THS6007

DUAL DIFFERENTIAL LINE DRIVERS AND LOW-POWER RECEIVERS

SLOS334– DECEMBER 2000

21

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 THS6007 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

harmonic distortion measurements are shown in Figures 30 and 31. It is commonly known that in the differential

driver configuration, the second order harmonics tend to be reduced by 6 dB or more. 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

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.

ADSL receive line noise

Per ANSI T1.413, the receive noise power spectral density for an ADSL line is –140 dBm/

√Hz. This results in

a voltage noise requirement of less than 31.6 nV/

√Hz for the receiver in an ADSL system with a 1:1 transformer

ratio.

Noise Power Spectral Density = –140 dBm/

√Hz

Power = 1e–17

× 1 Hz = 0.01 fW

For ADSL systems that use a 1:2 transformer ratio, such as central office line cards, the voltage noise

requirement for the receiver is lowered to 15.8 nV/

√Hz.

TRANSFORMER

RATIO

Vnoise ON LINE

1:1

31.6 nV/

√Hz

1:2

15.8 nV/

√Hz

The THS6007 receiver was designed to operate with 10 nV/

√Hz voltage noise, exceeding the noise

requirements for an ADSL system operating with 1:1 or 1:2 transformer ratios.