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MT8960AE Datasheet(PDF) 4 Page - Mitel Networks Corporation |
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MT8960AE Datasheet(HTML) 4 Page - Mitel Networks Corporation |
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4 / 22 page  MT8960/61/62/63/64/65/66/67 ISO2-CMOS 6-22 Functional Description Figure 1 shows the functional block diagram of the MT8960-67. These devices provide the conversion interface between the voiceband analog signals of a telephone subscriber loop and the digital signals required in a digital PCM (pulse code modulation) switching system. Analog (voiceband) signals in the transmit path enter the chip at VX, are sampled at 8kHz, and the samples quantized and assigned 8-bit digital values defined by logarithmic PCM encoding laws. Analog signals in the receive path leave the chip at VR after reconstruction from digital 8-bit words. Separate switched capacitor filter sections are used for bandlimiting prior to digital encoding in the transmit path and after digital decoding in the receive path. All filter clocks are derived from the 2.048 MHz master clock input, C2i. Chip size is minimized by the use of common circuitry performing the A to D and D to A conversion. A successive approximation technique is used with capacitor arrays to define the 16 steps and 8 chords in the signal conversion process. Eight-bit PCM encoded digital data enters and leaves the chip serially on DSTi and DSTo pins, respectively. Transmit Path Analog signals at the input (Vx) are firstly bandlimited to 508 kHz by an RC lowpass filter section. This performs the necessary anti-aliasing for the following first-order sampled data lowpass pre-filter which is clocked at 512 kHz. This further bandlimits the signal to 124 kHz before a fifth-order elliptic lowpass filter, clocked at 128 kHz, provides the 3.4 kHz bandwidth required by the encoder section. A 50/60 Hz third-order highpass notch filter clocked at 8 kHz completes the transmit filter path. Accumulated DC offset is cancelled in this last section by a switched-capacitor auto-zero loop which integrates the sign bit of the encoded PCM word, fed back from the codec and injects this voltage level into the non-inverting input of the comparator. An integrating capacitor (of value between 0.1 and 1 µF) must be externally connected from this point (ANUL) to the Analog Ground (GNDA). The absolute gain of the transmit filter (nominally 0 dB at 1 kHz) can be adjusted from 0 dB to 7 dB in 1 dB steps by means of three binary controlled gain pads. The resulting bandpass characteristics with the limits shown in Figure 10 meet the CCITT and AT&T recommended specifications. Typical atttenuations are 30 dB for 0-60 Hz and 35 dB for 4.6 kHz and above. The filter output signal is an 8 kHz staircase waveform which is fed into the codec capacitor array, or alternatively, into an external capacitive load of 250 pF when the chip is in the test mode. The digital encoder generates an eight-bit digital word representation of the 8 kHz sampled analog signal. The first bit of serial data stream is bit 7 (MSB) and represents the sign of the analog signal. Bits 4-6 represent the chord which contains the analog sample value. Bits 0-3 represent the step value of the analog sample within the selected chord. The MT8960-63 provide a sign plus magnitude PCM output code format. The MT8964/66 PCM output code conforms to the AT &T D3 specification, i.e., true sign bit and inverted magnitude bits. The MT8965/67 PCM output code conforms to the CCITT specifications with alternate digit inversion (even bits inverted). See Figs. 3 and 4 for the digital output code corresponding to the analog voltage, VIN, at VX input. The eight-bit digital word is output at DSTo at a nominal rate of 2.048 MHz, via the output buffer as the first 8-bits of the 125 µs sampling frame. Receive Path An eight-bit PCM encoded digital word is received on DSTi input once during the 125 µs period and is loaded into the input register. A charge proportional to the received PCM word appears on the capacitor array and an 8 kHz sample and hold circuit integrates this charge and holds it for the rest of the sampling period. The receive (D/A) filter provides interpolation filtering on the 8 kHz sample and hold signal from the codec. The filter consists of a 3.4 kHz lowpass fifth-order elliptic section clocked at 128 kHz and performs bandlimiting and smoothing of the 8 kHz "staircase" waveform. In addition, sinx/x gain correction is applied to the signal to compensate for the attenuation of higher frequencies caused by the capacitive sample and hold circuit. The absolute gain of the receive filter can be adjusted from 0 dB to -7 dB in 1 dB steps by means of three binary controlled gain pads. The resulting lowpass characteristics, with the limits shown in Figure 11, meet the CCITT and AT & T recommended specifications. Typical attenuation at 4.6 kHz and above is 30 dB. The filter is followed by a buffer amplifier which will drive 5V peak/peak into a 10k ohm load, suitable for driving electronic 2-4 wire circuits. |
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