

A/D

D/A

ANLG

IN +

ANLG IN –

GSX

ANLG LOOP

GSR

PCM OUT

Digitized PCM

Loopback

Response

PCM IN

Digitized PCM

Test Tone

Transmit

Voice

PWRO +

PWRO –

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.