THS6042_16 Datasheet, PDF (16/46 Pages) Texas Instruments – 350mA,12V ADSL CPE LINE DRIVERS

THS6042, THS6043
350 mA, ±12 V ADSL CPE LINE DRIVERS
SLOS264G − MARCH 2000 − REVISED DECEMBER 2001
APPLICATION INFORMATION
The THS6042/3 contain two independent operational amplifiers. These amplifiers are current feedback
topology amplifiers made for high-speed operation. They have been specifically designed to deliver the full
power requirements of ADSL and therefore can deliver output currents of at least 230 mA at full output voltage.
The THS6042/3 are fabricated using the Texas Instruments 30-V complementary bipolar process, HVBiCOM.
This process provides excellent isolation and high slew rates that result in the device’s excellent crosstalk and
extremely low distortion.
ADSL
The THS6042/3 were primarily designed as line drivers for ADSL (asymmetrical digital subscriber line). The
driver output stage has been sized to provide full ADSL power levels of 13 dBm onto the telephone lines.
Although actual driver output peak voltages and currents vary with each particular ADSL application, the
THS6042/3 are specified for a minimum full output current of 230 mA at ±6 V and 300 mA at the full output
voltage of ±12 V. This performance meets the demanding needs of ADSL at the client side end of the telephone
line. A typical ADSL schematic is shown in Figure 37.
The ADSL transmit band consists of 255 separate carrier frequencies each with its own modulation and
amplitude level. With such an implementation, it is imperative that signals put onto the telephone line have as
low a distortion as possible. This is because any distortion either interferes directly with other ADSL carrier
frequencies or creates intermodulation products that interfere with other ADSL carrier frequencies.
The THS6042/3 have been specifically designed for ultra low distortion by careful circuit implementation and
by taking advantage of the superb characteristics of the complementary bipolar process. Driver single-ended
distortion measurements are shown in Figures 7 − 15. In the differential driver configuration, the second order
harmonics tend to cancel out. Thus, the dominant total harmonic distortion (THD) is primarily due to the third
order harmonics. Additionally, distortion should be reduced as the feedback resistance drops. This is because
the bandwidth of the amplifier increases, which allows the amplifier to react faster to any nonlinearities in the
closed-loop system. Another significant point is the fact that distortion decreases as the impedance load
increases. This is because the output resistance of the amplifier becomes less significant as compared to the
output load resistance.
Even though the THS6042/3 are designed to drive ADSL signals that have a maximum bandwidth of 1.1 MHz,
reactive loading from the transformer can cause some serious issues. Most transformers have a resonance
peak typically occurring from 20 MHz up to 150 MHz depending on the manufacturer and construction
technique. This resonance peak can cause some serious issues with the line driver amplifier such as small
high-frequency oscillations, increased current consumption, and/or ringing. Although the series termination
resistor helps isolate the transformer’s resonance from the line-driver amplifier, additional means may be
necessary to eliminate the effects of a reactive load. The simplest way is to add a snubber network, also known
as a zoebel network, in parallel with the transformer as shown by R_(SNUB) and C_(SNUB) in Figure 36. At high
frequencies, where the transformer’s impedance becomes very high at its resonance frequency (ex: 1 kΩ @
100 MHz), the snubber provides a resistive load to the circuit. The value for R_(SNUB) should initially be set to
the impedance presented by the transformer within its pass-band. An example of this would be to use a 100-Ω
resistor for a 1:1 transformer or a 25-Ω resistor for a 1:2 transformer. The value for C_(SNUB) should be chosen
such that the –3 dB frequency is about 5 times less than the resonance frequency. For example,if the resonance
frequency is at 100 MHz, the impedance of C_(SNUB) should be equal to R_(SNUB) at 20 MHz. This leads to a value
of C_(SNUB) = 1 / (2 π f R_(SNUB)), or approximately 82 pF. This should only be used as a starting point. The final
values will be dictated by actual circuit testing.
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