MT8889CS datasheet(3/18 Pages) MITEL

3 / 18 page

MT8889C/MT8889C-1

4-109

Functional Description

The

MT8889C/MT8889C-1

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 as the 68HC11, 80C51 and TMS370C50, to

access the MT8889C/MT8889C-1 internal registers.

Input Configuration

The input arrangement of the MT8889C/MT8889C-1

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

RIN

IN+

IN-

VRef

VOLTAGE GAIN

(AV) = RF / RIN

MT8889C/

MT8889C-1

Figure 4 - Differential Input Configuration

0= LOGIC LOW, 1= LOGIC HIGH

Table 1. Functional Encode/Decode Table

FLOW

FHIGH

DIGIT

697

1209

697

1336

697

1477

770

1209

770

1336

770

1477

852

1209

852

1336

852

1477

941

1336

941

1209

941

1477

697

1633

770

1633

852

1633

941

1633

IN+

IN-

VRef

MT8889C/

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

MT8889C-1

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