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Page 6

Balanced Line Receiver ICs

Preliminary Information

kHz

nV

Hz

dBu

==

-

´

10

0775

20

130

20

173

()

,

.

The noise of the 249

W 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

nV

Hz

nV

Hz

nV

Hz

=+

=

(.

)

( .

)

.

173

2 06

2 68

22

The noise floor can then be calculated to be

Noise

dB

dBu

kHz

nV

Hz

=

é

ë

ê

ê

ù

û

ú

ú

=-

´

20

126 2

268

20

0775

log

.

.

.

u

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

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 k

W

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.