L9215A/G
Short-Loop Sine Wave Ringing SLIC
Data Sheet
September 2001
ac Applications (continued)
Design Examples (continued)
Set ZTG—Gain Shaping (continued)
RTGS CGS
Rx
RGX = 4750 Ω
–IT/R
318.25
0.1 µF
RT6
–
20
+
VTX
TXI
VITR
CODEC
OP AMP
CN
RT3
RHB
RN1
RCVN
RCVP
CODEC
OUTPUT
DRIVE
AMP
RRCV
RN2
RGP
5-6400.P (F)
Figure 24. Interface Circuit Using First-Generation Codec (Blocking Capacitors Not Shown)
Transmit Gain
TX (specified[dB]) is the specified transmit gain. 600 Ω is the
impedance at the PCM, and REQ is the impedance at
Transmit gain will be specified as a gain from T/R to
PCM, TX (dB). Since PCM is referenced to 600 Ω and
assumed to be 0 dB, and in the case of T/R being refer-
enced to some complex impedance other than 600 Ω
resistive, the effects of the impedance transformation
must be taken into account.
600
tip and ring. 20log
represents the power
----------
REQ
loss/gain due to the impedance transformation.
Note in the case of a 600 Ω pure resistive termination
600
REQ
600
= 0.
---------
Again, specified complex termination impedance at T/R
is of the form:
at T/R 20log
= 20log
----------
600
Thus, there is no power loss/gain due to impedance
transformation and TX (dB) = TX (specified[dB]).
R2
Finally, convert TX (dB) to a ratio, gTX:
TX (dB) = 20log gTX
R1
C
The ratio of RX/RT6 is used to set the transmit gain:
5-6396(F)
First, calculate the equivalent resistance of this network
at the midband frequency of 1000 Hz.
RX
RT6
318.25
20
1
M
----------
= gTX • ----------------- • ---- with a quad Agere codec
REQ =
such as T7504:
2
2
2
2
2
(2 πf) C1 R1R22 + R1 + R2
2 πfR2 C1
-----------------------------------------------------------------------------
--------------------------------------------------
2
+
RX < 200 kΩ
2
2
2
2
2
1 + (2 πf) R2 C1
1 + (2 πf) R2 C1
Using REQ, calculate the desired transmit gain, taking
into account the impedance transformation:
600
TX (dB) = TX (specified[dB]) + 20log ----------
REQ
40
Agere Systems Inc.
AGERE [ AGERE SYSTEMS ]
