THAT1240 Datasheet, PDF (6/8 Pages) List of Unclassifed Manufacturers – Balanced Line Receiver ICs

Page 6
e_{n line receiver}
= ^{10 ( -13200dBu) ´ 0,775} = 1.73
20kHz
nV
Hz
The noise of the 249W resistor is 2.05 nV/ÖHz.
We can assume that the noise contribution of R8 and
R19 will be negligible, and therefore, the total noise
density going into the input of the ADC will be
e_{n total} =
(1.73 ^nV )² + (2. 06 ^nV )² = 2. 68 ^nV
Hz
Hz
Hz
The noise floor can then be calculated to be
é 2.68 ^nV ´ 20kHz ù
Noise_dBu = 20 log ê
ëê
Hz
0.775
ú = -126. 2 dBu
ûú
Figure 7 shows an excellent method for control-
ling gain in a balanced system. There is often a
temptation in these systems to keep the signal bal-
anced and use two VCAs to independently control the
Balanced Line Receiver ICs
Preliminary Information
gain on each half of the balanced signal. Unfortu-
nately, this can results in common mode to differen-
tial mode conversion (degradation of CMRR) when
there are even slight differences in gain between the
VCAs. A better approach is to convert the signal to
single-ended, alter the gain, and then convert back to
balanced.
In Figure 7 we use a THAT 1243 -3 dB line re-
ceiver to do the balanced to single-ended conversion.
The VCA section also has a static gain of -3 dB due to
the ratio of R2 and R3. This circuit can accept
24 dBu, since the THAT 1243 output stage is capable
of 21 dBu without distortion. Reducing R3 to 14 kW
results in a 3 dB reduction in VCA output noise.
This arrangement results in 3 dB greater dynamic
range compared to the case where a -6 dB line re-
ceiver and a VCA with zero dB static gain are used.
After the VCA, the signal is restored to 24 dBu by the
THAT 1430.
THAT Corporation; 45 Sumner Street; Milford, Massachusetts 01757-1656; USA
Tel: [phone]; Fax: [phone]; Web: www.thatcorp.com