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TCM320AC46
GENERALPURPOSE AUDIO INTERFACE FOR DSP
SLWS001 D4091, JUNE 1993
POST OFFICE BOX 655303
DALLAS, TEXAS 75265
POST OFFICE BOX 1443
HOUSTON, TEXAS 77001
6
conversion laws
The TCM320AC46 provides
-law companding operation. The linear mode utilizes a 13-bit 2s complement
format.
transmit operation
microphone input
The microphone input amplifier is designed to simplify interface to electret-type microphone elements as shown
in Figure 1. The VMID buffer circuit provides a voltage (MICBIAS) equal to VCC/2 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 to the amplifier inverting input (MICIN) to set the amplifier gain.
VMID is brought out to provide a place to filter the VMID voltage.
microphone mute function
The MICMUTE input disables the microphone amplifier and attenuates the signal on the MICGS output to a level
that is 80 dB or more down from the signal on the MICIN input. MICMUTE also causes the digital circuitry to
transmit all zero code on DOUT.
TCM320AC46
2 k
17
VMID
18
MICIN
19
MICGS
20
MICBIAS
To Transmit Filter
Microphone Amplifier
VMID Buffer
>10 k
+
for Amplifiers
VMID Reference
1
F
VMID Generator
Microphone
Electret
+
0.1
F
MICMUTE
Microcontroller
6
+
Figure 1. Typical Microphone Interface
transmit filter
A low-pass antialiasing section is included on the device. This section provides 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.