LUCL9218AAR-D AGERE [Agere Systems], LUCL9218AAR-D Datasheet - Page 23

• Manufacturer Part Number: LUCL9218AAR-D
• Description: Low-Cost Line Interface
• Manufacturer: AGERE [Agere Systems]

Data Sheet
November 2001
Applications
ac Design
Receive Interface (continued)
With a third-generation codec, the line card designer
has different concerns. To design the ac interface, the
designer must first decide upon all termination imped-
ance, hybrid balances, and transmission-level points
(TLP) requirements that the line card must meet. In the
transmit direction, the only concern is that the SLIC
does not provide a signal that is too hot and overloads
the codec input. Thus, for the highest TLP that is being
designed to, given the SLIC gain, the designer, as a
function of voice band frequency, must ensure that the
codec is not overloaded. With a given TLP and a given
SLIC gain (if the signal will cause a codec overload),
the designer must insert some sort of loss, typically a
resistor divider, between the SLIC output and codec
input.
In the receive direction, the issue is to optimize S/N.
Again, the designer must consider all the considered
TLPs. The idea is, for all desired TLPs, to run the
codec at or as close as possible to its maximum output
signal, to optimize the S/N. Remember noise floor is
constant, so the hotter the signal from the codec, the
Example 1: Real Termination (First-Generation Codec)
ac equivalent circuits for real termination using a T7504 codec is shown in Figure 15.
Agere Systems Inc.
V
S
Z
T/R
Z
T
+
–
I
(continued)
V
T/R
R
R
T/R
P
P
RING
TIP
(continued)
CURRENT
SENSE
Figure 16. ac Equivalent Circuit
A
A
V
V
= –1
= 1
–
+
+
–
L9218 SLIC
–0.403 V/mA
A
3.93
V
better the S/N. The problem is, if the codec is feeding a
high-gain SLIC, either an external resistor divider is
needed to knock the gain down to meet the TLP
requirements, or the codec is not operating near maxi-
mum signal levels, thus compromising the S/N.
It appears the solution is to have a SLIC with a low
gain, especially in the receive direction. This will allow
the codec to operate near its maximum output signal
(to optimize S/N), without an external resistor divider
(to minimize cost).
Note also that some third-generation codecs require
the designer to provide an inherent resistive termina-
tion via external networks. The codec will then provide
gain shaping, as a function of frequency to meet the
return loss requirements. Further stability issues may
add external components or excessive ground plane
requirements to the design.
To meet the unique requirements of both types of
codecs, the L9218 offers two receive gain choices.
These receive gains are mask-programmable at the
factory and are offered as two different code variations.
For interface with a first-generation codec, the L9218A
is offered with a receive gain of 7.86. For interface with
a third-generation codec, the L9218G is offered with a
receive gain of 2. In either case, the transconductance
in the transmit direction, or the transmit gain is 403 .
=
VITR
–
+
RCVN
RCVP
L9218A/G Low-Cost Line Interface
R
G
R
T1
R
R
RCV
T2
R
HB1
VFR
VF
VF
X
X
IN
IP
1/4 T7504 CODEC
–
+
R
X
2.4 V
12-3581 (F).d
VGSX
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