TCM320AC56 Datasheet, PDF (15/23 Pages) Texas Instruments – VOICE-BAND AUDIO PROCESSORS (VBAP)

TCM320AC56, TCM320AC57
VOICE-BAND AUDIO PROCESSORS (VBAP)
PRINCIPLES OF OPERATION
SLWS016A – JUNE 1996 – REVISED APRIL 1997
conversion laws
The TCM320AC56 provides µ-law companding operation that approximates the CCITT G.711
recommendation. The TCM320AC57 provides A-law companding operation that approximates the CCITT
G.711 recommendation. The linear mode of operation uses a 13-bit two’s-complement format and is the same
for both the TCM320AC56 and the TCM320AC57.
transmit operation
microphone input
The microphone input amplifier is designed specifically to interface to electret-type microphone elements, as
shown in Figure 5. The VMID buffer circuit provides a voltage (MICBIAS) equal to 1/2 V_CC as a reference for
the microphone amplifier and a bias voltage to the electret microphone. The microphone amplifier output
(MICGS) is used in conjunction with a feedback network and applied to the amplifier inverting input (MICIN) to
set the amplifier gain. VMID appears at a terminal to provide a place to filter the VMID voltage.
1 µF
470 pF
VMID
17
MICBIAS
20
VMID Buffer
+
–
VMID Reference
For Amplifiers ^VDD
VMID
Generator
+
–
2 kΩ
3.3 µF
+
10 kΩ
Electret
Microphone
10 kΩ
MICGS
19
MICIN
18
MICMUTE
6
Microphone Amplifier
–
+
To Transmit Filters
TCM320AC56/57 VBAP
NOTE A: Terminal numbers shown are for the DW and N packages.
Figure 5. Typical Microphone Interface
microphone mute function
The MICMUTE input causes the digital circuitry to transmit all zero code on DOUT.
transmit filter
A low-pass antialiasing section is included on the device and achieves a 35-dB attenuation at the sampling
frequency. No external components are required to provide the necessary antialiasing function for the
switched-capacitor section of the transmit filter.
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