THS6007 Datasheet, PDF (21/36 Pages) Texas Instruments – DUAL DIFFERENTIAL LINE DRIVERS AND LOW-POWER RECEIVERS

THS6007
DUAL DIFFERENTIAL LINE DRIVERS AND LOW-POWER RECEIVERS
APPLICATION INFORMATION
SLOS334– DECEMBER 2000
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
the output signal produced a 2 V_O(PP) signal. Thus, the test was run at full signal and full load conditions.
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
Assume: R_L = 100 Ω
V_noise = √(P×R) = √(0.01 fW × 100 Ω) = 31.6 nV/√Hz
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
1:1
1:2
Vnoise ON LINE
31.6 nV/√Hz
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.
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