TCM320AC46

GENERALPURPOSE AUDIO INTERFACE FOR DSP

SLWS001 − D4091, JUNE 1993

POST OFFICE BOX 655303

POST OFFICE BOX 1443

6

conversion laws

The TCM320AC46 provides

µ-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

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

Ω

17

VMID

18

MICIN

19

MICGS

20

MICBIAS

To Transmit Filter

Microphone Amplifier

VMID Buffer

>10 k

Ω

+

−

for Amplifiers

VMID Reference

1

µF

VMID Generator

Microphone

Electret

−

+

0.1

µF

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.