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AFE1203 Datasheet(PDF) 6 Page - Texas Instruments |
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AFE1203 Datasheet(HTML) 6 Page - Texas Instruments |
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6 / 16 page  6 ® AFE1203 THEORY OF OPERATION The transmit channel consists of a D/A converter and a switched-capacitor pulse forming network followed by a differential line driver. The pulse forming network receives symbol data from the DSP and generates a 2B1Q output waveform. The output meets the pulse mask and power spectral density requirements defined in European Telecom- munications Standards Institute document RTR/TM-03036 for E1 mode and in sections 6.2.1 and 6.2.2.1 of Bellcore technical advisory TA-NWT-001210 for T1 mode. The differential line driver uses a composite output stage com- bining class B operation (for high efficiency driving large signals) with class AB operation (to minimize crossover distortion). The receive channel is designed around a fourth-order delta sigma A/D converter. It includes a difference amplifier designed to be used with an external compromise hybrid for first-order analog crosstalk reduction. A programmable gain amplifier with gains of 0dB to +9dB is also included. The delta sigma modulator operating at a 24X oversampling ratio produces 14 bits of resolution at output rates up to 584kHz. The basic functionality of the AFE1203 is illustrated in Figure 1. The receive channel operates by summing the two differen- tial inputs, one from the line (rxLINE) and the other from the compromise hybrid (rxHYB). The connection of these two inputs so that the hybrid signal is subtracted from the line signal is described in the paragraph titled “Echo Cancella- tion in the AFE”. The equivalent gain for each input in the difference amp is 1. The resulting signal then passes to a programmable gain amplifier which can be set for gains of 0dB through 9dB. The ADC converts the signal to a 14-bit digital word, rxD13-rxD0. SCALEABLE TIMING The AFE1203 scales operation with the clock frequency. All internal filters, the A/D converter, the D/A converter, and the pulse former change frequency with the clock speed so that the unit can be used at different frequencies by changing the clock speed. rxLOOP INPUT rxLOOP is the loopback control signal. When enabled, the rxLINEP and rxLINEN inputs are disconnected from the AFE. The rxHYBP and rxHYBN inputs remain connected. Loopback is enabled by applying a positive signal (Logic 1) to rxLOOP. ECHO CANCELLATION IN THE AFE The rxHYB input is designed to be subtracted from the rxLINE input for first-order echo cancellation. To accom- plish this, note that the rxLINE input is connected to the same polarity signal at the transformer (positive to positive and negative to negative) while the rxHYB input is con- nected to opposite polarity through the compromise hybrid (negative to positive and positive to negative) as shown in Figure 2. RECEIVE DATA CODING The data from the receive channel A/D converter is coded in Straight Offset Binary. FIGURE 1. Functional Block Diagram of AFE1203. rxGAIN1 rxGAIN0 GAIN 0 0 0dB 0 1 3.25dB 1 0 6dB 1 1 9dB ADC Pulse Former Programmable Gain Amp Difference Amplifier Differential Line Driver txLINE P txLINE N rxHYB P rxHYB N rxLINE P rxLINE N txDAT rxD13 - rxD0 14 D/A Converter ANALOG INPUT OUTPUT CODE (rxD13 - rxD0) Positive Full Scale 11111111111111 Negative Full Scale 00000000000000 RECEIVE CHANNEL PROGRAMMABLE GAIN AMPLIFIER The gain of the amplifier at the input of the Receive Channel is set by two gain control pins, rxGAIN1 and rxGAIN0. The resulting gain between 0dB and +9dB is shown below. |
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