Data Sheet
May 2001
L8576B Dual Ringing SLIC
Applications (continued)
ac Design (continued)
RTGS CTGS
Rx
RTGP = 24.4 kΩ
–IT/R
195
RT2
–
AX
+
VITR
CODEC
OP AMP
CN
RT1
RHB
RN1
RN2
RCVN
RCVP
CODEC
RRCV
OUTPUT
DRIVE
AMP
RGP
5-6400a(F)
Figure 21. ac 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
the T/R. 20log ---------- represents the power loss/gain
REQ
due to the impedance transformation.
Again specified complex termination impedance at T/R
is of the form:
Note in the case of a 600 Ω pure resistive termination
600
at T/R 20log ---------- = 20log --------- = 0.
REQ 600
600
R2
R1
Thus, there is no power loss/gain due to impedance
transformation and TX (dB) = TX (specified[dB]).
C
5-6396(F)
Finally, convert TX (dB) to a ratio, GTX:
TX (dB) = 20log GTX
First calculate the equivalent resistance of this network
at the midband frequency of 1000 Hz.
REQ =
The ratio of RX/RT2 is used to set the transmit gain:
2
2
2
2
2
(2 πf) C1 R1R22 + R1 + R2
----------------------------------------------------------------------------- + --------------------------------------------------
2
2 πfR2 C1
195
---------- = GTX x ---------
RX
RT2
2
2
2
2
2
1 + (2 πf) R2 C1
1 + (2 πf) R2 C1
M
Using REQ, calculate the desired transmit gain, taking
into account the impedance transformation:
with a dual Agere codec such as T8503
RX < 200 kΩ
600
REQ
TX (dB) = TX (specified[dB]) + 20log
----------
22
Agere Systems Inc.
AGERE [ AGERE SYSTEMS ]
