THS6062 Datasheet, PDF (18/31 Pages) Texas Instruments – LOW-NOISE ADSL DUAL DIFFERENTIAL RECEIVER

THS6062
LOW-NOISE ADSL DUAL DIFFERENTIAL RECEIVER
SLOS228B – JANUARY 1999 – REVISED JUNE 1999
APPLICATION INFORMATION
The ADSL remote terminal receive band consists of 255 separate carrier frequencies each with its own
modulation and amplitude level. With such an implementation, it is imperative that signals received off the
telephone line have as high a signal-to-noise ratio (SNR) as possible. This is because of the numerous sources
of interference on the line. The best way to accomplish this high SNR is to have a low-noise receiver on the
front-end. It is also important to have the lowest distortion possible to help minimize against interference within
the ADSL carriers. The THS6062 was designed with these two priorities in mind.
By taking advantage of the superb characteristics of the complimentary bipolar process (BICOM), the THS6062
offers extremely low noise and distortion while maintaining a high bandwidth. There are some aspects that help
minimize distortion in any amplifier. The first is to extend the bandwidth of the amplifier as high as possible
without peaking. This allows the amplifier to eliminate any nonlinearities in the output signal. Another thing that
helps to minimize distortion is to increase the load impedance seen by the amplifier, thereby reducing the
currents in the output stage. This will help keep the output transistors in their linear amplification range and will
also reduce the heating effects. This can be seen in Figures 17 to 20, which show a 1-kΩ load distortion is much
better than a 150 Ω load.
One client side terminal circuit implementation, shown in Figure 39, uses a 1:2 transformer ratio. While creating
a power and output voltage advantage for the line drivers, the 1:2 transformer ratio reduces the SNR for the
received signals. The ADSL standard, ANSI T1.413, stipulates a noise power spectral density of –140 dBm/Hz,
which is equivalent to 31.6 nV/√Hz for a 100 Ω system. Although many amplifiers can reach this level of
performance, actual ADSL system testing has indicated that the noise power spectral density may typically be
≤ –150 dBm/Hz, or ≤ 10 nV/√Hz. With a transformer ratio of 1:2, this number reduces to less than 5 nV/√Hz.
The THS6062, with an equivalent input noise of 1.6 nV/√Hz, is an excellent choice for this application. Coupled
with a very low 1.2 pA/√Hz equivalent input current noise and low value resistors, the THS6062 will ensure that
the received signal SNR will be as high as possible.
18
• POST OFFICE BOX 655303 DALLAS, TEXAS 75265