Data Sheet
May 2001
L8576B Dual Ringing SLIC
ZTER(low) is the specified termination impedance assum-
ing low frequency (C or C´ is open).
Applications (continued)
ac Design (continued)
Receive Gain
RP is the series protection resistor.
These two equations are best solved using a computer
spreadsheet.
Ratios of RRCV, RT1, RGP will set both the low-frequency
termination and receive gain for the complex case. In
the complex case, additional high-frequency compen-
sation, via CN, RN1, and RN2, is needed for the return
loss characteristic. For resistive termination, CN, RN1,
and RN2 are not used and RCVN is tied to ground and a
resistor.
Next, solve for the high-frequency return loss compen-
sation circuit, CN, RN1, and RN2:
2RP
CN x RN2 = ------------ CTGS x RTGP
1500
Determine the receive gain, GRCV, taking into account
the impedance transformation in a manner similar to
transmit gain.
1500
RTGS
RN1 = RN2 ------------ ------------- – 1
2RP RTGP
There is an input offset voltage associated with nodes
RCVN, RCVP. To minimize the effect of the mismatch of
this voltage at T/R, the equivalent resistance to ac
ground at RCVN should be approximately equal to that
at RCVP. Refer to Figure 22 (schematic with dc blocking
capacitors). To meet this requirement, RN2 = RGP || RT1.
REQ
600
RX (dB) = RX (specified[dB]) + 20log
----------
RX (dB) = 20log GRCV
Then:
6
Hybrid Balance
GRCV = -----------------------------------------------
RRCV RRCV
1 + --------------- + ---------------
Set the hybrid cancellation via RHB.
RT1
RGP
RX
RHB = ---------------------------------
GRCV × GTX
and low-frequency termination
1500
ZTER(low) = -------------------------------------------- + 2RP
If a +5 V only codec such as an Agere T8503 is used,
dc blocking capacitors must be added as shown in
Figure 22. This is because the codec is referenced to
+2.5 V and the SLIC to ground. With the ac coupling, a
dc bias at T/R is eliminated and power associated with
this bias is not consumed.
RT1
RT1
1 + ----------- + ---------------
RGP RRCV
RTGS CTGS
RX
RTSP = 24.4 kΩ
–IT/R
CB
CN
RT2
CC1
195
–
+
AX
VITR
CODEC
OP AMP
RHB
RN1
RN2
RT1
RCVN
RCVP
RRCV
CC2
RGP
CODEC OUTPUT
DRIVE AMP
5-8413 (F)
Figure 22. ac Interface Circuit Using First-Generation Codec (Including Blocking Capacitors)
Agere Systems Inc.
23
AGERE [ AGERE SYSTEMS ]
