THS6002

DUAL DIFFERENTIAL LINE DRIVERS AND RECEIVERS

SLOS202D– JANUARY 1998– REVISED JULY 1999

35

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 THS6002 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 Figure 23. 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. For this test, the load was 25

Ω and the output signal

resistor used for the test was 4 k

Ω, the distortion numbers are actually in a worst-case scenario. Distortion

should be reduced as the feedback resistance drops. This is because the bandwidth of the amplifier increases

dramatically, 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.