Datasheet

Functional Description (Continued)
TRANSMIT SECTION
The transmit section input is an operational amplifier with
provision for gain adjustment using two external resistors,
see
Figure 4 . The low noise and wide bandwidth allow gains
in excess of 20 dB across the audio passband to be real-
ized. The op amp drives a unity-gain filter consisting of RC
active pre-filter, followed by an eighth order switched-ca-
pacitor bandpass filter clocked at 256 kHz. The output of
this filter directly drives the encoder sample-and-hold circuit.
The A/D is of companding type according to m-law
(TP3054) or A-law (TP3057) coding conventions. A preci-
sion voltage reference is trimmed in manufacturing to pro-
vide an input overload (t
MAX
) of nominally 2.5V peak (see
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 (TP3057) or
m-law (TP3054) 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/
power amplifer capable of driving a 600X load to a level of
7.2 dBm. The receive section is unity-gain. Upon the occur-
rence 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
) periods. 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 ms.
4