Functional Description (Continued)
TRANSMIT SECTION
(due to encoding delay), which totals 290 ms. Any offset
voltage due to the filters or comparator is cancelled by sign
bit integration.
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-
RECEIVE SECTION
The receive section consists of an expanding DAC which
a fifth order switched-capacitor low pass filter
drives
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-
vide an input overload (t
) of nominally 2.5V peak (see
MAX
rence of FS , the data at the D input is clocked in on the
R
R
falling edge of the next eight BCLK (BCLK ) periods. At
table of Transmission Characteristics). The FS frame sync
X
R
X
the end of the decoder time slot, the decoding cycle begins,
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
and 10 ms later the decoder DAC output is updated. The
X
E
total decoder delay is
10 ms (decoder update) plus
at the next FS pulse. The total encoding delay will be ap-
X
proximately 165 ms (due to the transmit filter) plus 125 ms
110 ms (filter delay) plus 62.5 ms ((/2 frame), which gives
approximately 180 ms.
4
TI [ TEXAS INSTRUMENTS ]
