L9214A/G
Low-Cost Ringing SLIC
Preliminary Data Sheet
October 2001
Thus, it appears that 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).
ac Applications (continued)
ac Interface Network (continued)
In the receive direction, in order to control echo, the
gain is typically a loss, which requires a loss network at
the SLIC RCVN/RCVP inputs, which will reduce the
amount of gain that is available for termination imped-
ance. For this reason, a high-gain SLIC is required with
a first-generation codec.
To meet the unique requirements of both type of
codecs, the L9214 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 L9214 is
offered with a receive gain of 8. For interface with a
third-generation codec, the L9214 is offered with a
receive gain of 2. In either case, the transconductance
in the transmit direction or the transmit gain is 300 Ω,
(300 V/A).
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 point
(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 voiceband frequency, must ensure 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 resis-
tor divider, between the SLIC output and codec input.
This selection of receive gain gives the designer the
flexibility to maximize performance and minimize exter-
nal components, regardless of the type of codec cho-
sen.
Design Examples
First-Generation Codec ac Interface Network—
Resistive Termination
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. This feedback will increase
the signal at the codec input and increase the likeli-
hood that a resistor divider is needed in the transmit
direction. Further stability issues may add external
components or excessive ground plane requirements
to the design.
The following reference circuit shows the complete
SLIC schematic for interface to the Agere T7504 first-
generation codec for a resistive termination imped-
ance. For this example, the ac interface was designed
for a 600 Ω resistive termination and hybrid balance
with transmit gain and receive gain set to 0 dBm. For
illustration purposes, no PPM injection was assumed in
this example.
This is a lower feature application example and uses
single battery operation, fixed overhead, current limit,
and loop closure threshold.
In the receive direction, the issue is to optimize the
S/N. Again, the designer must consider all the 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 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 operated near maximum signal levels,
thus compromising the S/N.
Resistor RGN is optional. It compensates for any mis-
match of input bias voltage at the RCVN/RCVP inputs.
If it is not used, there may be a slight offset at tip and
ring due to mismatch of input bias voltage at the
RCVN/RCVP inputs. It is very common to simply tie
RCVN directly to ground in this particular mode of oper-
ation. If used, to calculate RGN, the impedance from
RCVN to ac ground should equal the impedance from
RCVP to ac ground.
30
Agere Systems Inc.
AGERE [ AGERE SYSTEMS ]
