M5913 datasheet(6/17 Pages) STMICROELECTRONICS

6 / 17 page

Encoding

The encoder internally samples the output of the

transmit filter and holds each sample on an inter-

nal sample and hold capacitor.

The encoder then 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.

An on-chip autozero circuit corrects for DC-offset

on the input signal to the encoder. This autozero

circuit uses the sign bit averaging technique. In

this way, all DC offset is removed from the en-

coder input waveform.

RECEIVE OPERATION

Decoding

The PCM word at the DR lead is serially fetched

on the first eight data clock bits of the frame.

A D/A conversion is performed on the digital word

and the corresponding analog sample is held on

an internal sample and hold capacitor. This sam-

ple is then transferred to the receive filter.

Receive Filter

The receive section of the filter provides pass-

band flatness and stopband rejection which fulfills

both the AT&T D3/D4 specification and CCITT

recommendation G.712. The filter contains the re-

quired compensation for the (sin X)/X response of

such decoders. The receive filter characteristics

and specifications are shown in the relative table.

Receive Output Power Amplifiers

A balanced output amplifier is provided in order to

allow maximum flexibility in output configuration.

Either of the two outputs can be used single

ended (referenced to GRDA) to drive single

ended loads. Alternatively, the differential output

will drive a bridged load directly. The output stage

is capable of driving loads as low as 300 ohms

single ended to a level of 12dBm or 600 ohms dif-

ferentially to a level of 15dBm.

The receive channel transmission level may be

adjusted between specified limits by manipulation

of the GSR input. GSR is internally connected to

an analog gain setting network. When GSR is

strapped to PWRO–, the receive level is mini-

mized; when it is tied to PWRO+, the level is mini-

mized. The output transmission level interpolates

between 0 and -12dB as GSR is interpolated

(with

potentiometer)

between

PWRO-

and

PWRO+. The use of the output gain set is illus-

trated in figure 4.

Transmission levels are specified relative to the

receive channel output under digital milliwatt con-

ditions, that is, when the digital input at DR is the

eight-code sequence specified in CCITT recom-

mendation G.711.

Figure 3: Transmit Filter Gain Adjustment.

Figure 4: Gain Setting Configuration.

M5913

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