Datasheet

 
  ±     
SLOS264G − MARCH 2000 − REVISED DECEMBER 2001
17
POST OFFICE BOX 655303 • DALLAS, TEXAS 75265
APPLICATION INFORMATION
ADSL (continued)
One problem in the ADSL CPE area is noise. 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 such as the THS6062 or
OPA2822 on the front-end. Even if the receiver has very low noise characteristics, noise could be dominated
by the line driver amplifier. The THS6042/3 were primarily designed to circumvent this issue.
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 be required to have ≤ –150
dBm/Hz, or ≤ 10 nV/√Hz
. With a transformer ratio of 1:2, this number reduces to less than 5 nV/√Hz. The
THS6042/3, with an equivalent input noise of 2.2 nV/√Hz
, is an excellent choice for this application. Coupled
with a low 2.1 pA/√Hz
noninverting current noise, a very low 11 pA/√Hz inverting current noise, and low value
resistors, the THS6042/3 ensures that the received signal SNR is as high as possible.