MT8880CS datasheet(3/18 Pages) MITEL

3 / 18 page

ISO2-CMOS

MT8880C/MT8880C-1

4-35

Functional Description

The MT8880C/C-1 Integrated DTMF Transceiver

architecture consists of a high performance DTMF

receiver with internal gain setting amplifier and a

DTMF generator which employs a burst counter such

that

precise

tone

bursts

and

pauses

can

synthesized. A call progress mode can be selected

such that frequencies within the specified passband

can

detected.

standard

microprocessor

interface allows access to an internal status register,

two control registers and two data registers.

Input Configuration

The input arrangement of the MT8880C/C-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

adjustment of gain. 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.

Figure 3 - Single-Ended 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 Fig. 7). These 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

RIN

IN+

IN-

VRef

VOLTAGE GAIN

(AV) = RF / RIN

MT8880C/C-1

Figure 4 - Differential Input Configuration

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.

Following the filter section is a decoder employing

digital

counting

techniques

determine

the

frequencies of the incoming tones and to verify that

they correspond to standard DTMF frequencies. A

complex averaging algorithm protects against tone

simulation by extraneous signals such as voice while

providing tolerance to small frequency deviations

and variations. This averaging algorithm has been

developed to ensure an optimum combination of

immunity to talk-off and tolerance to the presence of

interfering frequencies (third tones) and noise. When

the detector recognizes the presence of two valid

tones (this is referred to as the “signal condition” in

some industry specifications) the “Early Steering”

(ESt)

output

will

active

state.

Any

subsequent loss of signal condition will cause ESt to

assume an inactive state.

IN+

IN-

VRef

MT8880C/C-1

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

MT8880CS Datasheet(PDF) 3 Page - Mitel Networks Corporation

MT8880CS Datasheet(HTML) 3 Page - Mitel Networks Corporation

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

Datasheet Download

Link URL