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MT8880CC Datasheet(PDF) 3 Page - Mitel Networks Corporation |
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MT8880CC Datasheet(HTML) 3 Page - Mitel Networks Corporation |
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 be synthesized. A call progress mode can be selected such that frequencies within the specified passband can be detected. A 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 C RIN RF IN+ IN- GS 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 to 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 go to an active state. Any subsequent loss of signal condition will cause ESt to assume an inactive state. C1 C2 R1 R2 R3 R4 R5 IN+ IN- GS 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 |
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Similar Description - MT8880CC |
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