Datasheet

Functional Description (Continued)
table of Transmission Characteristics). The FS
X
frame sync
pulse controls the sampling of the filter output, and then the
successive-approximation encoding cycle begins. The 8-bit
code is then loaded into a buffer and shifted out through D
X
at the next FS
X
pulse. The total encoding delay will be ap-
proximately 165 ms (due to the transmit filter) plus 125 ms
(due to encoding delay), which totals 290 m s. Any offset
voltage due to the filters or comparator is cancelled by sign
bit integration.
RECEIVE SECTION
The receive section consists of an expanding DAC which
drives a fifth order switched-capacitor low pass filter
clocked at 256 kHz. The decoder is A-law (TP3067) or
m-law (TP3064) and the 5th order low pass filter corrects for
the sin x/x attenuation due to the 8 kHz sample/hold. The
filter is then followed by a 2nd order RC active post-filter
with its output at VF
R
O. The receive section is unity-gain,
but gain can be added by using the power amplifiers. Upon
the occurrence of FS
R
, the data at the D
R
input is clocked in
on the falling edge of the next eight BCLK
R
(BCLK
X
) peri-
ods. At the end of the decoder time slot, the decoding cycle
begins, and 10 ms later the decoder DAC output is updated.
The total decoder delay is
E
10 ms (decoder update) plus
110 ms (filter delay) plus 62.5 ms((/2 frame), which gives
approximately 180 m s.
RECEIVE POWER AMPLIFIERS
Two inverting mode power amplifiers are provided for direct-
ly driving a matched line interface transformer. The gain of
the first power amplifier can be adjusted to boost the
g
2.5V
peak output signal from the receive filter up to
g
3.3V peak
into an unbalanced 300X load, or
g
4.0V into an unbal-
anced 15 kX load. The second power amplifier is internally
connected in unity-gain inverting mode to give 6 dB of signal
gain for balanced loads.
Maximum power transfer to a 600X subscriber line termina-
tion is obtained by differentially driving a balanced trans-
former with a
S 2:1 turns ratio, as shown in Figure 4 . A total
peak power of 15.6 dBm can be delivered to the load plus
termination.
ENCODING FORMAT AT D
X
OUTPUT
TP3064
TP3067
m-Law
A-Law
(Includes Even Bit Inversion)
V
IN
ea
Full-Scale 1 000000010101010
V
IN
e
0V 1 111111111010101
Ð0 111111101010101
V
IN
eb
Full-Scale 0 000000000101010
4