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AFE1104E Datasheet(PDF) 6 Page - Burr-Brown (TI) |
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AFE1104E Datasheet(HTML) 6 Page - Burr-Brown (TI) |
6 / 10 page 6 ® AFE1104 THEORY OF OPERATION The transmit channel consists of a switched-capacitor pulse forming network followed by a differential line driver. The pulse forming network receives symbol data from SPAROW’S DAC+ line and generates a 2B1Q output wave- form. The output meets the pulse mask and power spectral density requirements defined in European Telecommunica- tions 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 combining 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 that can 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 AFE1104 is illustrated in Figure 1 shown below. 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. 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 in two’s complement code. FIGURE 1. Functional Block Diagram of AFE1104. 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 ANALOG INPUT OUTPUT CODE (rxD13 - rxD0) Positive Full Scale 01111111111111 Mid Scale 00000000000000 Negative Full Scale 10000000000000 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. rxGAIN1 rxGAIN0 GAIN 0 0 0dB 0 1 3.25dB 1 0 6dB 1 1 9dB |
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