THS6032

LOW-POWER ADSL CENTRAL-OFFICE LINE DRIVER

SLOS233C – APRIL1999 – REVISED MARCH 2000

13

POST OFFICE BOX 655303

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 THS6032 has been specifically designed for ultralow 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 11 – 15. 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.

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 central office signal is 1.65-V RMS at each driver’s output with a 1:2 transformer. But, to meet ADSL

requirements, the drivers must have a voltage peak-to-RMS crest factor of 5.6 in order to keep the bit-error

peak output voltage of 1.65 V

This high peak output voltage requirement, coupled with a low RMS voltage requirement, does not lend itself

to conventional high efficiency designs. One way to save power is to decrease the bias currents internal to the

amplifier. The drawback of doing this is an increase in distortion and a lower frequency response bandwidth.

This is where the THS6032 class-G architecture is useful. The class-G output stage utilizes both a high supply