THS6022 Datasheet, PDF (33/38 Pages) Texas Instruments – 250-mA DUAL DIFFERENTIAL LINE DRIVER

THS6022
250-mA DUAL DIFFERENTIAL LINE DRIVER
SLOS225C – SEPTEMBER 1998 – REVISED JANUARY 2000
APPLICATION INFORMATION
ADSL (continued)
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 it creates intermodulation products that interfere with ADSL carrier frequencies.
The THS6022 has 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 37 – 40. It is commonly known that in the differential driver
configuration, the second order harmonics tend to cancel out. Thus, the dominant total harmonic distortion
(THD) will be 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. This
is illustrated by Figure 40.
One problem that has been receiving a lot of attention in the ADSL area is power dissipation. One way to
substantially reduce power dissipation is to lower the power supply voltages. This is because the RMS voltage
of an ADSL remote terminal signal is 1.35-V RMS. But, to meet ADSL requirements, the drivers must have a
voltage RMS-to-peak crest factor of 5.6 in order to keep the bit-error probability rate below 10^–7. Hence, the
power supply voltages must be high enough to accomplish the peak output voltage of 1.35 V × 5.6 = 7.6 V_(PEAK).
If ±15-V power supplies are used for the THS6022 drivers in the circuit shown in Figure 61, the power dissipation
of the THS6022 is approximately 600 mW. This is assuming that part of the quiescent current is diverted back
to the load, which typically happens in a class-AB amplifier. But, if the power supplies are dropped down to
±12 V, then the power dissipation drops to appriximately 460 mW. This is a 23% reduction of power, which
ultimately lowers the temperature of the drivers and increases efficiency.
Another way to reduce power dissipation in the drivers is to increase the transformer ratio. The drawback in
doing this is that it increases the loading on the drivers and reduces the signals being received from the central
office. If this can be overcome, then a power reduction in the drivers will result. By going to a 1:2 transformer
ratio, the power supply voltages can drop to ± 6 V. The driver output voltage has now been reduced to 675-mV
RMS. But, the loading on the output of the drivers drops to 25 Ω. The power dissipated is now approximately
360 mW, a reduction of 22% over the previous example. But, the received signal is now 1/2 of the previous
example. This must be dealt with by requiring low-noise receivers. There are always trade offs when it comes
to dealing with power, so proper analysis of the system should always be considered.
general configurations
A common error for the first-time CFB user is to create a unity gain buffer amplifier by shorting the output directly
to the inverting input. A CFB amplifier in this configuration oscillates and is not recommended. The THS6022,
like all CFB amplifiers, must have a feedback resistor for stable operation. Additionally, placing capacitors
directly from the output to the inverting input is not recommended. This is because, at high frequencies, a
capacitor has a very low impedance. This results in an unstable amplifier and should not be considered when
using a current-feedback amplifier. Because of this, integrators and simple low-pass filters, which are easily
implemented on a VFB amplifier, have to be designed slightly differently. If filtering is required, simply place an
RC-filter at the noninverting terminal of the operational-amplifier (see Figure 62).
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