MT88L89AS datasheet(3/20 Pages) MITEL

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Advance Information

MT88L89

4-127

Functional Description

The MT88L89 Integrated DTMF Transceiver consists

of a high performance DTMF receiver with an

internal gain setting amplifier and a DTMF generator,

which employs a burst counter to synthesize precise

tone bursts and pauses. A call progress mode can

be selected so that frequencies within the specified

passband can be detected. The adaptive micro

interface

allows

microcontrollers,

such

the

68HC11, 80C51 and TMS370C50, to access the

MT88L89 internal registers.

Power Down

The MT88L89 provides enhanced power down

functionality to facilitate minimization of supply

current consumption. DTMF transmitter and receiver

circuit blocks may be independently powered down

via register control.

When asserted, RxEN control

bit powers down all analog and digital circuitry

associated solely with the DTMF and Call Progress

receiver. The TOUT control bit is used to disable the

transmitter and put all circuitry associated only with

the DTMF transmitter in power down mode. With the

TOUT control bit asserted, the TONE output pin is

held in a high impedance (floating) state. When both

power down control bits are asserted, circuits utilized

by both the DTMF transmitter and receiver are also

powered down. This includes the crystal oscillators,

and the VRef generator. In addition, the IRQ , TONE

output and DATA pins are held in a high impedance

state.

Input Configuration

The input arrangement of the MT88L89 provides a

differential-input operational amplifier as well as a

bias source (VRef), which is used to bias the inputs at

VDD/2. Provision is made for connection of a

feedback resistor to the op-amp output (GS) for gain

adjustment. In a single-ended configuration, the

input pins are connected as shown in Figure 3.

Figure 4 shows the necessary connections for a

differential input configuration.

Receiver Section

Separation of the low and high group tones is

achieved by applying the DTMF signal to the inputs

of two sixth-order switched capacitor bandpass

filters, the bandwidths of which correspond to the low

and high group frequencies (see Table 1). The filters

also incorporate notches at 350 Hz and 440 Hz for

exceptional dial tone rejection. Each filter output is

followed by a single order switched capacitor filter

section, which smooths the signals prior to limiting.

Limiting is performed by high-gain comparators

which are provided with hysteresis to prevent

detection of unwanted low-level signals. The outputs

of the comparators provide full rail logic swings at

the frequencies of the incoming DTMF signals.

Figure 3 - Single-Ended Input Configuration

Figure 4 - Differential Input Configuration

RIN

IN+

IN-

VRef

VOLTAGE GAIN

(AV) = RF / RIN

MT88L89

IN+

IN-

VRef

DIFFERENTIAL INPUT AMPLIFIER

C1 = C2 = 10 nF

R1 = R4 = R5 = 100 k

Ω

R2 = 60k

Ω, R3 = 37.5 kΩ

R3 = (R2R5)/(R2 + R5)

VOLTAGE GAIN

(AV diff) - R5/R1

INPUT IMPEDANCE

(ZINdiff) = 2 R1

2 + (1/

ωC)2

MT88L89

VDD

Positive power supply (3V typ.).

8,9

16,

3,5,

10-11

23-

No Connection.

Pin Description

Pin #

Name

Description

MT88L89AS Datasheet(PDF) 3 Page - Mitel Networks Corporation

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