MAS9090B datasheet(17/30 Pages) MICRONAS

17 / 30 page

DA9090B.001

January 14, 1998

FUNCTIONAL DESCRIPTION

x TX Audio Path

Analog front end provides three identical differential

inputs

(MIC1,

MIC2,

MIC3)

for

capacitive

connection

microphones

auxiliary audio

circuits. Desired input signal is selected with bits VS

and TE (6 and 7) of register CR4 and forwarded to a

low noise preamplifier.

Preamplifier has 15.7 dB gain and its output is fed

to the programmable gain amplifier which provides

an additional gain from 0 to 22.5 dB in 1.5 dB steps.

Gain is controlled with bits 4-7 of register CR5.

An active RC anti alias filter is used to prevent

signal folding during the sampling. Accurate analog

to digital conversion is done by using a sigma-delta

modulator followed by a decimation filter.

Digital multiplexer (bit DE (0) in CR 7) is used to

select the input of a digital bandpass filter (300-

3400 Hz). The input can be taken from the output of

the

decimator

from

internal

ring/tone

generator. The bandpass filter output contains hard

clipping saturation logic for signals exceeding

overload level (+3.14 dB). Highpass part of the

bandpass filter can be bypassed with bit HPT of

Output data can be compressed by using CCITT A-

law or

µ255-law coding. The compression code is

selected with bits CM (5), MA (4) and IA (3) of

x RX Audio Path

Received signal is transferred into RX register in 8

bit encoded format or in 16 bit linear format. The

data is expanded by using A-law or

µ-law signal

encoding according to CCITT A and

µ255 laws. The

expansion code is selected with bits CM (5), MA (4)

and IA (3) of register CR0. Signal is then passed

through a bandpass filter (bandpass 300-3400 Hz).

The high pass section of the filter can be bypassed

with bit HPR of register CR4.

The input signal of RX gain3 is controlled with bits

SI (5), RTE (2) and SE (0) of register CR4. Bit SI

activates the transmit side tone signal, bit RTE

activates

the

ring/tone

generator

and

bit

activates the received signal to be summed to the

gain input. RX gain3 can be programmed with bits

4-7 of register CR6 from 0 dB to -30 dB with -2dB

steps. It contains also hard-clipping saturation logic.

After gain adjustment the signal is fed to a digital

sigma-delta modulator followed by a switched

capacitor (SC) reconstruction filter and a continuous

time smoothing filter. Filtered analog signal can be

directed to a speaker amplifier (SP1) or to an extra

analog output amplifier (SP2) with bits OE1 (4) and

OE2 (3) of register CR4. Gains can be set with

steps.

Differential analog outputs (SP1, SP2) are capable

of directly driving output load of 30

Ω with power

level up to 66mW. Also ceramic receivers up to

50nF can be used. Power up transient noise

suppression is used in both outputs.

x Ring and tone generator

Ring/tone generator is able to generate one or two

sinewave or squarewave frequencies (including

DTMF tones) to the transmit (TX) receive (RX) or

buzzer

paths.

Generated

frequencies

can

programmed with registers CR8 and CR9. One of

the three frequency ranges can be selected with bits

DFT and HFT of register CR10. Output signal level

of the tone generator can be selected from 0 to -27

dB with -3dB steps with bits 4-7 of register CR7.

Single ended BZ output is used to drive a buzzer by

using an external bipolar transistor with pulse width

modulated (PWM) squarewave signal f1 (CR8).

This PWM signal can also be amplitude modulated

with signal f2 (CR9). Maximum load for BZ is 5 k

Ω

and 50pF. Implementation of tone generator is fully

digital.

Therefore

amplitude

frequency

response

variations

(at

output)

over

temperature, power supply or from unit to unit exist.

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