AM79Q4457J ETC1 [List of Unclassifed Manufacturers], AM79Q4457J Datasheet - Page 34

• Manufacturer Part Number: AM79Q4457J
• Description: Quad Subscriber Line Audio Processing Circuit-Non-Programmable (QSLAC-NP) Devices
• Manufacturer: ETC1 [List of Unclassifed Manufacturers]

CONSIDERATIONS FOR CONNECTION
TO SLICS
There are several factors to consider with the connec-
tion method used between the QSLAC-NP device and
the SLIC. The R
gain by establishing the current into the QSLAC-NP de-
vice’s I
gain in conjunction with the other SLIC circuit elements,
by establishing the current into the SLIC’s RSN pin. The
balance network provides a path for passing a repre-
sentative portion of the receive path signal back into the
transmit path for setting the transhybrid balance. Addi-
tionally, the capacitors used to provide DC isolation be-
tween the SLIC and the QSLAC device also have an
effect on system performance. Figure 13 shows the
connection scheme as described earlier in this docu-
ment. An alternative connection scheme is shown in
Figure 14. The only difference in these connection
methods is the placement of the balance network, but in
each case, there are specific factors to be considered.
Effects of C
While the purpose of the C
provide DC isolation, they have a finite impedance that
is a function of frequency. Nominal values of the R
and R
capacitors’ impedances at most voice band frequen-
cies; but at lower frequencies, the capacitor imped-
ances may have an effect. For example, a 0.1 uF
capacitor at 1000 Hz has an impedance of 1592 , but
at 300 Hz, that impedance increases to 5305
While this is still a small change compared to the re-
sistor values, it is not this change alone that may
need to be considered. For example, in Figure 14,
the I
sources feeding it: One is the Z
and the other is the series path of R
fed from the SLIC’s V
ground, so currents from the two source paths do not
effect one another. However, in Figure 13, the Z
work is connected between the C
nents. So long as the capacitor’s impedance is low, it
has little effect on signals from either Z
and both of those signal’s currents continue to flow into
the I
gins to become significant with lower frequencies, a
portion of the transmit path current from R
to flow into Z
QSLAC-NP device’s V
frequency attenuation to the transmit path signal. A
similar situation also exists in the receive path.
34
IN
IN
TX
virtual ground. As the capacitor’s impedance be-
IN
input pin of the QSLAC-NP device has two
pin. The RRX resistor controls the receive path
resistors typically are large compared to the
BAL
RX
toward the low impedance output of the
TX
and C
resistor controls the transmit path
TX
OUT
output. The I
TX
pin. The net effect is a low
RX
Capacitors
and C
BAL
TX
TX
TX
balance network,
IN
and R
and C
capacitors is to
BAL
pin is a virtual
Am79Q4457/5457 Data Sheet
or from R
TX
TX
TX
will begin
that are
compo-
BAL
net-
TX
RX
.
,
Placement of the Balance Network
The only difference in the two circuit connection meth-
ods shown in Figure 13 and Figure 14 is the placement
of the Z
both have different issues to consider for the overall
design performance. Depending on the SLIC and the
termination impedance for which balance is specified,
the circuit of Figure 13 may reduce the complexity
needed of the Z
is required. This would be especially true of short loop
applications where the actual termination in service is
a relatively constant resistance. Since in this configura-
tion the C
echo loop being canceled by Z
pendent echo responses due to their effects need not
be considered. In Figure 14, however, the capacitors
are in series with the receive and transmit paths. Since
the resistor values are unequal, the frequency rolloff
characteristics will likely be unequal without corre-
sponding changes of the C
quency dependent characteristic now implies that the
Z
components to maintain proper phase response.
SLIC Connection Consideration Summary
Two different interconnection schemes are described
in Figure 13 and Figure 14. The configuration shown in
Figure 13 may simplify the Z
even remove the need for it to contain capacitive ele-
ments, provided that the balance termination imped-
ance and SLIC characteristics are compatible. It may
be necessary in this configuration to use larger C
C
frequencies becomes a concern. The configuration
shown in Figure 14 may allow smaller transmit and re-
ceive path coupling capacitors, but may require a
slightly more complex Z
configurations from either of these figures are also pos-
sible. In any case, the designer must consider all of the
effects of SLIC characteristics, balance termination im-
pedance values, coupling capacitor values, and bal-
ance network values.
BAL
TX
capacitor values if frequency response at very low
network also contains the necessary complex
BAL
TX
network. Both methods have benefits and
and C
BAL
RX
network so that only a single resistor
capacitors are both external to the
BAL
RX
network. Variations of the
and C
BAL
BAL
network, or possibly
, the frequency de-
TX
values. This fre-
RX
or