
Data Sheet
September 2001
Short-Loop Sine Wave Ringing SLIC
L9215A/G
36
Agere Systems Inc.
ac Applications
(continued)
Design Examples
(continued)
First-Generation Codec ac Interface Network—
Resistive Termination
(continued)
This is a lower feature application example and uses
single battery operation, fixed overhead, current limit,
and loop closure threshold.
Resistor R
GN
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.
Example 1, Real Termination
The following design equations refer to the circuit in
Figure 22. Use these to synthesize real termination
impedance.
Termination Impedance:
z
T
=
Receive Gain:
Transmit Gain:
Hybrid Balance:
h
bal
= 20log
h
bal
= 20log
To optimize the hybrid balance, the sum of the currents
at the VFX input of the codec op amp should be set to
0. The expression for ZHB becomes the following:
I
T/R
–
------------
z
T
50
2
+
R
P
R
GP
1
--------
R
RCV
-----------
+
+
----------------------------------
+
=
g
rcv
V
FR
-----------
=
g
rcv
1
R
T1
-----------
R
GP
-----------
+
+
1
Z
T/R
--------
+
------------------------------------------------------------------
=
g
tx
V
T/R
----------
=
g
tx
X
R
T2
--------
Z
T/R
--------
×
=
R
HB
-----------
g
tx
–
g
rcv
×
V
FR
--------------
R
HB
k
(
)
×
g
tx
g
rcv
-------------------
=
12-2554.V (F)
Figure 22. ac Equivalent Circuit
R
P
Z
T
+
–
R
P
V
T/R
I
T/R
V
S
Z
T/R
+
–
RING
A
V
= –1
A
V
= 1
VITR
CURRENT
SENSE
TIP
+
–
R
T1
R
RCV
R
HB1
R
T2
RCVN
RCVP
R
X
VGSX
VF
X
IN
V
FR
1/4 T7504 CODEC
R
GP
+2.4 V
–0.300 V/mA
A
V
= 4
L9215
VF
X
IP
18
18
+
–
+
–