L9214A/G
Low-Cost Ringing SLIC
Preliminary Data Sheet
October 2001
Hybrid Balance
ac Applications (continued)
Design Examples (continued)
Set the hybrid cancellation via RHB.
RX
grcv × gtx
------------------------
RHB =
ac Interface Using First-Generation Codec (contin-
ued)
If a 5 V only codec such as the Agere T7504 is used,
dc blocking capacitors must be added as shown in Fig-
ure 16. 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.
Receive Gain
Ratios of RRCV, RT3, 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 via a
resistor.
Typically, values of 0.1 µF to 0.47 µF capacitors are
used for dc blocking. The addition of blocking capaci-
tors will cause a shift in the return loss and hybrid bal-
ance frequency response toward higher frequencies,
degrading the lower-frequency response. The lower
the value of the blocking capacitor, the more pro-
nounced the effect is, but the cost of the capacitor is
lower. It may be necessary to scale resistor values
higher to compensate for the low-frequency response.
This effect is best evaluated via simulation. A PSPICE®
model for the L9214 is available.
Determine the receive gain, grcv, taking into account the
impedance transformation in a manner similar to trans-
mit gain.
REQ
RX (dB) = RX (specified[dB]) + 20log
RX (dB) = 20log grcv
----------
600
Design equation calculations seldom yield standard
component values. Conversion from the calculated
value to standard value may have an effect on the ac
parameters. This effect should be evaluated and opti-
mized via simulation.
Then:
4
-----------------------------------------------
RRCV RRCV
grcv =
--------------- ---------------
1 +
+
RT3
RGP
and low-frequency termination
2400
--------------------------------------------
ZTER(low) =
+ 2RP + 36 Ω
RT3
RT3
----------- ---------------
1 +
+
RGP RRCV
ZTER(low) is the specified termination impedance assum-
ing low frequency (C or C´ is open).
RP is the series protection resistor.
36 Ω is the typical internal feed resistance.
These two equations are best solved using a computer
spreadsheet.
Next, solve for the high-frequency return loss compen-
sation circuit, CN, RN1, and RN2:
2RP
CNRN2 = ------------ CG RTGP
2400
RTGS
-------------
2400
2RP
RN1 = RN2 ------------
– 1
RTGP
There is an input offset voltage associated with nodes
RCVN and RCVP. To minimize the effect of 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 16 (with dc blocking capaci-
tors). To meet this requirement, RN2 = RGP || RT3.
36
Agere Systems Inc.
AGERE [ AGERE SYSTEMS ]
