T8531A/T8532 Agere Systems, Inc., T8531A/T8532 Datasheet - Page 47

• Manufacturer Part Number: T8531A/T8532
• Description: Multichannel Programmable Codec Chip Set
• Manufacturer: Agere Systems, Inc.

May 2001
Agere Systems Inc.
Applications
Common Voltage Reference
Every channel of the T8532 codec requires a 2.4 V reference (VRTX) for operation. Some SLICs provide this refer-
ence for the codec. An external circuit is required for SLICs without the reference voltage.
Figure 14 shows a circuit that can provide the 2.4 V reference for 16 or more channels. Even with the common ref-
erence voltage, interchannel crosstalk remains insignificant. The circuit employs a single supply op amp as a volt-
age follower. R1 and R2 set up the reference voltage. RL provides a reference bias when all channels are
programmed off and provides a discharge 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
VREFx
5 V
resistors provide the necessary bias for the VTX inputs.
+
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
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
TO
T8531A/2
12-3570a (F)
47