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AD8018 Datenblatt(PDF) 11 Page - Analog Devices |
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AD8018 Datenblatt(HTML) 11 Page - Analog Devices |
11 / 19 page REV. 0 AD8018 –11– Following these generic guidelines will improve the performance of the AD8018 in all applications. To optimize the AD8018’s performance as an ADSL differential line driver, locate the transformer hybrid near the AD8018 drivers and as close to the RJ11 jack as possible. Maintain differential circuit symmetry into the differential driver and from the output of the drivers through the transformer-coupled output of the bridge circuit as much as possible. CPE ADSL Application The low-cost, high-output current dual AD8018 xDSL driver amplifiers have been specifically designed to drive high fidelity xDSL signals to within 0.5 V of the power rails, the performance needed to provide CPE ADSL on a single 5 V supply. The AD8018 may be used in transformer-coupled bridge hybrid cir- cuits to drive modulated signals including Discrete MultiTone (DMT) upstream to the central office. Evaluation Board The AD8018ARU-EVAL evaluation board circuit in Figure 12 offers the ability to evaluate the AD8018 in a typical xDSL bridge hybrid circuit. The receiver circuit on these boards is typically unpopulated. Requesting samples of the AD8022AR with the AD8018ARU- EVAL board will provide the capability to evaluate the AD8018ARU along with other Analog Devices products in a typi- cal transceiver circuit. The evaluation circuits have been designed to replicate the CPE side analog transceiver hybrid circuits. The circuit mentioned above is designed using a one-transformer transceiver topology including a line receiver, line driver, line matching network, an RJ11 jack for interfacing to line simulators, and transformer-coupled inputs for single-ended-to-differential input conversion. AC-coupling capacitors of 0.01 µF, C8, and C10, in combina- tion with 10 k Ω resistors R24 and R25, will form a zero frequency at 1.6 kHz. Transformer Selection Customer premise ADSL requires the transmission of a +13 dBm (20 mW) DMT signal. The DMT signal can have a crest factor as high as 5.3, requiring the line driver to provide peak line power of 27.5 dBm (560 mW). 27.5 dBm peak line power translates into a 7.5 V peak voltage on the 100 Ω telephone line. Assuming that the maximum low-distortion output swing available from the AD8018 line driver on a 5 V supply is 4 V and, taking into account the power lost due to the termination resistance, a step-up transformer with turns ratio of 4.0 or greater is needed. In the simplified differential drive circuit shown in Figure 2, the AD8018 is coupled to the phone line through a step-up trans- former with a 1:4 turns ratio. R1 and R2 are back-termination or line-matching resistors, each 3.1 Ω (100 Ω/(2 × 42)), where 100 Ω is the approximate phone line impedance. The total dif- ferential load for the AD8018, including the termination resistors, is 12.5 Ω. Even under these conditions the AD8018 provides low distortion signals to within 0.5 V of the power rails. Stability Enhancements The CPE bridge hybrid circuit presents a complex impedance to the drive amplifiers, particularly when transformer parasitics are factored in. To ensure stable operation under the full range of load conditions, a series R-C network (Zoebel Network) should be connected between each amplifier’s output and ground. The recommended values are 10 Ω for the resistor and 1 nF for the capacitor to create a low impedance path to ground at frequen- cies above 16 MHz (see Figure 2). R33 and R34 are added to improve common-mode stability. Receive Channel Considerations A transformer used at the output of the differential line driver to step up the differential output voltage to the line has the inverse effect on signals received from the line. A voltage reduction or attenuation equal to the inverse of the turns ratio is realized in the receive channel of a typical bridge hybrid. The turns ratio of the transformer may also be dictated by the ability of the receive circuitry to resolve low-level signals in the noisy twisted pair tele- phone plant. Higher turns ratio transformers effectively reduce the received signal-to-noise ratio due to the reduction in the received signal strength. The AD8022, a dual amplifier with typical RTI voltage noise of only 2.5 nV/ √Hz and a low supply current of 4 mA/amplifier, is recommended for the receive channel. DMT Modulation, MultiTone Power Ratio (MTPR), and Out-of-Band SFDR ADSL systems rely on DMT modulation to carry digital data over phone lines. DMT modulation appears in the frequency domain as power contained in several individual frequency subbands, sometimes referred to as tones or bins, each of which is uniformly separated in frequency. A uniquely encoded, Quadra- ture Amplitude Modulation (QAM)-like signal occurs at the center frequency of each subband or tone. See Figure 9 for an example of a DMT waveform in the frequency domain, and Figure 10 for a time domain waveform. Difficulties will exist when decoding these subbands if a QAM signal from one subband is corrupted by the QAM signal(s) from other subbands, regardless of whether the corruption comes from an adjacent subband or harmonics of other subbands. Conventional methods of expressing the output signal integrity of line drivers, such as single-tone harmonic distortion or THD, two-tone InterModulation Distortion (IMD), and third order intercept (IP3), become significantly less meaningful when amplifiers are required to process DMT and other heavily modulated waveforms. A typical ADSL upstream DMT signal can contain as many as 27 carriers (subbands or tones) of QAM signals. MultiTone Power Ratio (MTPR) is the relative difference between the measured power in a typical subband (at one tone or carrier) versus the power at another subband spe- cifically selected to contain no QAM data. In other words, a selected subband (or tone) remains open or void of intentional power (without a QAM signal), yielding an empty frequency bin. MTPR, sometimes referred to as the “empty bin test,” is typically expressed in dBc, similar to expressing the relative difference between single-tone fundamentals and second or third harmonic distortion components. Measurements of MTPR are typically made on the line side or secondary side of the transformer. |
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Ähnliche Beschreibung - AD8018 |
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