T8531-TL-DB Agere Systems, Inc., T8531-TL-DB Datasheet - Page 45

• Manufacturer Part Number: T8531-TL-DB
• Description: Multichannel programmable codec chip set. Dry pack tray.
• Manufacturer: Agere Systems, Inc.

November 2000
Applications
Common Voltage Reference
Every channel of the T8532 codec requires a 2.4 V ref-
erence (VRTX) for operation. Some SLICs provide this
reference for the codec. An external circuit is required
for SLICs without the reference voltage.
Lucent Technologies Inc.
5 V
+
10 F
(continued)
0.1 F
2.37 k
R1
2.15 k
R2
Figure 14. Common 2.4 V Voltage Reference
2
3
–
+
5 V
8
4
1/2 LM2904
OR
EQUIVALENT
1
1.91 k
RL
Figure 14 shows a circuit that can provide the 2.4 V ref-
erence for 16 or more channels. Even with the com-
mon reference voltage, interchannel crosstalk remains
insignificant. The circuit employs a single supply op
amp as a voltage follower. R1 and R2 set up the refer-
ence voltage. RL provides a reference bias when all
channels are programmed off and provides a dis-
charge path for the reference filtering. The op amp
supplies an ample minimum of 20 mA. Each channel’s
VRTX node only requires a maximum of 340 A, and
VTX is a high-impedance input. The R
provide the necessary bias for the VTX inputs.
0.1 F
+
10 F
R
301 k
301 k
301 k
R
R
VREF15
VREF0
VREF1
VRTX15
VRTX1
VRTX0
VTX0
VTX1
VTX15
Codec Chip Set
VREFx
TO
T8531/2
resistors
12-3570 (F)x
45